Transmission I 03 200909 SDH Basics 126P

Transmission_I_03_200909 SDH Basics

Course Objectives:

To master the SDH frame structure and multiplexing/demultiplexing procedure

To master the overhead bytes and common alarms detected by the overhead byes

To master the NE type in SDH network and their features

To master the features of different SDH network topology

To master different protection principles of the self-healing network

To master the transmission performance

Reference:

Unitrans ZXSM Series SDH Equipment Training Manual

Contents

1 SDH Overview............................................................................................................................................1

1.1 SDH Concept.....................................................................................................................................1

1.2 SDH Generation Background............................................................................................................1

1.3 Limitations of PDH...........................................................................................................................2

1.3.1 Interfaces................................................................................................................................2

1.3.2 Multiplexing Method..............................................................................................................3

1.3.3 Operation and Maintenance....................................................................................................4

1.3.4 No Unified NMS Interface.....................................................................................................5

1.4 Advantages of SDH...........................................................................................................................5

1.4.1 Interfaces................................................................................................................................5

1.4.2 Multiplexing method..............................................................................................................6

1.4.3 Operation and Maintenance....................................................................................................7

1.4.4 Compatibility..........................................................................................................................7

1.5 Limitations of SDH...........................................................................................................................8

1.5.1 Low Utilization Ratio of Frequency Band..............................................................................8

1.5.2 Complicated Pointer Justification Mechanism.......................................................................8

1.5.3 Impact of Much Use of Software on System Security............................................................9

2 SDH Frame Structure and Multiplexing................................................................................................11

2.1 SDH frame structure........................................................................................................................11

2.1.1 Payload.................................................................................................................................13

2.1.2 Section Overhead (SOH)......................................................................................................13

2.1.3 Administrative Unit Pointer (AU-PTR)................................................................................14

2.2 Structure and Process of SDH Multiplexing...................................................................................14

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2.3 Concepts of Mapping, Aligning, and Multiplexing.........................................................................16

2.3.1 Mapping...............................................................................................................................16

2.3.2 Aligning................................................................................................................................16

2.3.3 Multiplexing.........................................................................................................................16

3 SDH Overhead and Pointer.....................................................................................................................18

3.1 SDH Overhead................................................................................................................................18

3.1.1 Concept of Overhead............................................................................................................18

3.1.2 Section Overhead Bytes.......................................................................................................18

3.1.3 STM-N Section Overhead....................................................................................................22

3.1.4 Path Overhead......................................................................................................................24

3.2 SDH Pointers...................................................................................................................................28

4 Logic Structure of SDH Equipment........................................................................................................31

4.1 Common Network Elements in SDH Network...............................................................................31

4.1.1 TM — Terminal Multiplexer................................................................................................31

4.1.2 ADM — Add/Drop Multiplexer...........................................................................................32

4.1.3 REG — Regenerator.............................................................................................................33

4.1.4 DXC — Digital Cross-Connect Equipment..........................................................................33

4.2 Logical Functional Blocks of SDH Equipment...............................................................................35

5 Topology and Protection of SDH Network.............................................................................................39

5.1 Significance of Network Protection................................................................................................39

5.2 Basic SDH Network Topologies......................................................................................................39

5.3 Concept and Classification of Self-healing.....................................................................................41

5.3.1 Overview..............................................................................................................................41

5.3.2 Self-healing Concept............................................................................................................42

5.3.3 Self-healing Classification....................................................................................................43

5.4 Chain Network Protection...............................................................................................................44

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5.4.1 Overview..............................................................................................................................44

5.4.2 Basic Chain Network Protection Types................................................................................45

5.5 Self-healing Ring Protection...........................................................................................................46

5.5.1 Self-healing Ring Classification...........................................................................................46

5.5.2 Two-fiber Unidirectional Path Protection Ring....................................................................47

5.5.3 Two-fiber Bidirectional Path Protection Ring,.....................................................................50

5.5.4 Two-Fiber Bidirectional MS Protection Ring.......................................................................51

5.5.5 Four-fiber Bidirectional MS Protection Ring.......................................................................53

5.5.6 Comparison of Common Self-healing Rings........................................................................56

5.6 Dual Node Interconnection (DNI) Protection..................................................................................57

5.6.1 Terminologies.......................................................................................................................57

5.6.2 DNI Principle.......................................................................................................................59

5.6.3 Application Instance.............................................................................................................60

5.7 Error Connection and Error Squelch...............................................................................................63

5.7.1 Error Connection..................................................................................................................63

5.7.2 Error Squelch of Error Connection.......................................................................................63

5.8 Logical Subnet Protection...............................................................................................................64

5.8.1 Overview..............................................................................................................................64

5.8.2 Basic Principles....................................................................................................................65

5.8.3 Categorization......................................................................................................................66

5.8.4 Application Instance.............................................................................................................66

5.9 Topology and Features of Complicated Network............................................................................72

5.9.1 T Network.............................................................................................................................72

5.9.2 Ring-chain Network.............................................................................................................73

5.9.3 Tributary Cross-Over of Ring Subnets.................................................................................74

5.9.4 Tangent Rings.......................................................................................................................74

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5.9.5 Intersected Rings..................................................................................................................75

5.9.6 Hinge Network.....................................................................................................................76

5.10 Overall Architecture of SDH Network..........................................................................................76

6 Timing and Synchronization...................................................................................................................81

6.1 Synchronization Modes...................................................................................................................81

6.1.1 Pseudo Synchronization.......................................................................................................81

6.1.2 Master/Slave Synchronization..............................................................................................82

6.2 Working Modes of Sub-Clock in Master/Slave Synchronous Network..........................................83

6.2.1 Normal Working Mode - Track and Lock the Upper Level Clock.......................................84

6.2.2 Hold-on Mode......................................................................................................................84

6.2.3 Free Run Mode – Free Oscillation Mode.............................................................................84

6.3 Network Synchronization Requirements of SDH............................................................................84

6.4 Clock Source Types of SDH NE.....................................................................................................85

6.5 Selection Principle of Clock in SDH Network................................................................................86

6.5.1 Synchronization Principle of SDH Network........................................................................87

6.5.2 Instance.................................................................................................................................88

7 Optical Interfaces.....................................................................................................................................93

7.1 Optical Interface Types....................................................................................................................93

7.2 Optical Interface Parameters...........................................................................................................94

7.2.1 Optical Line Code Pattern....................................................................................................95

7.2.2 S Point Specifications-Specifications of Optical Transmitter...............................................95

7.2.3 R Point Specifications-Specifications of Optical Receiver...................................................96

8 Transmission Performance......................................................................................................................99

8.1 Bit Error Characteristics..................................................................................................................99

8.1.1 Generation and Distribution of Bit Error..............................................................................99

8.1.2 Measurement of Bit Error Performance.............................................................................100

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8.1.3 Bit Error Specifications Related to Digital Section............................................................101

8.1.4 Measures to Reduce Bit Error............................................................................................102

8.2 Availability Parameters..................................................................................................................102

8.3 Jitter/Wander Performance............................................................................................................103

8.3.1 Generation Principles of Jitter/Wander...............................................................................103

8.3.2 Jitter Performance Specifications.......................................................................................104

8.3.3 Measures to Reduce Jitter...................................................................................................106

8.3.4 Notes...................................................................................................................................106

9 Test...........................................................................................................................................................109

9.1 SDH Test Method..........................................................................................................................109

9.2 SDH Tested Items..........................................................................................................................109

10 Introduction to Network Management...............................................................................................111

10.1 TMN Fundamentals.....................................................................................................................111

10.1.1 TMN Management Frame.................................................................................................111

10.1.2 Physical Structure of TMN...............................................................................................112

10.1.3 TMN Interfaces.................................................................................................................113

10.1.4 TMN Layers Division.......................................................................................................114

10.2 SDH Management Network (SMN)............................................................................................114

10.2.1 SMN and TMN.................................................................................................................114

10.2.2 SDH Management Interfaces............................................................................................115

10.3 SDH Management Functions.......................................................................................................116

10.4 OSI Model and ECC Protocol Stack...........................................................................................116

10.4.1 OSI Concept.....................................................................................................................116

10.4.2 ECC Protocol Stack Description.......................................................................................117

v

1 SDH Overview

Key points

Limitations of PDH

Advantages of SDH

Limitations of SDH

1.1 SDH Concept

Synchronous Digital Hierarchy or SDH is an international standard for wide band

transmission hierarchy, which defines the transmission rate, frame structure,

multiplexing mode, and optical interface specifications of digital signal transmission.

1.2 SDH Generation Background

The high developing information society nowadays requires the ability of

communication networks to provide various telecommunication services. The amount

of information transmitted, switched, and processed by telecommunications network

keeps increasing requiring modern communication networks to develop towards

digitalization, integration, intelligentization and personalization.

Transmission system is an important part of communication networks. The quality of

transmission system makes a direct effect on the development of communication

network. Lots of countries are developing information highway by constructing optical

transmission network with bigger capacity. The optical transmission network based on

SDH/WDM is the basic physical platform of the information highway. The

transmission network should have universal unified interface specifications, so that

every user in the world can communicate conveniently anytime and anywhere.

The multiplexing method used by PDH cannot satisfy the transmission requirements of

bigger capacity. The regional specifications of PDH make network interconnections

difficult, and restrict the transmission network development to higher rates.

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1.3 Limitations of PDH

Limitations of traditional PDH mainly lie in the following aspects.

1.3.1 Interfaces

1. PDH only has regional specifications for electrical interfaces, and no universal

standard.

The current PDH system has three different signal rate standards: European,

North American, and Japanese systems. They have different electrical interface

rate levels, signal frame structures, and multiplexing methods, which makes

international inter-working very difficult and does not adapt to the development

of current communication industry. The electrical interface rate levels of the

three systems are shown in Fig. 1.2-1.

565Mbit/s

139Mbit/s

34Mbit/s

8Mbit/s

2Mbit/s

1.6Gbit/s

400Mbit/s

100Mbit/s

6.3Mbit/s

1.5Mbit/s

274Mbit/s

45Mbit/s

6.3Mbit/s

× 4 × 4

× 4

× 4× 4

× 4

× 4

× 4

× 6

× 7

× 3

European system

Japanese system

North American system

× 5

32Mbit/s

Fig. 1.2-1 PDH rate levels

2. PDH does not have universal unified standards for optical interfaces.

Different manufacturers use their own line code pattern to monitor the

transmission performance of optical lines. The typical instance is the mBnB

code, where mB is the information code, and nB is the redundant code. The

function of redundant code is to monitor the transmission performance of

optical lines, which makes the signal rate at the same level of optical interface

greater than the standard signal rate at electrical interface. However, it adds

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Chapter 10 Introduction to Network Management

requirement for the transmission bandwidth of optical channels. Meanwhile,

different manufacturers add different redundant codes to the information codes

when coding line signals, resulting in different code patterns and rates for the

same optical interfaces at the same rate level from different manufacturers, and

the equipments from different manufacturers are incompatible with each other.

Thus the hybrid networking by equipments from multiple manufacturers is

restricted in the transmission network and the cost of network construction and

operation is increased, which brings difficulty to networking application,

network management and interconnection.

1.3.2 Multiplexing Method

In PDH system, only PCM equipment adopts synchronous multiplexing method to

multiplex 64 kbit/s signals to a basic group rate; while all other groups adopts the

“Plesiochronous Multiplexing” method. Because the signals at all levels of PDH rates

are asynchronous, positive signal rate justification is required to adapt and

accommodate the rate difference of tributary signals at various levels.

Since PDH uses asynchronous multiplexing method, when low-speed signals are

multiplexed into high-speed signals, their locations in the frame structures of the high-

speed signals do not have a regular pattern. In other words, low-speed signals cannot

be located easily in high-speed signals, resulting in low-speed signals being unable to

be directly dropped from or added to high-speed signals. For example, 2 Mbit/s signals

cannot be directly added to or dropped from 140 Mbit/s signals, which causes two

problems:

1. The low-speed signals have to be dropped from or added to high-speed signals

level by level. For example, to drop/add 2 Mbit/s signals from/to 140 Mbit/s

signals, we need to follow the procedure shown in Fig. 1.2-2.

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2Mbit/s (electrical signal)

140/34(Mbit/s)

34/8(Mbit/s) 8/34(Mbit/s)

34/140(Mbit/s)

8/2(Mbit/s) 2/8(Mbit/s)

PDHOptical signal

Optical/

electrical

Dem

ultiplex

Dem

ultiplex

Dem

ultiplex

Multiplex

Multiplex

Multiplex

Electrical

/Optical

Fig. 1.2-2 Drop/add 2Mbit/s signals from/to 140Mbit/s signals

The figure shows that adding/dropping 2 Mbit/s signals to/from 140 Mbit/s

signals use lots of “back-to-back” equipment. 2 Mbit/s signals are dropped

from 140 Mbit/s signals by three levels of demultiplexing equipment, and then

2 Mbit/s signals are added to 140 Mbit/s signals by three levels of multiplexing

equipment. One 140 Mbit/s signal can be demultiplexed to sixty-four 2 Mbit/s

signals. Even if only one 2 Mbit/s signal needs to be dropped from the 140

Mbit/s signal, a full set of three-level multiplexing/demultiplexing equipments

are required. This not only increases the equipment volume, cost, and power

consumption but also reduces the equipment reliability.

2. To drop/add 2 Mbit/s signals from/to 140 Mbit/s signals, we need to follow the

process of multiplexing/demultiplexing level by level, which can damage

signals and cause deterioration in transmission performance. For large-capacity

long-distance transmission, such defect is intolerable.

1.3.3 Operation and Maintenance

The PDH signal frame structure has very few overhead bytes for Operation,

Administration and Maintenance (OAM). This is why we need to add redundant codes

to monitor line performance when coding optical line signals. And this is unfavorable

for hierarchical management, performance monitoring, real-time service scheduling,

and control of transmission bandwidth, alarm analysis, and troubleshooting of the

transmission network.

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1.3.4 No Unified NMS Interface

PDH has no network management function, and no unified NMS (Network

Management System) interface, which is unfavorable to build unified TMN

(Telecommunication Management Network).

PDH transmission hierarchy becomes more and more unsuitable for transmission

network development because of the above defects. Therefore, Bell Communication

Institute of the U.S. first introduced the Synchronous Optical Network (SONET)

hierarchy which consists of a full set of leveled standard digital transmission structure.

CCITT accepted SONET in 1988, and renamed it Synchronous Digital Hierarchy

(SDH), making it the general technical hierarchy applicable not only to optical fiber

transmission but also to microwave and satellite transmission.

1.4 Advantages of SDH

The inherent disadvantages of PDH pave the way for the steady development of SDH

as a brand new generation of transmission hierarchy.

The main objective of SDH is to construct a digital communication network from the

aspect of uniform national telecommunications network and international

interconnection. Take the case of Integrated Services Digital Network (ISDN)

especially Broadband ISDN (B-ISDN), SDH plays an important role since SDH based

network is a highly uniform, standardized, intelligent network. It adopts a universally

uniform interfaces to make equipment from different manufacturers compatible,

manage and operate all networks efficiently and coordinately, implement flexible

networking and service scheduling, implement network self healing, improve the

utilization ratio of network resource, and saves expenses for equipment operation and

maintenance.

We will describe the advantages of SDH from the following aspects: interfaces,

multiplexing method, operation and maintenance, and compatibility.

1.4.1 Interfaces

1. Electrical interfaces

Interface standardization is the key point in determining equipment

interconnection from different manufacturers. SDH made uniform standards for

Network Node Interface (NNI). The standards include rate levels of digital

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signal, frame structure, multiplexing method, line interface, and monitoring

management making it easy for interconnection of SDH equipment from

different manufacturers, i.e. equipment of different manufacturers can be

installed on one transmission line, which presents transverse compatibility.

SDH has a set of standard hierarchy of information structure, i.e. a set of

standard rate levels. Its basic information structure hierarchy is STM-1

(Synchronous Transport Module) with a corresponding rate of 155 Mbit/s. The

higher-level digital signal series such as 622 Mbit/s (STM-4) and 2.5 Gbit/s

(STM-16) can be formed by synchronously multiplexing the information

module (e.g. STM-1) of basic rate level through byte interleaving. The number

of multiplexing is a multiple of four, e.g. STM-4 = 4×STM-1, STM-16 =

4×STM-4, STM-64 = 4×STM-16.

2. Optical interfaces

The line interfaces (optical interfaces) adopt a universally uniform standard as

well. The line coding of SDH signals only does scrambling and does not insert

redundant code.

The scrambling standard is universally uniform, which enables optical interface

interconnection of SDH equipment from different manufacturers by adding

standard scrambler to the terminal equipment. Scrambling prevents too many

consecutive “0” or “1”, making it easy to extract clock signal from line signals.

Since line signals are only scrambled, the optical signal rate of SDH line is the

same as the standard signal rate of SDH electrical interface, thus not increase

the transmission bandwidth of optical channel.

Currently, ITU-T officially recommends scrambled NRZ code to be the uniform

code for SDH optical interfaces.

1.4.2 Multiplexing method

Since lower-speed SDH signals are multiplexed into frame structure of higher-speed

SDH signals through byte-interleaving and multiplexing, the locations of lower-speed

SDH signals in the higher-speed SDH signal are regular and predictable. Thus we can

directly drop/add lower-speed SDH signal such as 155 Mbit/s (STM-1) signal from/to

higher-speed SDH signal such as 2.5 Gbit/s (STM-16) signal. The simplification of

6


signal multiplexing/demultiplexing makes SDH particularly suitable for optical fiber

communication system of high speed and bigger capacity.

Besides, SDH uses synchronous multiplexing method and flexible mapping structure,

allowing PDH low-speed tributary signal to be multiplexed within the SDH signal

frame (STM-N). Therefore the locations of low-speed tributary signals in the STM-N

frame are predictable; we can directly drop/add low-speed tributary signal from/to

SDH signal. Thus saves lots of multiplexing/demultiplexing equipment (back-to-back

equipment), increases system reliability, reduces signal damage, reduces equipment

cost and power consumption, and simplifies service dropping/adding.

SDH integrates the advantages in both software and hardware; realizes the “one-step”

multiplexing of low-speed tributary signal (e.g. 2 Mbit/s) into STM-N signal; enables

maintenance personnel to schedule services flexibly and conveniently by using only

software. The SDH multiplexing method makes it easier to implement digital cross

connect function; provides the network with strong self-healing ability; and makes it

easier for network operators to dynamically construct network according to actual

needs.

1.4.3 Operation and Maintenance

The SDH frame structure provides abundant overhead bytes for Operation,

Administration, and Maintenance (OAM), which greatly enhance the monitoring

function for the networks and improve the automation of maintenance. PDH signal has

few overhead byte, thus redundant bits need to be added during line coding for

performance monitoring of line. Taking PCM30/32 signal as example, only TS0 and

TS16 timeslots are used for overhead function in its frame structure.

SDH frame has abundant overhead bytes, which account for 1/20 of the whole

bandwidth. Thus OAM functions are enhanced and system maintenance expenses are

reduced. According to statistics, the combined cost of SDH system is only 65.8% of

PDH system, in which the reduction of maintenance expenses plays an important role.

1.4.4 Compatibility

SDH having a strong compatibility can coexist with PDH networks, hence during

construction of SDH network existing PDH equipment can be kept. That is to say, SDH

network can transmit PDH services. In addition, SDH network can also transmit

7


Asynchronous Transfer Mode (ATM) signals, and Fiber Distributed Data Interface

(FDDI) signals.

The basic transport module of SDH signal (STM-1) can accommodate various rates of

PDH tributary signals and other digital signals, such as ATM, FDDI, and DQDB; thus

exhibiting forward and backward compatibility. SDH particularly designed application

methods such as STM-N concatenation to adapt to requirements of transmitting new

services such as ATM and IP.

Various patterns (tributaries) are mapped and multiplexed at the network interface

(start point) into the STM-N frame structure, and then demultiplex the tributaries at the

SDH network boundary (end point); thus allowing transmission of signals of various

patterns in SDH transmission network.

1.5 Limitations of SDH

Limitations always come along with benefits. SDH system is not perfect either. It has

the following three limitations.

1.5.1 Low Utilization Ratio of Frequency Band

A major advantage of SDH is its enhanced reliability and enhanced automation of

OAM. This is due to a great number of overhead bytes added in the STM-N frame of

SDH. This will certainly increase the transmission rate and bandwidth, and PDH

signals occupy a lower transmission rate and bandwidth than SDH signals when

transmitting the same valid information. For example, an STM-1 signal of SDH can

accommodate sixty-three 2 Mbit/s or three 34 Mbit/s (equivalent to 48×2 Mbit/s) or one

140 Mbit/s (equivalent to 64×2 Mbit/s) PDH signals. Only when PDH signals are

multiplexed into the frames of STM-1 signals as 140 Mbit/s signals, can the STM-1

signal accommodate the information quantity of 63×2 Mbit/s. However, STM-1 (155

Mbit/s) is higher than the E4 signal (140 Mbit/s) having the same information quantity.

In other words, STM-1 occupies more transmission bandwidth than that of PDH E4

signals, when transmitting the same quantity of information.

1.5.2 Complicated Pointer Justification Mechanism

SDH system allows low-speed signals (e.g. 2 Mbit/s) to be directly dropped from high-

speed signals (e.g. STM-1) in “one step”, without the level-by-level

8


multiplexing/demultiplexing process. Such function is implemented through pointer

mechanism. The pointers indicate the locations of low-speed signals all the times, so

that the required low-speed signals can be extracted correctly. We can say that the

pointer technology is a major feature of SDH system.

However, the implementation of pointer function increases the complexity of the

system. The most important problem is the generation of a particular jitter to SDH - a

compound jitter resulting from pointer justification. Such jitter often occurs at the

network boundary (SDH/PDH), and has a low frequency and large amplitude causing

low-speed signals to degrade in transmission performance after they are disassembled.

In addition, it is very difficult to filter such jitter.

1.5.3 Impact of Much Use of Software on System Security

A major characteristic of SDH is its high OAM automation, which means that the

software accounts for a great part in the system. On one hand, this makes the system

susceptible to computer viruses, which is predominant nowadays. On the other hand,

man-made incorrect operations and software faults on the network layer could be fatal

to the system. SDH system is heavily dependent on software hence the security for

running SDH system has become an important subject that should be addressed.

SDH system is an emergent novelty. Despite its drawbacks, it has exhibited powerful

vitality in the development of transmission network. Therefore, the shift from PDH to

SDH has become an irreversible trend for the transmission network.

Summary

This chapter describes the technical background and features of SDH, with the main

aim to help you understand the overall concept of SDH.

Exercises

1. Why did SDH becomes the transmission technology used today?

2. What are the limitations of SDH?

3. What services can you transmit through SDH other than PDH?

4. Why it is a disadvantage of SDH to rely heavily on software?

5. What are OAM bytes and why are they used?

9


6. Why SDH is not frequency efficient compare to PDH?

7. How many E1, E3, and E4 can be transmitted through a single STM-1?

8. What is meant by Byte Interleave and why it is used in SDH?

9. What are the data rates of STM-64, STM-16, and STM-4?

10. What is Scrambling and why it is used in SDH frames?

11. Is it possible to locate a particular E1 inside a STM-1 frame?

10

2 SDH Frame Structure and Multiplexing

Key points

SDH frame structure

Process of multiplexing 2 M/34 M/140 M PDH signals into STM-N

Concepts of mapping and aligning

2.1 SDH frame structure

SDH signal frame structure arranges low-speed tributary signal evenly and regularly in

one frame, so that it is easy to implement synchronous multiplexing, cross-connection,

adding/dropping, and switching of tributary signals since it aims to conveniently

add/drop low-speed tributary signals to/from high-speed signals. With this, ITU-T

specified STM-N frame in a rectangular block structure with the unit of byte (eight

bits), as shown in Fig. 2.1-1.

Regenerator Section OverHead

(RSOH)

Administrative Unit Pointer (AU PTR)

Multiplex Section OverHead (MSOH)

STM-N net load (Payload)

9×N columns (bytes)261×N columns (bytes)

270×N columns

9 rows

Transmission direction

125μ s

1

3

5

9

4

Fig. 2.1-1 SDH frame structure

11

As shown in the figure, STM-N frame consists of (270×N) columns × 9 rows of bytes,

where N is the N in STM-N with the value range of 1, 4, 16, and 64. N means that the

signal is formed by byte-interleaving and multiplexing of N STM-1 signals. The frame

structure of STM-1 signal is a block of 9 rows × 270 columns. When N STM-1 signals

form STM-N signal through byte-interleaving and multiplexing, only the columns of

STM-1 signals are processed with byte-interleaving and multiplexing, and the row

number is constant.

1. Transmission mode

Serial transmission transmits signals bit by bit in the line and STM-N signal

transmission also conforms to this mode. SDH signal transmit frame bytes from

left to right frame, then top to bottom frame, byte by byte, and bit by bit. After one

row is finished, the next row follows; after one frame is transmitted, the next

frame follows.

2. Frame frequency

ITU-T specified 8000 frames/second as the frame frequency for any level of STM

signal. The time cycle of the frame is 125 μs.

3. Transmission rate of STM-N

The transmission rate of STM-1 is:

270 (270 columns for each frame) × 9 (9 rows all together) × 8 bit (8 bits per byte)

× 8000 (8000 frames/second) = 155.520 Mbit/s

Since the frame’s time cycle is constant, the rate of STM-N signal is regular. For

example, the STM-4 transmission rate constantly equals to four times of STM-1

transmission rate, the STM-16 transmission rate constantly equals to sixteen times

of STM-1 transmission rate. The regularity of SDH signal rate makes it easy to

directly drop/add low-speed tributary signal from/to high-speed STM-N signal

stream. And this is the advantage of byte-based synchronous multiplexing of

SDH. Table 2.1-1 lists the SDH rate levels.

Table 2.1-1 SDH rate levels

STM-1 STM-4 STM-16 STM-64

Rate 155.520 Mbit/s 622.080 Mbit/s 2488.320 Mbit/s 9953.280 Mbit/s

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As shown in Fig. 2.1-1, the frame of STM-N consists of three parts: Section Overhead

(SOH), including Regenerator Section Overhead (RSOH) and Multiplex Section

Overhead (MSOH); Administrative Unit Pointer (AU-PTR); and net information load

(Payload).

We will describe the functions of the three parts.

2.1.1 Payload

The payload area of STM-N frame stores user information block. It functions as the

carriage of the STM-N train, and the cargo in the carriage is the packed low-speed

signal that is to be transported. In order to monitor the cargo (packed low-speed signal)

damage, in real time during transmission, monitoring overhead bytes are added, i.e.

Path Overhead (POH) bytes, into the package of the low-speed signal. POH is loaded

into the STM-N train as part of the payload to be transported in the SDH network; it is

responsible to monitor, manage, and control the path (lower path) performance of

packaged cargo.

2.1.2 Section Overhead (SOH)

SOH are mandatory additional bytes in the STM-N frame to ensure normal

transmission of the payload, and are primarily used for OAM of the network. For

example, SOH can monitor damages of all the cargoes in the STM-N train, while POH

is used to judge which cargo is damaged when cargo damage occurs in the STM-N

train. In other words, SOH monitors the cargoes as a whole, and POH monitors one

specific cargo. SOH and POH also have other management functions.

SOH is divided into RSOH and MSOH, which monitor their corresponding section.

Actually, section functions as a transmission path, and RSOH and MSOH monitor this

transmission path.

RSOH and MSOH have different management range. For example, if 2.5 G signals are

transmitted in fiber, then RSOH monitors the transmission performance of the whole

STM-16; while MSOH monitors the transmission performance of each STM-1 in the

STM-16 signal.

RSOH is located at column (1 ~ 9×N) × row (1 ~ 3), which are 3×9×N bytes all

together. MSOH is located at column (1 ~ 9×N) × row (5 ~ 9), which are 5×9×N bytes

all together.

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2.1.3 Administrative Unit Pointer (AU-PTR)

AU-PTR is located at column 1 ~ 9×N of the fourth row, which are 9×N bytes all

together. AU-PTR indicates the exact location of the first byte of the payload in the

STM-N frame, so that the payload can be disassembled correctly according to the

indicator at the receiving end.

Pointers include higher-order and lower-order pointers. High-order pointer is AU-PTR,

and low-order pointer is TU-PTR (Tributary Unit Pointer). The function of TU-PTR is

similar to AU-PTR function, while it points to smaller payload.

2.2 Structure and Process of SDH Multiplexing

There are two cases for SDH multiplexing: one case is multiplexing STM-1 signals

into STM-N signal; the other case is multiplexing PDH tributary signals (such as 2

Mbit/s, 34 Mbit/s, and 140 Mbit/s) into STM-N SDH signal.

1. Multiplexing STM-1 signals into STM-N signal

The multiplexing is implemented by byte-interleaving, with the multiplexing base of

four, i.e. 4×STM-1→STM-4, 4×STM-4→STM-16. The frame frequency is constant

(8000 frames/second) during multiplexing, which means that the rate of upper-level

STM-N signal is four times the lower-level STM-N signal. During byte-interleaving

multiplexing, the payload and pointer bytes of each frame are interleaved and

multiplexed using the original values, while ITU-T specified special standards for

SOH. For the STM-N frame composed by synchronous multiplexing, the SOHs of

STM-N are not formed by multiplexing and interleaving all the SOHs of lower-order

STM-N frames, instead some SOHs of lower-order frames are abandoned, for which

there are special specifications. For SOH details of various level STM-N frame, refer to

chapter 3 SDH Section Overhead and Pointers.

2. Multiplexing PDH tributary signals into STM-N signal

The compatibility of SDH network requires SDH multiplexing method satisfy both

asynchronous multiplexing (e.g. multiplex PDH tributary signals into STM-N SDH

signal) and synchronous multiplexing (e.g. STM-1→STM-4), and it should be easy for

dropping/adding low-speed signal from/to high-speed STM-N signal without causing

much signal time-delay or slipping damage. To satisfy these requirements, SDH has a

unique set of multiplexing process and structure. In its multiplexing structure, SDH

14


uses pointer justification and alignment to replace the 125 μs buffer to justify the

frequency offset of positive tributary signal, and to align the phase. All service signals

will go through the three steps to be multiplexed into STM-N frame: mapping,

aligning, and multiplexing.

ITU-T defines a complete set of multiplexing structure (i.e. multiplexing routes),

through which digital signals of the three PDH systems can be multiplexed into STM-

N signals in multiple ways, as shown in Fig. 2.2-1.

STM-N AUG AU-4 VC-4

AU-3 VC-3

TUG-3

TUG-2 TU-2

TU-12

TU-11

TU-3 VC-3

VC-2

VC-12

VC-11

C-4

C-3

C-2

C-12

C-11

139264kbit/s

44736kbit/s34368kbit/s

6312kbit/s

2048kbit/s

1544kbit/s

×1

×7×3

×1

×3

×4

×7

×3

×1×N

Pointer Processing

Multiplexing

Aligning

Mapping

Fig. 2.2-1 SDH multiplexing structure defined by ITU-T

In Fig. 2.2-1, the multiplexing structure includes some basic multiplexing unit: C-

Container, VC-Virtual Container, TU-Tributary Unit, TUG- Tributary Unit Group, AU-

Administrative Unit, AUG- AU-Administrative Unit Group. The numbers of these

multiplexing units identify their corresponding signal levels. In Fig. 2.2-1, there are

multiple routes from one payload to STM-N, i.e. multiple multiplexing methods. For

example, there are two routes to multiplex 2 Mbit/s signal into STM-N signal, i.e. there

are two multiplexing methods. As a special note, 8 Mbit/s PDH tributary signal cannot

be multiplexed into STM-N signal.

Although there are multiple routes to multiplex a signal into SDH STM-N signal, the

technical system of Chinese optical synchronous transmission network defined the

PDH system based on 2 Mbit/s signal as the SDH payload, and selected AU-4

multiplexing route, as shown in Fig. 2.2-2.

15


STM-N AUG AU-4 VC-4

TUG-3

TUG-2

TU-12

TU-3 VC-3

VC-12

C-4

C-3

C-12

139264kbit/s

34368kbit/s

2048kbit/s

×7

×3

×1

×3

×1×N

Pointer processing

Multiplexing

Aligning

Mapping

Fig. 2.2-2 Basic SDH multiplexing mapping structure of China

2.3 Concepts of Mapping, Aligning, and Multiplexing

In this section, we will describe the three different steps during the process of

multiplexing low-speed PDH tributary signals into STM-N signal: mapping, aligning,

and multiplexing.

2.3.1 Mapping

Mapping is the process to adapt tributary signal into virtual container at the SDH

network border (e.g. SDH/PDH network boundary). The process of mapping involves

rate adjustment of the various PDH tributary signals (140/34/2/45 Mbit/s) first, then

loading these signals to their corresponding standard container C. Then, VC (Virtual

Container) is formed from the containers by adding corresponding path overhead. The

reverse process of mapping is called demapping.

To adapt to various network applications, there are three mapping methods:

asynchronization, bit synchronization, and byte synchronization; and two mapping

modes: floating VC and locked TU.

2.3.2 Aligning

Aligning is the procedure by which the frame offset information is incorporated into

the tributary unit or the administrative unit when adapting to the frame reference of the

supporting layer. Aligning depends on TU-PTR or AU-PTR functions. It is always

accompanied synchronously by pointer justification event.

16


2.3.3 Multiplexing

Multiplexing is the process to adapt multiple signals of lower-order path layer to

higher-order path layer (e.g. TU-12 (×3) → TUG-2 (×7) → TUG-3 (×3) → VC-4), or

to adapt multiple signals of higher-order path layer to multiplex section layer (e.g. AU-

4 (×1) → AUG (×N) → STM-N). The basic method of SDH multiplexing is to

interleave bytes of lower-order signal first, and then add some stuffing bits and

specified overheads to form the higher-order signal. The reverse process of

multiplexing is called demultiplexing.

Summary

This chapter describes the SDH frame structure and the functions of its main parts, and

the steps to multiplex PDH (2 M, 34 M, and 140 M) signals into STM-N frame.

Exercises

1. One STM-1 signal can accommodate 140 M signals, 34 M signals, 2

M signals.

2. Give the different components of the SDH frame and its function.

3. How does a 2M signal mapped to an STM-N frame?

4. Why alignment is required in SDH frames?

5. What are the two types of SOH?

6. How many bytes are used for AU-PTR in a STM-1 frame?

7. How many bytes are contained in a RSOH of a STM-1 frame?

8. What is the difference between AU-PTR and TU-PTR?

9. What is the difference between SOH and POH?

10, How a Container (C) is different from Virtual Container (VC)?

11. What is the frame frequency of a SDH frame?

12. How many rows and columns are contained in a SDH frame of STM-16?

13. How SDH multiplexing structure used in China is different from / similar to

SDH multiplexing structure defined by ITU-T?

17

3 SDH Overhead and Pointer

Key points

SDH overheads

SDH pointers

3.1 SDH Overhead

3.1.1 Concept of Overhead

Overhead is the general designation of overhead bytes/bits. It includes all bytes in the

STM-N frame except payload that carries service information. Overhead is used to

support OAM function of the transmission network. It implements layered monitoring

and management, and it can be divided into section monitoring and path monitoring.

Section monitoring includes monitoring of regenerator section and multiplex section;

while path monitoring includes monitoring of higher-order path and lower-order path.

To illustrate this take 2.5 G system as an example, regenerator section overhead

monitors the whole STM-16 frame, while multiplex section overhead monitors any of

the sixteen STM-1s in the STM-16 frame. Higher-order path overhead monitors VC-4

of every STM-1, while lower-order path overhead monitors any of the sixty-three VC-

12s in the VC-4.

3.1.2 Section Overhead Bytes

Section overhead (SOH) is located at column (1 ~ 9×N) × row (1 ~ 9) except the fourth

row which is the AU-PTR. To describe the various functions of the overhead bytes,

take the STM-1 signal as an example. For STM-1 signal, the SOH includes the RSOH

located at column (1 ~ 9) × row (1 ~ 3), and the MSOH located at column (1 ~ 9) ×

row (5 ~ 9), as shown in Fig. 3.1-1.

18


A1 A1 A1 A2 A2 A2 JO × * × *

B1 ? E1 ? F1 × ×

D1 ? ? D2 ? D3

AU-PTR

B2 B2 B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M1 E2 × ×

? Bytes related with transmission medium (temporarily used) × Bytes reserved for national use * Non-scrambled bytes for national use

All the unmarked bytes are to be determined by international specifications (for medium-related application, additional national use, and other usage)

9 bytes

9 rows

RSOH

MSOH

?

Fig. 3.1-1 Arrangements of STM-1 SOH bytes

Fig. 3.1-1 shows the locations of RSOH and MSOH in an STM-1 frame. As mentioned

previously, each has a different monitoring range. RSOH monitors bigger scope as

STM-N, i.e. it monitors every regenerator section; while MSOH monitors small parts

as STM-1 within the bigger scope, i.e. it monitors every multiplex section.

3.1.2.1 Framing Bytes: A1 and A2

The framing bytes indicate the beginning of the frame, so that the frames of receiving

end and transmitting end are kept synchronous.

3.1.2.2 Regeneration Section (RS) Trace Byte: J0

J0 is used to repetitively transmit a Section Access Point Identifier so that a section

receiver can verify its continued connection from the intended transmitter. Within the

networks of the same operator, J0 can be set as any character; while at the border of

networks from different operators, J0 bytes must match. Through J0 byte, the operator

can find and solve faults in advance to shorten the network recovery time.

3.1.2.3 Data Communication Channel (DCC) Bytes: D1 ~ D12

One of SDH features is its automatic OAM function. SDH can send commands to an

NE and query data of the NE through the network management terminal, and can

19


perform functions such as real-time service scheduling, alarm/fault locating, and online

test. These OAM functions are all transmitted by D1~D12 bytes in STM-N frame. Data

Communication Channel (DCC) is the physical layer of Embedded Control Channel

(ECC), and it transmits OAM information among NEs, thus constructing a

transmission channel for SDH Management Network (SMN).

D1~D3 bytes are Regenerator Section DCC (DCCR), with the rate of 3×64 kbit/s=192

kbit/s, used to transmit OAM information among regenerator section terminals;

D4~D12 bytes are Multiplex Section DCC (DCCM), with the rate of 9×64 kbit/s＝576

kbit/s, used to transmit OAM information among multiplex section terminals.

The total rate of DCC channel is 768 kbit/s providing SMN with a powerful private

data communication channel.

3.1.2.4 Orderwire Bytes: E1 and E2

E1 and E2 can each provide an orderwire channel of 64 kbit/s for voice

communication.

E1 belongs to RSOH, used for orderwire communication between regenerator sections;

E2 belongs to MSOH, used for direct orderwire communication between multiplex

section terminals.

3.1.2.5 RS User Channel Byte: F1

F1 provides a data/voice channel with a rate of 64 kbit/s. It is reserved for user

(generally network provider) for orderwire communication of specific maintenance

purpose, or for transmission of 64 kbit/s special data.

3.1.2.6 RS Bit Interleaved Parity 8-bit Code (BIP-8) Byte: B1

B1 byte is used to monitor bit errors for the RS layer located at row 2, column 1 of the

RSOH.

3.1.2.7 Multiplex Section (MS) Bit Interleaved Parity (N×24)-bit Code (BIP- N×24) Byte: B2

The working principle of B2 byte is the same as that of B1. But B2 detects the bit

errors for the MS layer. One STM-N frame has only one B1 byte. While B2 byte

monitors bit errors for each STM-1 frame in STM-N frame, and every three B2 bytes

corresponds to one STM-1 frame, thus one STM-N frame has N×3 B2 bytes.

20


3.1.2.8 MS Remote Error Indication (MS-REI) Byte: M1

M1 byte is a reply message and is sent back to the transmitter from the receiver. M1

byte carries the count of error blocks detected by the receiver through B2 byte, and it

reports MS-REI (MS Remote Error Indication) in the current performance management

of the transmitter, so the transmitter can learn about bit error status at the receiver.

3.1.2.9 Automatic Protect Switching (APS) Channel Bytes: K1, K2 (b1~b5)

K1 and K2 (b1~b5) are used to transmit Automatic Protection Switch (APS)

information, to support automatic switching of equipment in case of fault, so that the

network service can recover automatically (self-healing). They are used specially for

MS APS.

3.1.2.10 MS Remote Defect Indication (MS-RDI) Byte: K2 (b6~b8)

K2 (b6~b8) is used for feedback message of MS far end alarm. The feedback message

is sent back to the transmitter (information source) from the receiver (information

destination), indicating that the receiver detected fault at the receiving direction or that

the receiver is receiving MS alarm indication signal. That is to say, when the received

signal at the receiver degrades, it sends MS-RDI alarm back to the transmitter, so that

the transmitter can learn about the receiver status. If the received b6~b8 bits of K2 are

“111”, it is MS Alarm Indication Signal (MS-AIS) and indicates that the end itself need

to send MS-RDI to the opposite end by writing “110” into b6~b8 bits of K2.

3.1.2.11 Synchronization Status Message Byte: S1 (b5~b8)

SDH MSOH uses bits 5~8 of S1 byte to represent different clock quality levels

specified by ITU-T, so that the equipment can judge the clock quality received via S1

and then judge whether to switch to a clock source of higher quality. S1 byte structure

is shown in Fig. 3.1-2.

b1 b2 b3 b4 b5 b6 b7 b8

Synchronous Status Message

Fig. 3.1-2 S1 byte structure

The four bits can compose sixteen different codes, which can represents sixteen

21


different synchronous quality levels, as shown in Table 3.1-1. Among these

combinations, only four combinations are currently used to transmit clock quality

information.

Table 3.1-1 SSM Codes

S1(b5~b8)SDH Synchronous Quality

Level DescriptionS1(b5~b8)

SDH Synchronous Quality

Level Description

0000

Unknown synchronous

quality level (existing

synchronous network)

1000 G.812 local office clock signal

0001 Reserved 1001 Reserved

0010 G.811 clock signal 1010 Reserved

0011 Reserved 1011Synchronous Equipment

Timing Source (SETS)

0100G.812 transition office clock

signal1100 Reserved



0111 Reserved 1111Should not be used for

synchronization

The detailed clock source switching will be described in chapter 6.

3.1.2.12 Bytes Related with Transmission Medium: ∆

Bytes marked with ∆ are especially used for special functions of a certain transmission

medium, e.g. this byte can be used to identify signal direction when using a single

optical fiber to implement bi-directional transmission.

3.1.2.13 Bytes Reserved for National Use: ×

Bytes marked with × are reserved for national use.

Functions of all the unmarked bytes are to be determined by international

specifications.

3.1.3 STM-N Section Overhead

N STM-1 frames form an STM-N frame via byte-interleaving and multiplexing. The

bytes in the AU-4 of each STM-1 are interleaved and multiplexed without changing the

bytes themselves, while the SOH multiplexing obeys a special regulation. Overhead

22


bytes except A1, A2, and B2 need termination process before being inserted into

corresponding STM-N overhead bytes; while A1, A2, and B2 bytes are interleaved and

multiplexed based on byte into the STM-N. Fig. 3.1-3 shows the SOH structure of

STM-4 frame.

A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1 A1

B1

A2 A2 A2 A2 A2 A2 A2 A2 A2 A2 A2 A2

1 5 9 13 17 21 25 29 33

J0 *Z0

*Z0

*Z0

* * * * * * * *

E1 F1

D1

K1 K2B2 B2 B2 B2 B2 B2 B2 B2 B2 B2 B2 B2

D2 D3

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M1 E2

1 2 3 4 5 6 7 8 9

36 bytes

RS

OH

MS

OH

9 ro

ws

AU-PTR

Multiplexed columnsReserved bytes for national use

* Non-scrambled bytes

All the unmarked bytes are to be determined by international specifications (for medium-related application, additional national use, and other usage)

Fig. 3.1-3 SOH of STM-4 frame

The SOH bytes of the STM-N frame are a layout of the SOHs of N STM-1 frames after

the interleaving process. During this operation, only the SOH of the first STM-1 is

completely retained, while for the rest N-1 STM-1 frames, only A1, A2 and B2 are

retained. Hence, one STM-N frame only has one B1 byte, but N×3 B2 bytes (since B2

is the result of BIP-24 check, every STM-1 frame has three B2 bytes), one byte each

for D1~D12, one M1 byte, one byte each for K1 and K2. Fig. 3.1-4 shows the byte

arrangements of STM-16 SOH.

23


A1

B2

A1

B2

A2

E1

D2

K1

D5

D8

D11

A2 A2 J0C1

F1

D3

K2

D6

D9

D12

E2

*x

x

x

*x

x

x

A1

B1

D1

B2

D4

D7

D10

S1

A1 A1 A1 A2 A2 A2 Z0 C1

*x

x x x

B2 B2 B2

x x x

AU-PTR

RSOH

MSOH

9 rows

144 bytes

M1 ...

Note: x Reserved bytes for national use * Non-scrambled code for national use Z0 and all the unmarked bytes are to be determined by international specifications (for medium-related application, additional national use, and other usage)

*x

x

Fig. 3.1-4 Byte arrangements of STM-16 SOH

3.1.4 Path Overhead

Section Overhead (SOH) is responsible for OAM functions for the section layer, while

Path Overhead (POH) is responsible for OAM functions for the path layer.

POH falls into two categories: lower-order path POH (LP-POH) and higher-order path

POH (HP-POH). HP-POH monitors VC-4 level path, i.e. it can monitor the

transmission status of 140 Mbit/s signals in the STM-N frame. And LP-POH

implements the OAM functions of VC-12 level path, i.e. it monitors the transmission

performance of 2 Mbit/s signals in the STM-N frame.

3.1.4.1 High-order Path Overhead: HP-POH

HP-POH locates at the first column of VC-4 frame, with nine bytes all together as

shown in Fig. 3.1-5.

24


J1

B3

C2

G1

F2

H4

F3

K3

N1

Higher-order path trace byte

Higher-order path bit error monitoring BIP-8 byte

Higher-order path signal label byte

Higher-order path status byte

Higher-order path user channel byte

TU multiframe location indicator byte

Higher-order path user channel byte

Automatic Protection Switching (APS) channel, allocated for future use

Network operator byte VC-3 or VC-4

Fig. 3.1-5 HP-POH structure

1. J1: Higher-order path trace byte

AU-PTR points at the location of the first byte of VC-4, so that the receiver can

accurately extract VC-4 from the AU-4 according to AU-PTR value. J1 is the

first byte of VC-4, so AU-PTR points at the location of J1 byte.

The function of J1 is similar to that of J0. It is used to repetitively send

identifier for higher-order path access point, so that the receiver in the path can

verify its continued connection to the intended transmitter (this path is

continuously connected). J1 byte can be configured or modified according to

actual needs, but the requirement is that J1 bytes at the receiver and transmitter

should match.

2. B3: Higher-order path bit error monitoring byte (BIP-8)

Using BIP-8, B3 byte is responsible to monitor bit error performance during

VC-4 transmission, i.e. it monitors the bit error performance during 140 Mbit/s

signal transmission. The monitoring mechanism is similar to that of B1 and B2,

with the difference that B3 performs BIP-8 check for VC-4 frame.

3. C2: Higher-order path signal label byte

C2 is used to indicate the characteristics of multiplexing structure and net

information load, such as if the path is equipped, the loaded services types and

25


their mapping method. For example, C2=00H indicates that the VC-4 path is

not equipped with signal, and code of all “1”s (TU-AIS) should be inserted into

the net load TUG3 of the VC-4 path, so that higher-order path unequipped

alarm i.e. VC4-UNEQ will occur in the equipment. C2=02H indicates that the

net load equipped in the VC-4 is multiplexed via the TUG structure. China

adopts TUG structure to multiplex 2 Mbit/s signals into VC-4. C2=15H

indicates that the net load of the VC-4 are signals of FDDI (Fiber Distributed

Data Interface) format.

4. G1: Higher-order path status byte

G1 is used to convey the path terminating status and performance back to VC-4

originating path equipment. Therefore the status and performance of the bi-

directional path in its entirety can be monitored, from either end or any point of

the path. G1 byte actually transmits the reply message, i.e. the message sent

back to the transmitter from the receiver, so that the transmitter can learn the

status of the VC-4 path signal received by the receiver. Arrangements of G1 bits

are shown in Fig. 3.1-6.

1 2 3 4 5 6 7 8

FEBBE RDI Reserved Standby

Fig. 3.1-6 Arrangements of G1 bits

5. F2, F3: User channel bytes

These two bytes provide orderwire communication between path units (related

to net load) and are seldom used currently.

6. H4: TU multiframe location indicator byte

H4 indicates the multiframe type of the payload and the location of the net load.

For example, it acts as the indicator byte for TU-12 multiframe, or as the

indicator for cell border when ATM net load enters a VC-4.

7. K3: Automatic Protection Switching (APS) channel

Bits b1~b4 of K3 byte are used to transmit the command of higher-order path

protection switching (APS).

26


8. N1: Network operator byte

N1 is used to provide a Tandem Connection Monitoring (TCM) of higher-order

path.

3.1.4.2 Lower-order Path Overhead: LP-POH

Lower-order path refers to the VC-12 path overhead. It monitors the transmission

performance of the VC-12 level path, i.e. it monitors the transmission status of 2 Mbit/s

PDH signals in STM-N.

Fig. 3.1-7 shows the structure of a VC-12 multiframe, which consists of four VC-12

base frames. LP-POH is located at the first byte of every VC-12 base frame, and one

group of LP-POHs has four bytes: V5, J2, N2, and K4.

V5 J2 N2 K4

VC12 VC12VC12VC12

1 4

1

9

500us VC12 multiframe

Fig. 3.1-7 LP-POH structure

1. V5: Path status and signal label byte

V5 is the first byte of TU-12 multiframe. TU-PTR points at the location of V5

byte in the TU-12 multiframe.

V5 has the functions of bit error detection, signal label and VC-12 path status

indication. Therefore, V5 functions are similar to those of higher-order path

overhead bytes B3, C2 and G1. Table 3.1-2 lists the V5 byte structure.

Table 3.1-2 Structure of VC-12 POH (V5)

27


Bit Error Monitoring

(BIP-2)

Far End Background

Block Error

(FEBBE)

Remote Failure

Indication

(RFI)

Signal Label

Remote Defect

Indication

(RDI)

1 2 3 4 5 6 7 8

Bit Error Monitoring:

Transmit Bit Interleaved

Parity code BIP-2: Bit 1

configuration should

enable the parity check

of all odd bits in last

VC-12 multiframe to be

even.

Bit 2 configuration should

enable the parity check

of all even bits to be

even.

Far End Background

Block Error: Send

“1” if BIP-2 detects

block error;

otherwise, send “0”.

Remote Failure

Indication: Send

“1” for fault;

otherwise send

“0”.

Signal Label:

It indicates the net load

equipment status and

mapping method. It has three

bits which can have eight

binary value.

000: VC path unequipped

001: VC path equipped, but

payload is not specified.

010: Asynchronous floating

mapping

011: Bit synchronous floating

100: Byte synchronous

floating

101: Reserved

110: O.181 test signal

111: VC-AIS

Remote Defect

Indication (equivalent

to FERF used

before):

Send “1” for

receiving failure;

otherwise, send “0”.

If the receiver detects block error via BIP-2, it will indicate the block error

number detected via BIP-2 in the local end performance event of V5-BBE.

Meanwhile, it will send V5-FEBBE back to the transmitter via bit b3 of V5.

Then the performance event V5-FEBBE at the transmitter indicates the

corresponding block error number. Bit b8 of V5 is used for Remote Defect

Indication of VC-12 path. When the receiver receives TU-12 AIS signal, it will

send one VC12-RDI (LP-POH Remote Defect Indication) signal back to the

transmitter.

2. J2: VC-12 path trace byte

The function of J2 is similar to that of J0 and J1. It is used to repetitively send

the identifier of lower-path access point negotiated by the receiver and

transmitter, so that the receiver can verify its continued connection in the path

from the intended transmitter.

3. N2: Network operator byte

N2 is used to provide a Tandem Connection Monitoring (TCM) for the lower-

28


order path.

4. K4: Automatic protection switching path

3.2 SDH Pointers

Pointers provide alignment function. Through alignment, the receiver can accurately

extract corresponding VC from STM-N signal stream, and then extract low-speed PDH

signal by unpacking VC and C packages; thus implementing the function of extracting

low-speed tributary signals from STM-N signal directly.

Alignment is the procedure by which the frame offset information is incorporated into

the tributary unit or the administrative unit, i.e. using the pointer attached to VC to

indicate and determine the location of the beginning of the lower-order VC frame in the

TU payload (or the location of beginning of higher-order VC frame in the AU payload).

The pointer value is adjusted when the relative frame phase offset causes VC frame

beginning to float, so as to ensure that the pointer always traces and indicates the

beginning of the VC frame. For VC-4, AU-PTR points at the location of J1 byte; for

VC-12, TU-PTR points at the location of V5 byte. TU-PTR or AU-PTR can provide a

flexible and dynamic method for aligning VC in TU or AU frame; because TU-PTR or

AU-PTR can accommodate not only the phase difference of VC and SDH, but also

their rate difference.

Summary

This chapter describes various overheads of SDH frame; the mechanism of layered

monitoring using RSOH, MSOH, HP-POH, and LP-POH; and pointer alignment

principle.

The major points to master are the functions of overhead bytes, and their relations with

alarm and performance detection.

Exercises

1. MS-AIS and MS-RDI are detected by which byte?

2. What is the detection principle of A1 and A2 byte?

3. What bytes are used to implement monitoring and alarm of bit error at different

29


SDH layers?

4. What is the use of K1 and K2 bytes?

5. What is the need of S1 byte in a SDH frame?

6. What is the significance of E1 and E2 bytes?

7. How many bytes are contained in a VC-4 POH?

8. What is the use of M1 byte?

9. Which byte indicates the type and composition of VC-4 tributary information?

10. What is the difference between J0 and J1 bytes?

11. What is the purpose of having overhead bytes in SDH?

12. How many VC-12 frames are combined together to form a multi-frame?

30

4 Logic Structure of SDH Equipment

Key points

Common NEs in SDH network

Logical functional blocks of SDH equipment

4.1 Common Network Elements in SDH Network

SDH transmission network is composed of various types of network elements (NE)

which are connected by optical cables. These NEs can perform transmission functions

of SDH network like service add/drop, service cross-connect, and network fault self-

healing. The following contents describe the characteristics and basic functions of the

common NEs in an SDH network.

4.1.1 TM — Terminal Multiplexer

TM is located at the terminal site of the network with only one optical direction, as


TM

2Mbit/s 34Mbit/s

STM-M

140Mbit/s

STM-NW

Note: M<N

Fig. 4.1-1 TM model

The functions of a TM are to multiplex low-speed signals at a tributary port into the

high-speed STM-N signal at a line port, or to drop low-speed tributary signals from

STM-N signals.

31

Note:

The line port may only input/output one channel of STM-N signal, while tributary port

may input/output multiple channels of low-speed tributary signal.

TM performs the cross-connect function when multiplexing low-speed tributary signals

into the STM-N frame. For example, we can multiplex one STM-1 tributary signal into

any position of a STM-16 line signal, i.e. multiplex STM-1 to any position of the

sixteen STM-1s of STM-16. And we can multiplex one tributary 2 Mbit/s signal into

any position among the sixty-three VC-12s of an STM-1.

4.1.2 ADM — Add/Drop Multiplexer

ADM is used at the transition office in the SDH transmission network, such as the

middle node of a chain or a node in a ring. It is the most frequently used and most

important network element in an SDH network, as shown in Fig. 4.1-2.

ADM

2Mbit/s 34Mbit/sSTM-M

140Mbit/s

STM-N STM-NW E

Note: M<N

Fig. 4.1-2 ADM model

ADM has 2 line sides and 1 tributary side. For convenience of description, we call

them the west (W) line port and the east (E) line port. The ADM tributary side connects

with the tributary ports, and the tributary port signals are the added/dropped services

to/from the line side STM-N signal. The functions of an ADM are to cross-connect and

multiplex low-speed tributary signal to the east/west line, or to drop the low-speed

tributary signal from the line signal received from the east/west line port; in addition,

ADM can cross-connect the STM-N signals of the east/west line. For example, cross-

connect the third STM-1 of east STM-16 with the 15th STM-1 of west STM-16.

32


ADM is the most important NE in SDH since it may be used as the equivalence of

other network elements, i.e. it can accomplish the functions of other network elements.

For example, one ADM is equivalent to two TMs.

4.1.3 REG — Regenerator

The characteristic of REG is that it only regenerates optical signals without

adding/dropping electrical line service. There are two kinds of REGs in SDH

transmission network: one is pure optical REG, which regenerates the optical power so

as to extend the optical transmission distance; the other is electrical REG for pulse

regeneration and reshaping, which performs Optical/Electrical (O/E) conversion,

electrical signal sampling, determination, regeneration and reshaping, and E/O

conversion to eliminate accumulated line noise and thus ensures good waveform of the

line signals being transmitted.

Hereinafter we only discuss the latter REG. The REG is an equipment with two sides,

which connect with the west line port (W) and east line port (E) respectively, as shown

in Fig. 4.1-3:

REG STM-NSTM-N W E

Fig. 4.1-3 REG model

The REG processes optical signals at the W/E side by O/E conversion, sampling,

determination, regeneration and reshaping, E/O conversion and sends the processed

optical signal out at the E/W side. Compared with ADM, REG lacks the tributary ports

side. Therefore, ADM is also equivalent to a REG when it does not add/drop local

electrical line service.

REG only processes RSOH in the STM-N frame, and has no cross-connect function (it

only needs to connect W and E directly); while ADM and TM process not only RSOH,

but also MSOH, since they need to multiplex the low-speed tributary signals to the

STM-N frame.

33


4.1.4 DXC — Digital Cross-Connect Equipment

DXC mainly accomplishes the cross-connection of the STM-N signals. It is an

equipment with multiple ports. ADM is actually equal to a cross matrix completing the

cross-connection of signals, as shown in Fig. 4.1-4.

DXCm n

Equivalent to

Input lines: m

Output lines: n

Fig. 4.1-4 DXC model

The DXC can cross-connect the input m-channel STM-N signals to the output n-

channel STM-N signals. The figure above indicates there are m lines of input fiber, and

n lines of output fiber. The core function of DXC is to cross-connect. A powerful DXC

can perform the low-level cross-connect (e.g. cross-connect of VC-4 or VC-12 level) of

high-speed signals (e.g. STM-16) within the cross-connect matrix.

DXCm/n is generally used to denote the type and performance of a DXC (m≥n), where

m refers to the maximum rate level of the DXC, and n refers to the minimum rate level

that the cross-connection matrix can handle. The bigger m is, the greater the DXC

bearing capacity is; the smaller n is, the stronger the DXC flexibility is. Zero represents

for 64 kbit/s rate of electrical line. The numbers of 1, 2, 3, and 4 respectively represents

for rate of level 1 to level 4 group in PDH system; where 4 also represents for rate of

STM-1 level in SDH system. 5 and 6 respectively represents for rate of STM-4 and

STM-16 level in SDH system. For example, DXC4/1 indicates that the maximum rate

at the access port is STM-1, while the minimum rate of cross-connect is that of the

PDH primary group signal. The values of m and n and their corresponding meanings

are listed in Table 4.1-1.

Table 4.1-1 Relations between rate and m/n

m/n 0 1 2 3 4 5 6

Rate 64 kbit/s 2 Mbit/s 8 Mbit/s 34 Mbit/s 140 Mbit/s 622 Mbit/s 2.5 Gbit/s

34


155 Mbit/s

DXC with small capacity can be implemented by ADM. For example, ZTE 2.5 G

equipment has the cross-connect capacity equivalent to DXC6/1.

4.2 Logical Functional Blocks of SDH Equipment

SDH system requires that equipment of different manufacturers be compatible with

each other transversely. In order to realize interconnection, ITU-T standardizes SDH

equipment by adopting a functional reference model to break down the functions

performed by the equipment into basic standard functional blocks. The

implementations of functional blocks are independent of the physical implementation

of the equipment. Different equipments are flexible combinations of these basic

functional blocks to perform different functions. The standardization of the basic

functional blocks enables standardization of equipment and makes the specifications

universal with clear and simple descriptions.

We take the TM equipment as an example to describe its typical functional blocks and

its functions, as shown in Fig. 4.2-1.

SPI RST MST MSP MSA

PPI LPA HPT

PPI LPA LPT LPC HPT

HPC

STM A B C D E F

TTFW

140Mb/s G.703 M L

HOI

G F

2Mb/s34Mb/s

G.703 K HPAFGHHIJ

LOI

HOA

OHA OHA interface

Note: Takes 2 Mbit/s as example

SEMF MCF

SETS SETPI

P N

Q interfaceF interface

External synchronization

D4-D12 D1-D3

Fig. 4.2-1 Logical functional blocks of TM equipment

The full names of functional blocks in Fig. 4.2-1 are given below:

SPI: SDH Physical Interface TTF: Transmission Terminal Function

35


RST: Regenerator Section Terminal HOI: Higher-Order Interface

MST: Multiplex Section Terminal LOI: Lower-Order Interface

MSP: Multiplex Section Protection HOA: Higher-Order Assembler

MSA: Multiplex Section Adaptation HPC: Higher-Order Path Connection

PPI: PDH Physical Interface OHA: Overhead Access Function

LPA: Lower-Order Path Adaptation LPT: Lower-Order Path Terminal

MCF: Message Communication Function LPC: Lower-Order Path Connection

HPT: Higher-Order Path Terminal HPA: Higher-Order Path Adaptation

SETS: Synchronous Equipment Timing Source

SEMF: Synchronous Equipment Management Function

SETPI: Synchronous Equipment Timing Physical Interface

Among them, SPI, TTF, RST, HOI, MST, LOI, MSP, HOA, MSA, HPC, PPI, LPA,

LPT, LPC, HPA and HPT are basic functional blocks of an equipment. Different

equipments can be formed by flexible combination of basic functional blocks, such as

REG, TM, ADM and DXC. SEMF, MCF, OHA, SETS, SETPI are auxiliary functional

blocks which helps basic functional blocks to implement the required functions of an

equipment.

Fig. 4.2-1 shows the functional block diagram of a TM. The signal flow is: the STM-N

signal on the line enters the equipment from reference point A, and are disassembled

into 140 Mbit/s PDH signals by passing through A->B->C->D->E->F->G->L->M; And

then by passing through A->B->C->D->E->F->G->H->I->J->K, it is disassembled into

2 Mbit/s or 34 Mbit/s PDH signals (here we take 2 Mbit/s signal as example). These

two routes are defined as the receiving direction of the equipment. The transmitting

direction is the reverse of these two routes where 140 Mbit/s, 2 Mbit/s, and 34 Mbit/s

PDH signals are multiplexed into the STM-N signal frames on the line.

Summary

This chapter describes the common SDH NEs, and logical functional blocks of SDH

equipment.

36


Exercises

1. What alarms may cause HP-RDI?

2. What is the function of TTF functional block?

3. What does DXC4/1 mean?

4. What does DXC4/1 mean?

5. How is it possible to achieve interface compatibility among SDH nodes made by

different vendors?

6. What are the roles of REG?

7. Is it possible to have DXC features embedded in an ADM?

8. Can you add or drop any electrical tributary signal (PDH) into a REG?

9. Is it possible to add or drop any electrical tributary signal (PDH) into a DXC?

10. Why MSOH is not processed by REG?

37

5 Topology and Protection of SDH Network

Key points

Significance of network protection

Basic topologies of SDH network

Concept and categories of self-healing

Chain network protection

Self-healing ring protection

Dual node interconnection protection

Error connection and error squelch

Logical subnet protection

Topologies of complicated network

Overall hierarchy of SDH network

5.1 Significance of Network Protection

Our lives and work become more and more dependent on communication with the

development of technology. According to statistics, communication interruption for one

hour can cause loss of twenty thousand dollars for an insurance company, loss of 2.5

million dollars for an airline company, and loss of 6 million dollars for an investment

bank. Communication interruption for two days can lead to bankruptcy of a bank.

Therefore, the survivability of communication network has become one of the key

factors for modern network design and operation.

5.2 Basic SDH Network Topologies

SDH network is constructed by interconnecting SDH NEs with optical cables. The

geometrical arrangements of network node equipment (NE) and transmission lines

39

form the network topology. The validity (channel utilization ratio), reliability and

economical efficiency of the network are largely related to the topology.

There are five basic network topologies: chain, star, tree, ring, and mesh, as shown in

Fig. 5.2-1.

TM DXC/ADM TM

TM TM TM

ADMDXC/ADM ADM TM

TM

TM

ADM

ADM

ADM

ADM

DXC/ADM DXC/ADM

DXC/ADM DXC/ADM

TM ADM ADM TM(a) Chain

(b) Star

(c) Tree

(d) Ring

(e) Mesh

Fig. 5.2-1 Basic network topologies

1. Chain network

The chain network topology is to connect all nodes serially, with the two ends

open. The characteristic of chain network is that it is relatively economical. It is

mostly applied in the early stages of SDH network, and mainly applied in private

networks (e.g. railway network).

2. Star network

The star network topology is to make an NE of the network as the central node

connected with the other nodes, while the other nodes are not connected with

each other. All services need to be transited through this special node. The

40


characteristic of star network is that it can uniformly manage other network

nodes through the central node, thus facilitates bandwidth allocation and saves

costs. However, the central node has some potential bottleneck problems for

security protection and processing capacity. The role of the central node is

similar to the tandem office of the switching network. Star topology is mostly

applied in local networks (access network and subscriber network).

3. Tree Network

Tree network topology can be considered as a combination of the chain and star

topologies. Its central node also has some potential bottleneck problems for

security protection and processing capacity.

4. Ring Network

Actually, the ring network topology is to connect the two ends of the chain

network topology, hence any one NE node of the network is not open. Currently,

the ring network topology is very popular because of its powerful survivability,

i.e. powerful self-healing function. The ring network is generally applied in local

networks (access network and subscriber network), inter-office relay network,

etc.

5. Mesh network

The mesh network is to connect all nodes with each other. This network

topology provides multiple transmission routes between two NE nodes, which

improves network reliability and eliminates bottleneck problem and failure

problem. However, high system redundancy will surely reduce the system

validity. Its cost is high and the structure is complicated. Mesh network is mainly

applied in the toll network to improve network reliability.

Currently, the chain and ring networks are employed the most, which can form more

complicated networks through flexible combinations. This chapter mainly describes the

structure and characteristics of chain networks, and the working principles and

characteristics of major self-healing methods.

41


5.3 Concept and Classification of Self-healing

5.3.1 Overview

According to traffic flow, services on the transmission network can be classified into

unidirectional and bidirectional services.

Taking the ring network as example, the difference between the unidirectional and

bidirectional services is shown in Fig. 5.3-1.

ADM

ADM

ADM

ADM

Fig. 5.3-1 Ring network

If there is a service between A and C, suppose the service route from A to C is

A→B→C, and the service route from C to A is C→B→A, then the route of A to C and

C to A is identical, This is called consistent route.

In the above example, if the route from C to A is C→D→A, then the route of A to C is

different from the route of C to A. This is called separate route.

The service of the consistent route is called bidirectional service, while the separate

route is called unidirectional service. Service directions and routes of common network

topologies are listed in Table 5.3-1.

Table 5.3-1 Service directions and routes of common network topologies

Network Type Route Service Direction

Chain network Consistent route Bidirectional

Ring

network

Bidirectional path ring Consistent route Bidirectional

Bidirectional multiplex

section (MS) ringConsistent route Bidirectional

Unidirectional path ring Separate route Unidirectional

42


Unidirectional MS ring Separate route Unidirectional

5.3.2 Self-healing Concept

Self-healing means that network can automatically restore its carried services from a

network fault without manual intervention within a very short period of time (ITU-T

specifies the recovery time should be no more than 50 ms), so that subscribers will not

realize network fault.

Its basic principle is that networks should be able to find out a substitute transmission

route and re-establish the communication in case of network fault.

The substitute route can make use of the redundancy of the standby equipment or the

currently working equipment to satisfy the recovery demands of all the services or the

designated priority services. Therefore, the preconditions for network self-healing

capability include redundant route, powerful cross capability of the NE and intelligence

of the NE.

Self-healing can only recover the failed services through the standby channel, but

cannot repair or replace the failed components or lines. Thus, the troubleshooting is

still to be completed by manual intervention, e.g. broken cable needs to be connected

manually.

5.3.3 Self-healing Classification

There are multiple ways to classify self-healing network. According to network

topologies, self-healing networks can be classified as follows:

1. Chain network service protection mode

1) 1+1 path protection

2) 1+1 multiplex section (MS) protection

3) 1:1 MS protection

2. Ring network service protection mode

1) Two-fiber unidirectional path protection ring

2) Two-fiber bidirectional path protection ring

3) Two-fiber unidirectional MS protection ring

4) Two-fiber bidirectional MS protection ring

43


5) Four-fiber bidirectional MS protection ring

3. Inter-ring service protection mode

1) Dual Node Interconnection (DNI protection mode)

2) Multi-node interconnection changed to dual node interconnection

5.4 Chain Network Protection

5.4.1 Overview

Chain networks has timeslot multiplexing function, that is to say, the VC with certain

sequence number of an STM-N line signal can be reused on different transmission

sections.

TM ADM ADM TM

Tributary service

Tributary service

Tributary service

Tributary service

A B C D

STM-N

Y timeslot

X timeslot

X timeslot

X timeslot

X timeslot

X timeslot

X timeslot

Y timeslot

Fig. 5.4-1 Chain network schematic

As shown in Fig. 5.4-1, there are services between A and B, B and C, C and D, A and

D. The services between A and B occupy timeslot X of optical cable section A—B (VC

with sequence number of X, for example, the 48th VC-12 of 3rd VC-4), and the service

between B and C occupy timeslot X of optical cable section B—C (the 48 th VC-12 of

the 3rd VC-4), and the services between C and D occupy timeslot X of optical cable

section C—D (the 48th VC-12 of the 3rd VC-4). The above illustration is called timeslot

reuse. Since timeslot X of the optical cable has been occupied, the services between A

and D can only occupy timeslot Y of the optical cable, for example, the 49th VC-12 of

the 3rd VC-4, or the 48th VC-12 of the 7th VC-4.

44


The time slot reuse function of chain network can expand network service capacity.

Network service capacity refers to the total service amount than can be transmitted on

the network. It is related to the network topology, network self-healing mode, and

service distribution relation between the NEs.

The minimum traffic of the chain network occurs when the end stations of the chain

network act as the service host station. The service host station exchanges services with

all the other NEs, and all the other NEs do not exchange services with each other.

Taking Fig. 5.4-1 as example, if A is the service host station, there is no service among

B, C, and D. Yet B, C, D can communicate with A . Since the maximum capacity of the

optical cable section from A to B equals to STM-N (suppose the system rate level is

STM-N), the service capacity of the network is STM-N.

The condition for chain networks to reach maximal service capacity is that service only

exists between neighboring NEs. As shown in Fig. 5.4-1, services only exist between A

and B, B and C, C and D, and no service exists between A and D. At this time, timeslot

can be reused, and the service on each optical cable section can occupy all the time

slots of the whole STM-N. Provided that the chain network has M NEs, the maximum

service capacity of the network would reach (M-1)× STM-N, where (M-1) refers to the

number of optical cable sections.

Common chain networks include:

Two-fiber chain: It cannot provide the service protection (self-healing) function.

Four-fiber chain: It generally provides 1+1 or 1:1 service protection. Two optical fibers

act as the active transmitting/receiving channel, and the other pair acts as the standby

transmitting/receiving channel.

5.4.2 Basic Chain Network Protection Types

5.4.2.1 1+1 Path Protection

1+1 path protection is based on the path. Whether to switch or not is determined by the

signal quality of each path.

1+1 path protection adopts the principle of “Concurrent Transmission and Preferred

Receiving”. When adding services, the path service signal will be sent simultaneously

to the working and protection channels. When dropping services, it will be received

simultaneously two path signals from the working and protection channels. In both

45


situations, the signal with better quality will be added or dropped.

It generally adopts PATH-AIS signal as the switching proof without APS protocol. The

switching time should be no more than 10 ms.

5.4.2.2 1+1 Multiplex Section Protection

Multiplex section protection is based on the multiplex section. Whether to switch or

not is determined by the signal quality of the multiplex section between two stations.

When the multiplex section is faulty, the service signal in the whole station will be

switched to the protection channel for protection purpose.

In 1+1 multiplex section protection mode, the service signal simultaneously crosses

over the working and protection channels for transmission.

Under normal status, the signal of the working channel is used. When the system

detects LOS, LOF, MS-AIS, or the alarm of bit error >10E-3, it will switch to the

protection channel to receive the service signal.

5.4.2.3 1:1 Multiplex Section Protection

In 1:1 multiplex section protection mode, the service signal does not always cross over

the working and protection channels simultaneously. Thus, it can transmit the

additional low priority service in the protection channel.

Upon fault of the working channel, the protection channel will discard the additional

service, and perform cross-over and switching to protect service signals according to

the APS protocol.

When working normally, 1:1 protection is equivalent to 2+0 protection.

5.5 Self-healing Ring Protection

5.5.1 Self-healing Ring Classification

The self-healing ring can be classified according to different standards:

1. According to the service direction of the ring:

Unidirectional ring and bidirectional ring.

2. According to the number of the optical fibers between NE nodes:

46


Two-fiber ring (one pair of receiving/transmitting optical fibers) and four-fiber

ring (two pairs of receiving/transmitting optical fibers).

3. According to the protected service level:

Path protection ring and multiplex section protection ring.

The differences between the path protection ring and multiplex section

protection ring are as listed in Table 5.5-1.

Table 5.5-1 Differences between the path ring and the multiplex section ring

Path Protection Ring Multiplex Section (MS) Switching Ring

Protection Unit Service protection is based on the path, that

is, protect one VC of the STM-N signal.

Whether to switch or not is determined by

the signal transmission quality of the path

of the ring.

Service protection is based on the multiplex section.

Whether to switch or not is determined by the signal

quality of the multiplex section of the ring.

Switching

Condition

PATH-AIS;

Whether to switch or not is generally

determined by the receiver when it detects

TU-AIS signal.

Switching is started by the APS protocol carried via K1

and K2 bytes. The switching conditions of the MS

protection are LOF, LOS, MS-AIS or MS-EXC alarm

signals.

Switching

Mode

Taking the STM-16 ring as example, if the

48th TU-12 of the 4th VC4 received has TU-

AIS, only this TU-12 path is switched to

the standby channel.

When the MS is faulty, the whole STM-N or 1/2 STM-

N service signals of the ring will all be switched to the

standby channel.

Optical Fiber

Utilization

Ratio

The path protection ring is generally a

dedicated protection. In normal

circumstances, the protection channel is

also used to transmit the active service (1+1

service protection), so the channel

utilization ratio is low.

The MS protection ring adopts public protection. In

normal circumstances, the primary channel is used to

transfer the primary service. Adopting 1:1 protection

mode, the standby channel is used to transmit additional

service, so the channel utilization ratio is high.

Note:

As the STM-N frame has only one K1 and one K2, the multiplex section protection

switching is not to switch only one of the paths, but to switch all primary STM-N

(four-fiber ring) or 1/2 STM-N (two-fiber ring) services to the standby channel.

47


5.5.2 Two-fiber Unidirectional Path Protection Ring

The two-fiber unidirectional path protection ring consists of two rings made up by two

optical fibers, one is S1 which is the primary ring, and the other is P1 which is the

standby ring. The service flow directions of the two rings must be opposite. The

protection function of the path protection ring is realized through the switching

function of the NE tributary card. The tributary card concurrently transmits the

tributary service to S1 and P1. Services of the two rings are identical but the flow

directions are opposite. Normally the NE tributary card drops tributary service from the

primary ring as shown in Fig. 5.5-1.

CA AC

CA AC

S1P1

P1

S1

A

DC

B

Fig. 5.5-1 Two-fiber unidirectional path switching ring

If there is service between NEs A and C in the ring network, A and C will concurrently

transmit the tributary services to the S1 and P1 rings. Services are transmitted to C

through S1 optical fiber (primary ring service) via NE D, and are concurrently

transmitted to C by P1 optical fiber (standby ring service) through NE B. Under normal

conditions, tributary card at NE C chooses to receive the service from the S1 ring,

which is the primary ring. The service transmission from C to A is similar to that from

A to C, like S1: C->B->A, and P1: C->D->A.

Receiving end selects service on S1 ring: C->B->A

Even if the optical fibers between B and C are cut off, the concurrent transmission

function of the NE tributary card will not change, that is to say, the services on S1 and

P1 are still identical, as shown in Fig. 5.5-2.

48


CA AC

CA AC

S1P1

P1

S1

AD

CB

Switching

Fig. 5.5-2 Two-fiber unidirectional path switching ring (in case of fault)

The service from A to C is concurrently transmitted to S1 and P1 optical fibers by the

tributary card of NE A, of which, service on S1 is transmitted to C via D, and service

on P1 is transmitted via B. As the optical cable between B and C is cut off, the service

on P1 cannot reach C. However, since C selects to receive the service on S1 by default,

the service from A to C is not interrupted, and the tributary card of NE C will not

perform protection switching.

The tributary card of NE C concurrently transmits the service to NE A onto S1 and P1

rings. The service from C to A on P1 is transmitted to A via NE D. The service from C

to A on S1 cannot be transmitted to A due to the broken optical cable between B and C.

NE A chooses to receive the service on S1 by default. As the service from C to A on S1

cannot reach A, the tributary card of NE A will receive the TU-AIS alarm signal from

S1 ring, and it will immediately switch to receive the service from the P1 ring, thus the

service from C to A is transmitted to A and the ring service path protection is

completed. At this time the tributary card of NE A is in the path protection switching

status, that is, switches to receive the standby ring signals.

The advantage of two-fiber unidirectional path protection ring is its fast switching

speed. Since services on the rings are concurrently transmitted and preferred received,

the path service protection mode is actually 1+1 protection. The service flow direction

is simple and clear, and the service is easy to configure and maintain.

The disadvantage of this protection mode is that the network service capacity is not

large. The service capacity of two-fiber unidirectional ring constantly equals to STM-

N, which is irrelevant to the node number of the ring and the service distribution

among the NEs.

49


For instance, when certain service between NE A and NE D occupies timeslot X. Since

the service is unidirectional, the service from A to D will occupy timeslot X of the

optical cable section from A to D of the primary ring (and occupy timeslot X of the

optical cable section of A to B, B to C, and C to D of the standby ring). The service

from D to A will occupy timeslot X of D to C, C to B, and B to A of the primary ring

(and occupy timeslot X of optical cable section from D to A of the standby ring.). In

other words, the service occupying timeslot X of A to D will occupy the timeslot X of

all optical cables of the ring (both the primary ring and standby ring), and other

services cannot use this timeslot (there is no function of timeslot reuse). When the

traffic between A and D equals to STM-N, other NEs cannot transmit service with each

other any more, because all the timeslot resources of the STM-N are occupied already.

Therefore, the largest service capacity of the unidirectional protection ring is STM-N.

The two-fiber unidirectional path ring is mostly applied when the ring has a service

host station or service concentration station.

Note:

The service flow direction of S1 and P1 must be opposite when making up the path

ring. Otherwise the ring network has no protection function. The path protection ring

only switches one path.

5.5.3 Two-fiber Bidirectional Path Protection Ring,

The service of two-fiber bidirectional path protection ring is bidirectional (consistent

route), and the protection principle is “Concurrent Transmission and Preferred

Receiving”. It adopts the 1+1 service protection mode. The service capacity equals to

that of the two-fiber unidirectional path protection ring. It is shown in Fig. 5.5-3.

50


Fig. 5.5-3 Two-fiber bidirectional path protection ring

The path protection rings of ZTE equipment are non-revertive.

Tips:

When self healing occurs in the network, service will switch to the standby channel for

transmission. There are two modes for switching: revertive mode and non-revertive

mode.

Revertive mode means that when the primary channel is faulty, service will switch to

the standby channel; and when the primary channel recovers, the service will switch

back to the primary channel. Generally, it is necessary to wait for a while (a few

minutes or so) so as to switch the service back from the standby channel to the primary

channel until the transmission performance of the primary channel becomes stable.

Non-revertive mode means that when the primary channel is faulty, service will switch

to the standby channel; and when the primary channel recovers, the service will not

switch back to the primary channel. The original primary channel now serves as

standby channel, and the original standby channel now serves as primary channel. Only

when the original standby channel has fault, will the service switch back to the original

primary channel.

5.5.4 Two-Fiber Bidirectional MS Protection Ring

The two-fiber bidirectional MS protection/switching ring (also known as two-fiber

bidirectional MS shared ring) adopts timeslot protection method. It uses the first half of

timeslots in each fiber (e.g. 1st ~ 8th AU4s in STM-16) as working timeslots to

51


transmit primary service; and the other half (e.g. 9th ~ 16th AU4s in STM-16) as

protection timeslots to transmit additional service and protect primary service. In other

words, it uses the protection timeslots of one fiber to protect the primary service of

another fiber. Therefore, there are no dedicated primary or standby fibers in a two-fiber

bidirectional MS protection ring. Instead, the first half of timeslots in each fiber are

primary channel, and the other half are standby channel, and the service flow directions

of the two fibers are opposite.

When the network is normal, the service flow directions are shown in Fig. 5.5-4.

Fig. 5.5-4 Two-fiber bidirectional MS protection ring

When the network is normal, the primary service from A to C is transmitted using

timeslots S1 of S1/P2 fiber (for STM-16 system, primary service can only use 1st ~ 8th

AU4s); it is transmitted to C through B in S1/P2 fiber, and NE C receives the service in

timeslots S1 of S1/P2 fiber. The primary service from C to A is transmitted using

timeslots S2 of S2/P1 fiber; it is transmitted to A through B in S2/P1 fiber, and NE A

extracts the service from timeslots S2 of S2/P1 fiber.

When the optical cable between B and C is cut off, the service flow directions are


52


Fig. 5.5-5 Two-fiber bidirectional MS protection ring (in case of fault)

When the optical cable between B and C is cut off, the primary service from A to C is

transmitted to B in S1/P2 fiber; And NE B performs switching (the NE adjacent to the

fault location performs switching), which switches all the service in timeslots S1 of

S1/P2 fiber to timeslots P1 of S2/P1 fiber (e.g. in STM-16 system, it switches all the

service in 1st ~ 8th AU4s of S1/P2 fiber to 9th ~ 16th AU4s of S2/P1 fiber). And then

the primary service is transmitted to NE C through NE A and D via S2/P1 fiber. NE C

(fault end point) will also perform switching, which switches the primary service from

A to C in timeslots P1 of S2/P1 fiber back to the timeslots S1 of S1/P2 fiber; then NE C

will extract service from timeslots S1 and completes receiving of primary service from

A to C.

The primary service from C to A in timeslots S2 is first switched by NE C to timeslots

P2 of S1/P2 fiber; and then it is transmitted to NE B through D and A via S1/P2 fiber;

NE B will perform switching, which switches the primary service from C to A in

timeslots P2 of S1/P2 fiber back to the timeslots S2 of S2/P1 fiber; then NE A will

extract service from timeslots S2 and completes receiving of primary service from C to

A.

Through the above method, the self-healing of ring network is completed.

Timeslots P1 and P2 can be used to transmit additional service under normal condition.

In case of fault, the additional service is interrupted, and timeslots P1 and P2 are used

as protection timeslots to transmit primary services.

Compared with path protection ring, MS protection ring needs to use APS protocol,

which costs more protection switching time. As per ITU-T specifications, the

protection switching time should be less than 50 ms.

53


The service capacity (i.e. the maximum traffic amount) of the two-fiber bidirectional

MS protection ring equals to (K/2)×STM-N, where K refers to the number of NEs

(K≤16). This is the maximum traffic amount when there is service only between

adjacent nodes. Under this circumstance, every optical cable section is used privately

by the two adjacent NEs. For instance, cable section A-D only transmits the

bidirectional service between A and D, and cable section D-C only transmits the

bidirectional service between D and C. The service between two adjacent NEs does not

occupy timeslot resource of other optical cable section, so that every cable section can

transmit maximum traffic of 1/2×STM-N (timeslot can be reused). And the number of

optical cable section equals to that of the nodes in the ring network, therefore, the

service capacity under this circumstance reaches the maximum traffic amount: (K/2) ×

STM-N.

The MS protection mode of ZTE equipment is revertive, with the default protection

switching recovery time as eight minutes.

5.5.5 Four-fiber Bidirectional MS Protection Ring

The four-fiber bidirectional MS protection ring consists of four fibers: S1, P1, S2, and

P2. Among which, S1 and S2 are primary fibers which transmit the primary service; P1

and P2 are standby fibers which transmit protected service. In other words, P1 and P2

protect the primary service of S1 and S2 in case of primary fault. Please pay attention

to the service flow directions of these four fibers: the service flow directions of S1 and

S2 are opposite (consistent route, bidirectional ring); the service flow directions of S1

and P1 are opposite, and those of S2 and P2 are also opposite; the service flow

direction of S1 and P2 are the same, and those of S2 and P1 are also the same.

54


Note:

Each node of the four-fiber ring is configured as double ADM system. Because one

ADM only has two line ports of E/W (east/west), but the NE node of four-fiber ring has

two line ports each for E/W direction. Therefore, the NE node needs to be configured

as double ADM system.

As shown in Fig. 5.5-6, when the ring network works normally, the primary service

from A to D is transmitted to D through B via S1 fiber; and the primary service from D

to A is transmitted to A through B via S2 fiber (bidirectional service). NE A and D

exchanges primary services by receiving the service in the primary fibers.

Other nodes and corss-sections

Node A Node B

Node CNode D

Add/drop service

Add/drop service

WP

WP

WP

WP

WWPP

P

P

P

P

W

W

W

W

S1

S2 P2

P1

Fig. 5.5-6 Normally the service between A and D passes B and C

If faults occur to optical cables between B and C, the cross-section switching or cross-

ring switching will happen to the service in the ring. The trigger conditions and

switching procedures are as follows:

1. Cross-section switching

For the four-fiber ring, if the fault only affects working channel, service can be

recovered by switching to the cross-section protection channel.

55


As shown in Fig. 5.5-7, when the working fiber S1 between B and C is broken

and the other three fibers wok normally, the service from A to D will be

transmitted to B via S1 fiber, then B will perform cross-section switching to

switch service from S1 to P2; when the service reaches C, C will perform cross-

section switching to switch service from P2 back to S1; then the service will be

transmitted via S1 to D and dropped at D. The service from D to A will also be

switched cross-section at node C and B. Therefore, the service routes are the

same before/after cross-section switching, which are: A→B→C→D and

D→C→B→A.

Other nodes and cross-sections

Node A Node B

NodeCNode D

Add/drop service

Add/drop service

Cross-section

BrCross-section

Sw

Cross-section

Br

Cross-section

Sw

WP

WP

WP

WP

X

WWPP

P

P

P

P

W

W

W

W

Fig. 5.5-7 Route example for the cross-section switching in case of fault

2. Cross-ring switching

For the four-fiber ring, if the fault affects both working channel and protection

channel, the service can be recovered by cross-ring switching.

As shown in Fig. 5.5-8, when S1 and P2 fibers between B and C are both

broken, the service between A and D is transmitted to B via S1 fiber, and B will

perform cross-ring switching to switch service from S1 to P1; then the service is

transmitted back to A via P1, and is transmitted on to D and C; C performs

cross-ring switching again to switch the service from P1 back to S1, and the

56


service is transmitted via S1 till it reaches D where it is dropped. Therefore, the

routes of the bi-directional service between A and D change after cross-ring

switching, which are respectively A→B→A→D→C→D and

D→C→D→A→B→A.

Other nodes and cross-sections

Node A Node B

Node CNode D

Add/drop service

Add/drop service

Cross-ring Br

Cross-ring Sw

Cross-ring Br

Cross-ring Sw

WP

WP

WP

WP

XWP

P

P

P

P

W

W

W

W

X

Fig. 5.5-8 Route example of the cross-ring switching in case of fault

The cross-section switching has higher priority than the cross-ring switching. If one

fiber section has requests for both of them, the system will respond to the cross-section

switching, because the service will reach the designated end via longer path after cross-

ring switching, which will seize protection path of other service. Therefore, cross-

section switching request is prioritized. Only when the service cannot be recovered

using cross-section switching, will cross-ring switching is used.

The service capacity, that is the maximum traffic amount of the four-fiber bidirectional

MS protection ring equals to K×STM-N, where K is the number of NEs (K≤16).

5.5.6 Comparison of Common Self-healing Rings

Among the above five protection modes of self-healing ring, three modes are

commonly used for networking. Table 5.5-2 compares these three protection modes

and lists their protection methods and characteristics.

57


Table 5.5-2 Comparisons of three commonly used modes for self-healing ring

Item Two-fiber Unidirectional Path

Ring

Two-fiber Bidirectional MS

Ring

Four-fiber Bidirectional MS

Ring

Node number K K K

Line rate STM-N STM-N STM-N

Ring transmission

capacity

STM-N K/2×STM-N K×STM-N

APS protocol No Yes Yes

Switching time <30 ms 50 ms ~200 ms 50 ms ~200 ms

Node cost Low Medium High

System complexity Simple Complicated Complicated

Major application

occasion

Access network, relay network

(centralized service)

Relay network, toll network

(distributed service)

Relay network, toll network

(distributed service)

5.6 Dual Node Interconnection (DNI) Protection

5.6.1 Terminologies

Drop-and-Continue: It refers to a function of the ring node, where the service signal

will be dropped from the working channel (Drop) of the ring and also continue to be

transmitted forward along the ring (Continue).

Dual Hubbed: The dual hubbed service can be led to the two central offices or any one

of the offices (or similar sites). Once one of the two junction points is faulty, the dual

hubbed service can be recovered.

Dual Node Interconnection: It refers to the structure between two rings. Two nodes of

one ring are interconnected with two nodes of the other ring.

Hold-off Time: It refers to the time interval from claiming the signal failure or signal

deterioration to starting the protection switching algorithm.

Path Selector: It is used in the SNCP architecture to select the working channel from

one side of the node or from the other side of the node to drop the service according to

the channel level criteria.

58


Primary Node: It is the node used in the MS-Ring interconnection architecture to

provide service signal selection and D&C function for certain tributary. Different

tributaries can have different primary nodes.

Propagation of Switching: One switching results in another switching. From the point

of maintenance, the propagation of switching is always unwelcome.

Ring Interconnection: It refers to an architecture between two rings, where one or

several nodes of each ring are interconnected with the other ring.

Ring Interworking: It refers to a network topology, where two rings are

interconnected via two nodes on each ring. This topological operation mode can

prevent any service loss on the ring when any one of the nodes is faulty, as shown in

Fig. 5.6-1.

Secondary Circuit: It is the replaceable route for the service to be transmitted from

one ring to another ring in the MS shared protection ring interworking architecture. It is

used when the service circuit is interrupted.

Secondary Node: It is the node that can provide replaceable interworking route for the

tributary in the MS shared protection ring interworking architecture.

Service Circuit: It is the original route preferentially selected for the service to be

transmitted from one ring to another ring in the MS shared protection ring interworking

architecture.

Service Selector: It is used for node function of ring interworking in the MS shared

protection ring architecture. It determines to select the service from one side of the

node or another side of the node according to some criteria.

Single Node Interconnection: It refers to an architecture between two rings. One node

of each ring is interconnected with one node of another ring.

Termination Node: It refers to the node where one tributary enters into or leave the

ring. (it can not be the primary node or secondary node.)

59


5.6.2 DNI Principle

A

Fig. 5.6-1 Ring interworking

Dual Node Interconnection (DNI) is a structure between two rings where each ring

provides two nodes to interconnect with the other ring. It provides protection for the

services of one ring crossover another ring by allocating the two interconnections

between the two rings. One special mode of dual node interconnection is called ring

interworking. The ring interworking is a network topology where the two rings are

interconnected via two nodes on each ring. The topological operation mode can prevent

any service loss on the ring if any one of the nodes is faulty. As shown in Fig. 5.6-1,

one tributary can be added and dropped at node A of the upper ring, or at node Z of the

lower ring. The meanings of the characters are described as follows:

· TA = the transmitted signal at node A.

· RA = the received signal at node A.

· TI1 = the transmitted signal of one node of the two interconnection nodes.

· RI1 = the received signal of one node of the two interconnection nodes.

· TI2 = the transmitted signal of another node of the two interconnection nodes.

· RI2 = the received signal of another node of the two interconnection nodes.

60


In the ring interworking, the interface relationship of the two sets of interconnection

nodes are described as follows:

· RI1=RI2=TA

· TI1=T I2

· RA=T I1 or T I2.

In other words, the signal from node A to node Z is transmitted to the two

interconnection nodes. Similarly, the signal sent back from node Z to node A is also

transmitted to the two interconnection nodes. Finally, only one of the two mutually

repetitive signals at the two interconnection nodes is selected by node Z or A.

5.6.3 Application Instance

Fig. 5.6-2 shows a DNI network consisting of two MS rings and a DNI network

consisting of one MS ring and one path ring.

1. Two MS rings

61


SS

SS

P S

SS

P S

MS shared protection ring

P Primary nodeS Secondary node

Service Selector


Fig. 5.6-2 Interworking of two MS shared protection rings

2. MS ring and path ring

62


P S

SS

PS

PS

PS

PS

SS


P Primary nodeS Secondary node

Service selector

Path selector

SNC protection ring

Fig. 5.6-3 Interworking of MS shared protection ring and path ring

63


5.7 Error Connection and Error Squelch

5.7.1 Error Connection

As shown in Fig. 5.7-1, there are two services in the ring: A<-> C and A<->F.

A<->C: A and C both use the 1st AU 1st TU12 of the optical board.

A<->F: A and F both use 1st AA 1st TU12 of the optical board (A uses another optical

board).

A

Fig. 5.7-1 Error connection example: 2.5G MS Ring

When node A is faulty, F will switch to the protection timeslot 9th AU 1st TU12 of the

protection optical board to receive and transmit the services of A<->F. As C is not the

end of the disconnected optical fiber, the service of A<->C is switched to the protection

time slot 9th AU 1st TU12 at node B. Now C is connected with F and becomes an error

connection.

5.7.2 Error Squelch of Error Connection

The principle for handling error connection is to disconnect it.

The methods for handling error connection: For one service, if it has been detected that

the target point does not exist already, insert AIS in the protection timeslot of the

64


protection optical board. Then for node F and B (the two ends of the disconnected

optical fiber), insert AIS in the protection timeslot 9th AU1 1st TU12. Although error

connection occurs, the service that is wrongly connected will not go through, so as to

squelch the error connection.

5.8 Logical Subnet Protection

5.8.1 Overview

Logical subnet is a method of splitting network based on the logical topology of the

network. It is a subnet developed after splitting the channel layer and section layer

horizontally based on the service and function features of the circuit-layer network.

In view of the disadvantage of splitting the network based on the physical topology of

the network, we regard the SDH transmission network from a logical viewpoint, and

split the network based on the logical topology of the network. This way, a large-

capacity complex physical network is split into several logical subnets with

independent functions and services, so that it is far easier for the system to manage and

protect the logical subnets.

The SDH network in the intersection ring structure can be simplified into: Use the

over-ring service and non-ring service as a basis of splitting the logical subnet, and

split the physical ring logically into different logical subnets according to the service or

protection mode. Now we use the intersection ring shown in Fig 5.8-1 as an example to

describe the logical splitting of intersection ring. Fig 5.8-1 splits the intersection ring

into two independent MS rings. The two logical MS rings can apply the bidirectional

shared protection mode to configure and protect the services. This highly improves the

resource utilization.

65


Fig 5.8-1 Split the Intersection Ring into Independent Logical Rings

5.8.2 Basic Principles

In the actual networking of SDH, due to limitation of fiber distribution, multiple

transmission network may share one fiber section, and the MS ring protection mode is

configured for each transmission network. Therefore, this shared fiber section should

be split for each transmission network, so as to build multiple logical subnets logically,

and provide the MS protection for multiple logical subnets.

The logical subnet protection is only limited to the expansion of the MS-Spring mode.

Logical MS-Spring means changing the rule of “splitting the line bandwidth into

working channel and protection channel evenly (measured in AUG)” to this rule:

Define some AUGs as working channels as specifically required, and define other

AUGs as protection channels, still measured in AUG, but the splitting rule is: It can be

split flexibly only if the number of AUGs of the protection channels of any logical NE

in the logical subnet is not less than the number of AUGs of the working channel of

any logical NE.

Specifically, the logical subnet protection function is to carry multiple logical optical

ports in one physical optical port. Each logical optical port can combine with other

logical optical ports or SDH devices to form MS ring network, i.e., logical subnet. The

logical subnet implements the corresponding MS protection according to the

corresponding MS protection excitation, and tries to provide protection for the services

affected by line faults.

66


A maximum of 4 logical subnets can be carried on a fiber section.

5.8.3 Categorization

From perspective of application mode, the logical subnet can be divided into three

types:

· Dedicated mode

· Shared mode

· Extra service mode

After combining with the MS ring network type, the logical subnet of each mode can

be implemented in the four following ways:

· 2-fiber ring + 2-fiber ring




5.8.4 Application Instance

5.8.4.1 STM-16 Rate Two-fiber Ring and STM-4 Rate Two-fiber Ring

Network consisting of one high-rate two-fiber ring and one low-rate two-fiber ring is

shown in Fig. 5.8-5. NE A, B, C, D and NE E, F, C, D respectively compose two logic

multiplex section rings. The two rings share the optical fiber cross-section connecting

C with D, with the bandwidth of STM-16.

67


A

D

E

B

C

F

Logic MS ring 1 (STM-16)


2#2#

2#

2#

7#7#

7#

7#

1#

1#

1#

8#

8#

8#

Fig. 5.8-5 Combination of high-rate two-fiber ring and low-rate two-fiber ring

The allocation for the bandwidth of the shared cross-section is shown in Fig. 5.8-6.

From left to right in the figure, they respectively represents for: the bandwidth

allocation when configuring only one MS ring, the dedicated bandwidth allocation

when configuring two logic MS rings, the optimized dedicated bandwidth allocation

when configuring two logic MS rings, and the shared bandwidth allocation when

configuring two logic MS rings.

W

P

W1

P1

W2

P2

W1

W2

P1

P2

W1

W2

P0

Fig. 5.8-6 Allocation modes for the shared bandwidth when high-rate two-fiber ring combines with

low-rate two-fiber ring

When adopting optimized dedicated allocation to allocate the shared cross-section

bandwidth, the detailed configurations of the two logic subnets are shown in Fig. 5.8-7.

In the logic MS ring 1, the 1st~8th AUs of the optical board in slot #7 between A and B

are the working AUs, while the 9th~16th AUs are the protection AUs. The 1st~8th AUs

of the optical board in slot #2 between A and D are the working AUs, while the 9~16

AUs are the protection AUs. The marked shadow area refers to the detailed bandwidth

allocation mode of the physical shared cross-section.

68


Fig. 5.8-7 Protection configuration of each NE in the two logic MS rings when the high-rate two-fiber ring combines with

the low-rate two-fiber ring

5.8.4.2 STM-16 Rate Four-fiber Ring and STM-4 Rate Four-fiber Ring

Network consisting of one high-rate four-fiber ring and one low-rate four-fiber ring is



C with D, with the bandwidth of (STM-16)×2.

A

D

E

B

C

F



2#2#

2#

2#

7#7#

7#

7#

1#

1#

1#

11#8#

8#

8#

11#16#

16#

16#

16# 11#

11#

10#

10#

10#

17#

17#

17#

Fig. 5.8-8 Combination of high-rate four-fiber ring and low-rate four-fiber ring

The allocations for the bandwidth of the shared cross-section are shown in Fig. 5.8-9.

From left to right, they respectively represents for: the bandwidth allocation mode

when configuring only one MS ring, the dedicated bandwidth allocation when

configuring two logic MS rings, the optimized dedicated bandwidth allocation when

configuring two logic MS rings, and the shared bandwidth allocation when configuring

two logic MS rings.

69


W

P

W1

W2

P1

P2

W1

W2

P2

P1

W1

W2

P0

Fig. 5.8-9 Allocation modes for the shared bandwidth when high-rate four-fiber ring combines with

low-rate four-fiber ring

When adopting the optimized dedicated allocation to allocate the shared cross-section

bandwidth, the detailed configurations of the two logic subnets are shown in Fig.

5.8-10. In the logic MS ring 1, the 1st~16th AUs of the optical board in slot #7 between

A and B are the working AUs, while the 1st~16th AUs of the optical board in slot #16

are the protection AUs. The 1st~16th AUs of the optical board in slot #2 between A and

D are the working AUs, while the 1st~16th AUs of the optical board in slot #11 are the

protection AUs. The marked shadow area refers to the detailed bandwidth allocation

mode of the physical shared cross-section.

Fig. 5.8-10 Protection configuration of each NE in the two logic MS rings when the high-rate four-fiber ring combines with

the low-rate four-fiber ring

5.8.4.3 STM-16 Rate Four-fiber Ring and STM-4 Rate Two-fiber Ring

Network consisting of one high-rate four-fiber ring and one low-rate two-fiber ring is



C with D, with the bandwidth of (STM-16)×2.

70


A

D

E

B

C

F



2#2#

2#

2#

7#7#

7#

7#

1#

1#

1#

11#8#

8#

8#

11#16#

16#

16#

16# 11#

11#

Fig. 5.8-11 Combination of high-rate four-fiber ring and low-rate two-fiber ring

The bandwidth allocation of the shared cross-section is shown in Fig. 5.8-12. From left

to right, they respectively represents for: the bandwidth allocation when configuring

only one MS ring, the ordinary dedicated bandwidth allocation when configuring two

logic MS rings, the optimized dedicated bandwidth allocation when configuring two

logic MS rings.

W

P

W1

P1

W1

W2

P2

P1

W2

P2

Fig. 5.8-12 Allocation modes for the shared bandwidth when high-rate four-fiber ring combines with

low-rate four-fiber ring




A and B are the working AUs, while the 1st~16th AUs of the optical board in slot #16

are the protection AUs. The 1st~16th AUs of the optical board in slot #2 between A and

D are the working AUs, while the 1st~16th AUs of the optical board in slot #11 are the

71


protection AUs. The marked shadow area refers to the detailed bandwidth allocation

mode of the physical shared cross-section.

Fig. 5.8-13 Protection configuration of each NE in the two logic MS rings when the high-rate four-fiber ring combines with

the low-rate two-fiber ring

5.8.4.4 STM-16 Rate Two-fiber Ring and STM-4 Rate Four-fiber Ring

Network consisting of one high-rate two-fiber ring and one low-rate four-fiber ring is



C with D, with the bandwidth of STM-16.

A

D

E

B

C

F



2#2#

2#

2#

7#7#

7#

7#

1#

1#

1#

11#8#

8#

8#

16#10#

10#

10#

17#

17#

17#

Fig. 5.8-14 Combination of high-rate two-fiber ring and low-rate four-fiber ring

The bandwidth allocation of the shared cross-section is shown in Fig. 5.8-15. From left

to right, they respectively represents for: the bandwidth allocation when configuring

only one MS ring, the dedicated bandwidth allocation when configuring two logic MS

72


rings, the optimized dedicated bandwidth allocation when configuring two logic MS

rings, and the shared bandwidth allocation when configuring two logic MS rings.

W

P

W1

P1

W2

P2

W1

W2

P2

P1

W1

W2

P0

Fig. 5.8-15 Allocation modes for the shared bandwidth when high-rate two-fiber ring combines with

low-rate two-fiber ring




A and B are the working AUs, while the 9th~16th AUs are the protection AUs. The

1st~8th AUs of the optical board in slot #2 between A and D are the working AUs,

while the 9th~16th AUs are the protection AUs. The marked shadow area refers to the

detailed bandwidth allocation mode of the physical shared cross-section.

Fig. 5.8-16 Protection configuration of each NE in the two logic MS rings when the high-rate two-fiber ring combines with

the low-rate four-fiber ring

5.9 Topology and Features of Complicated Network

The combinations of chains and rings can compose some more complicated network

topologies. This section describes several topologies commonly used for networking.

5.9.1 T Network

T network is actually a tree network, as shown in Fig. 5.9-1.

73


TM

TM

TMADM ADM

ADM

ADM

STM-16

STM-4

A

Fig. 5.9-1 T network topology

Suppose it is an STM-16 system on the trunk, and STM-4 system on the tributary lines.

The function of T network is to add/drop tributary STM-4 services to/from the trunk

STM-16 system via NE A. Tributary lines are connected to the tributaries of NE A.

These tributary services are regarded as low-speed tributary signals of NE A, and are

added/dropped via NE A.

5.9.2 Ring-chain Network

The network topology of ring-chain network is shown in Fig. 5.9-2.

TMADM

ADM

ADM ADM

ADM

STM-4

STM-16

C

A B D

Fig. 5.9-2 Ring-chain network topology

The ring-chain network consists of basic topologies of ring and chain networks, which

are connected together via NE A, as shown in Fig. 5.9-2. The STM-4 service of the

chain is the low-speed tributary service of NE A, and is added/dropped via NE A. The

STM-4 service on the chain has no protection, while the service on the ring is

protected. For example, suppose there is service between NE C and NE D in the figure.

If the optical cable between A and B is broken, the service on the chain will be

interrupted; if the optical cable between A and C is broken, the service between C and

D will not be interrupted because of the ring protection function.

74


5.9.3 Tributary Cross-Over of Ring Subnets

The network topology is shown in Fig. 5.9-3.

ADM

ADMADM

ADM

ADM

ADM

ADM

ADM

STM-1/4STM-16 STM-16

AB

Fig. 5.9-3 Network topology of tributary cross-over of ring subnets

Two STM-16 rings are connected together through the tributary path between A and B.

Any NE in the two rings can exchange service with each other through the tributary

between A and B, there are multiple routes for choices, and the system redundancy is

high. Sine all services between the two rings must be transmitted through the low-

speed tributary between A and B, the speed bottleneck of low-speed tributary and

security problem exist.

5.9.4 Tangent Rings

The network topology is shown in Fig. 5.9-4.

75


ADM

ADM ADM

ADM

ADM

ADM

ADM

ADM 2500

STM-1 STM-1

STM-1

ADM

STM-16

STM-16

STM-16

A

Ring Ⅱ

Ring III

155

622

STM-4

STM-4

STM-4

DXC/ADM

RingⅠ

Fig. 5.9-4 Tangent rings topology

The three rings are tangent with each other through NE A. A DXC or an ADM (ring II

and ring III are both low-speed tributaries for NE A) can be used. This networking

mode can enable NEs of the rings to exchange services freely, with greater service

dispatching ability than a ring network with tributary cross-over. It provides services

with more routes for choice, and the system redundancy is higher. However, this

networking mode has problems of security and protection for the central node (NE A).

5.9.5 Intersected Rings

The tangent rings can be extended to be intersected rings in order to provide the backup

central (important) node with more routes for choice, and improves system reliability

and redundancy. The intersected rings topology is shown in Fig. 5.9-5.

76


ADM

ADM

ADM

ADM

ADM

ADM

STM-16

STM-16

STM-16

622

STM-4

STM-4

STM-4

DXC/ADM

DXC/ADM

Fig. 5.9-5 Intersected rings topology

5.9.6 Hinge Network

The hinge network topology is shown in Fig. 5.9-6. NE A is the hinge node, and chains

or rings of STM-1 or STM-4 can connect to the tributary side of NE A. Through the

cross-connect function of NE A, the tributary service can be added/dropped to/from the

trunk, and tributaries can exchange services between each other; thus avoiding adding

direct route and equipment between tributaries, and avoiding occupying resource of the

trunk network.

ADM

ADM ADM

ADM

ADM

ADM

ADMADM

ADM

ADM

STM-16STM-16

STM-1

STM-1

STM-16

STM-4

STM-4

DXC/ADM

A

STM¡ ª4/1

Fig. 5.9-6 Hinge network topology

5.10 Overall Architecture of SDH Network

SDH has great advantages compared with PDH. However these advantages can only be

exhibited when constructing SDH network.

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In traditional networking, improving the utilization ratio of transmission equipment is

the most important issue. To improve utilization ratio and security of lines, many direct

routes are added between the nodes, which results in too complicated network

structure. With the development of modern communication, the most important task is

to simplify network structure, build a strong OAM functions, reduce transmission cost,

and support the development of new services.

The SDH network structure of China includes four layers, as shown in Fig. 5.10-1.

DXC16/16

DXC16/16

DXC64/64

DXC64/64

DXC4/4

ADM

DXC16/16

DXC4/4

DXC4/1 ADM ADM DXC4/1 ADM ADM

ADM

ADM ADM

OLT OLT

ADM

OLT

OLT OLT

ADM

OLT

OLT

DXC4/1

Second-level trunk network

User access network

Ring

Star

Fi rst-level trunk network

STM64 or STM16

ADM

OLT

Relay network

Fig. 5.10-1 SDH network structure of China

The top layer is the long-distance first-level trunk network. Major capital cities of

provinces and the tandem cities with large traffic are equipped with DXC4/4, and high-

speed optical fiber lines of STM-4/STM-16 connect these cities, giving the national

mesh backbone network with bigger capacity and high reliability, assisted by a small

amount of linear networks. Since DXC4/4 has 140 Mbit/s interfaces for PDH, the

original PDH 140 Mbit/s and 565 Mbit/s systems can also be accommodated into the

long-distance first-level trunk network managed uniformly by DXC4/4.

The second layer is the second-level trunk network. Its major tandem nodes are

equipped with DXC4/4 or DXC4/1, STM-1/STM-4 transmission chains connect these

78


nodes, thus composing province internal mesh network or ring backbone network,

assisted by a small amount of linear networks. Since DXC4/1 has 2 Mbit/s, 34 Mbit/s,

and 140 Mbit/s interfaces, the original PDH system can also be accommodated into the

second-level trunk network managed uniformly, with flexible circuit dispatching

ability.

The third layer is the relay network (i.e. the transmission part between toll terminating

office and local office, or between local offices), which can be divided into several

rings according to different regions. ADMs compose self-healing ring of STM-1/STM-

4, or dual-node ring in route backup mode. These rings have strong survivability and

service dispatching function. The ring network mainly adopts MS protection switching

ring, whether to use four-fiber or two-fiber is determined by the traffic amount and

cost. The rings communicate with others through DXC4/1, to dispatch services and

implement other management functions. The rings can serve as the gateway or

interface between toll network and relay network, and between relay network and user

network. They can also serve as the gateway between PDH and SDH.

The bottom layer is the user access network. It’s located at the boundary of the whole

network, there are few service capacity requirements for it, and most of its traffic is

concentrated on one node (terminating office), therefore path switching ring and star

network are both good applications to this layer. The equipment needed for this layer

include ADM and OLC (Optical User Loop Carrier system). The rate is STM-1/STM-

4. The interface can be: STM-1 optical/electrical interface; 2 Mbit/s, 34 Mbit/s, or 140

Mbit/s interface; ordinary telephone user interface; small-scale switch interface; 2B+D

or 30B+D interface; and metropolitan network interface.

The user access network is the largest and most complicated part of SDH network,

which occupies 50% of the whole communication network investment. Applying

optical fibers to the user access network is a step-by-step process. FTTC (Fiber To The

Curb), FTTB (Fiber To The Building) and FTTH (Fiber To The Home) are different

stages of this process. China currently needs to consider adopting the integrated

SDH/CATV network when popularizing optical fiber user access network, which is to

provide not only telecommunication services, but also CATV service.

Summary

79


This chapter describes the basic topologies, self-healing principle, and networking and

features of complicated networks of SDH network. They key points to master are:

working principles, application scopes, and service capacities of the unidirectional path

protection ring and two-fiber bidirectional MS protection ring.

Exercises

1. The switching condition of unidirectional path protection ring is alarm.

2. The switching condition of two-fiber bidirectional MS protection ring are: ,

, alarms.

3. Relate two fiber unidirectional path protection ring and two fiber bidirectional

path protection ring.

4. Which one is faster; path protection or multiplex section protection?

5. What is the difference between Cross-ring Switching and Cross-connection

Switching?

6. How returnable mode is different from non-returnable mode?

7. How DNI works?

8. What are the advantages of logical subnet protection?

9. Why it is not possible to provide protection over a two fiber chain network?

10. What is the maximum and minimum number of nodes that can be protected by

multiplex section protection method in a ring topology?

.

80

6 Timing and Synchronization

Key points

Synchronization methods

Working mode of the clock in master/slave synchronization network

Synchronization methods of SDH network

Protection switching principles of the clock in SDH network

6.1 Synchronization Modes

Network synchronization is one of the major problems to be solved in digital network,

because we need to ensure that the transmitter put the pulse at a certain time position

(timeslot) when transmitting the digital pulse signal, and the receiver should be able to

read this pulse from the certain timeslot, so that the transmitter and receiver can

communicate with each other normally. The above function is implemented by

synchronizing clocks of the transmitter and receiver. Therefore, the purpose of network

synchronization is to restrict the frequency and phase of clock at each node within the

pre-defined allowable range to avoid transmission performance degradation

(impairment) caused by inaccurate synchronization of transmitter/receiver in the digital

transmission system.

There are different modes for digital network synchronization, among which two

modes are commonly used: pseudo synchronization and master/slave synchronization.

6.1.1 Pseudo Synchronization

It means that the clock of each digital exchange in the digital switching network is

independent from each other and without any relation, while the clock of each digital

exchange is of extreme precision and stability. Usually the cesium atom clock is used

due to its high precision. Since the clock has high precision, although the clock of each

exchange in the network is not completely the same (in frequency and in phase), the

error is very small and the network is close to synchronization, so it is called pseudo-

synchronization.

81

This method is generally applied in international digital network, that is, the digital

network between one country and another country. For example, there is one cesium

atom clock each at China international exchange and U.S.A international exchange, and

these two clocks adopt pseudo-synchronization method.

6.1.2 Master/Slave Synchronization

Master/slave synchronization is to set up a clock master office equipped with high-

precision clock in the network, and each exchange within the network is controlled by

the master office (i.e. track the clock of the master office and use it as timing

reference). The upper-level exchanges control the lower-level exchanges, until reaching

the end network element in the network – the terminal exchange.

The master/slave synchronization method is generally used for the internal digital

network of a country or region. Its characteristics are: there is only one master office

clock in the country or region, other network elements within the network all rely on

this master office clock as their timing reference.

The principles of master/slave synchronization and pseudo-synchronization are shown

in Fig. 6.1-1.

To overseas international exchang

International exchange

International exchange

Local exchange

National exchange

National exchange

National exchange

National exchange

Local exchange

Local tandem exchange




Terminal exchange

Terminal exchange

Terminal exchange

Terminal exchange

MS MS MS MS

MS MS MS MSMS

MS MSMS MS MS

MS: Master/Slave synchronization

Pseudo-synchronization

.

.

.

.

.

.

Fig. 6.1-1 Master/slave synchronization and pseudo-synchronization

In order to enhance the reliability of the master/slave synchronization system, a vice

clock may be set up in the network by adopting the hierarchical master/slave control

mode. Both clocks adopt the cesium clock. In normal cases, the primary clock works as

82


the timing reference of the network, and the vice clock also relies on the primary clock

for timing reference. When the primary clock encounters fault, the vice clock will

provide timing reference for the whole network. After the primary clock recovers, it

will switch back to the primary clock to provide the network timing reference.

The synchronization mode adopted by China is the hierarchical master/slave

synchronization mode of which the primary clock is in Beijing and the vice clock is in

Wuhan. When adopting the master/slave synchronization, the timing signals of the

upper level NEs are transmitted to the lower level NEs via a certain route – via the

synchronous link or by affixing to the line signals. The NE of the upper level extracts

the clock signal, tracks and locks the clock using its own phase-locked oscillators, and

uses the clock as reference to generate local clock signal for itself. Meanwhile it

transmits the clock via the synchronous link or the transmission line (i.e. affixing the

clock information to the line signals for transmission) to the lower level NEs for clock

tracking and locking. If one NE fails to receive the reference clock transmitted from the

upper level NE, it can use its external timing reference or start its internal crystal

oscillator to provide the local clock used by itself, and it will transmit the local clock

signal to its lower level NEs.

Besides the pseudo synchronization and master/slave synchronization modes, there are

other synchronization modes in digital network, including mutual synchronization,

external reference implantation.

The external reference implantation mode backs up the clock at important nodes in the

network to avoid situation when the primary clock reference at the important node is

lost, and the quality of its internal clock is not good enough, so that the normal work of

a wide range of NEs will be influenced. The external reference implantation mode

adopts the GPS (Global Position System), and sets up GPS receivers at important NE

offices to provide high-precision timing, and to form the Local Primary Reference

clock (LPR). Once the primary clock reference is lost, other lower level NEs in the

region still adopts the master/slave synchronization mode to track the reference clock

offered by the GPS.

83


6.2 Working Modes of Sub-Clock in Master/Slave Synchronous Network

In the master/slave synchronous digital network, the clocks of sub-stations (lower level

stations) have three types of working modes.

6.2.1 Normal Working Mode - Track and Lock the Upper Level Clock

The clock reference tracked and locked by the sub-station is transmitted from the upper

level station which may be the primary clock of the network, or the internal clock of

the upper level NEs transmitted to this NE, or the local GPS clock.

Compared with the other working modes of sub-clocks, this type of working mode

offers sub-clocks with highest precision.

6.2.2 Hold-on Mode

When all the timing references are lost, the sub-clocks will enter the holdover mode

where the clock sources of the sub-stations will use the last frequency information

saved before losing the timing reference signals for its timing reference. In other

words, the sub-clocks have the “memory” function which can offer the timing signals

relatively matching the original timing references to ensure a fairly small frequency

error between the sub-clock frequency and the reference clock frequency for a long

time. However, the inherent oscillation frequency of the oscillator will gradually

wander, so the relatively high precision offered by this working mode cannot last very

long. The clock precision of this working mode is just less than that of the normal

working mode.

6.2.3 Free Run Mode – Free Oscillation Mode

When the sub-clock loses all its external timing references including the timing

reference memory or it remains in the holdover mode for too long, the internal

oscillator of the sub-clock will work in the free oscillation mode.

This mode offers the lowest clock precision.

6.3 Network Synchronization Requirements of SDH

The synchronization performance of digital network is critical to the normal work of

the network. The introduction of SDH network raises more requirements for network

84


synchronization. When the network is in normal working mode, every NE is

synchronized to one reference clock, there is only phase offset and no frequency offset

between NEs clocks, therefore only occasional pointer justification event will occur

(point justification event does not happen often when the network is synchronized).

When one NE loses its synchronous reference clock and enters holdover mode or free-

oscillation mode, the frequency offset between its local clock and the network clock

will occur, resulting in continuous pointer justifications and affecting normal

transmission of service in the network.

SDH and PDH networks will coexist for a long time. Jitter and wander at the

SDH/PDH boundary mainly comes from pointer justification and payload mapping

process. The pointer justification frequency at SDH/PDH border is closely related to

the synchronization performance of the gateway node.

If the SDH input gateway which execute asynchronous mapping loses its

synchronization, the frequency offset and wander of the clock at this node will result in

continuous pointer justification of the whole SDH network, and deteriorate the

synchronization performance.

If the last node connecting with SDH network loses its synchronization, the SDH

network output will still have pointer justification and affect synchronization

performance.

If the middle network node loses synchronization, as long as the input gateway is still

synchronized with the PRC (Primary Reference Clock), the network unit that is next to

the faulty node and is still synchronized or the output gateway can correct the pointer

move of the middle network node, so that there will be no net pointer move at the last

output gateway and will not affect the synchronization performance.

6.4 Clock Source Types of SDH NE

The clock sources of SDH NE include four types as follows:

· External clock source: input interface provided by SETPI functional block

· Line clock source: extracted from the STM-N line signals by SPI functional

block

· Tributary clock source: extracted from PDH tributary signals by PPI functional

block, but this clock is seldom used since the pointer justification at the

85


boundary of SDH/PDH networks will influence the clock quality

· Internal clock source of equipment: provided by SETS functional block

Meanwhile, SDH NE provides the external with the output interface of clock source

through the SETPI functional block.

6.5 Selection Principle of Clock in SDH Network

The service add/drop and rerouting functions of SDH network bring unprecedented

flexibility and high survivability to network applications, and makes network

synchronous timing selection more complicated. In SDH network, the timing reference

allocation between nodes is realized through a large number of lower level SDH NE

clocks. Therefore, the quality of the timing reference must be properly identified. The

Synchronization Status Message (SSM) is used to indicate the quality of timing

reference.

SDH MSOH makes use of the 5th to 8th bits of byte S1 to transmit SSM message,

which can represents sixteen different synchronization quality levels. Refer to “3.1.2.11

Synchronization Status Message Byte: S1 (b5~b8)” for details.

In SDH network, the timing reference allocation between nodes is realized through a

large number of SDH NE clocks. With the increasing number of NEs in the

synchronous link, the quality of the timing reference signal is gradually deteriorating.

Therefore, if there are multiple synchronous paths of the same quality level for an NE

to choose from, adopting the synchronous path that passes through the least number of

NEs will help to improve the timing performance of the SDH network. According to

this principle, ZTE designed the S1 byte patent algorithm which enables NEs to choose

the clock reference signal of the highest quality level and shortest synchronous path.

The clock selection should follow the rules below:

1. If an NE can select from multiple valid clock sources, it will first select the

clock with the highest quality level according to the quality level information of

the clock sources.

2. If the quality levels of clock sources are the same, the NE will select the clock

source passing through the least number of NEs along the transmission path.

3. The NE forwards to the downstream NE the quality level information of the

currently adopted clock source and the quantity of passed NE via the S1 byte,

86


and sends the “unavailable” status information to the upstream NE.

Note:

The upstream and downstream NEs are relative. If NE B extracts clock from NE A, NE

A is the upstream NE of NE B, and NE B is the downstream NE of NE A.

6.5.1 Synchronization Principle of SDH Network

6.5.1.1 Master/Slave Synchronization Mode

Synchronous digital network in China adopts hierarchical master/slave synchronization

mode. That is using a single reference clock to control the synchronization of the whole

network through the synchronous link of the synchronous distribution network. Within

the network, a series of hierarchical clocks are adopted, and the clocks of each level are

synchronous with the clocks of the upper level or of the same level.

According to the precision level, the master/slave synchronous clocks of SDH network

can be divided into four types (levels) corresponding to different application scopes.

ITU-T standardizes each level of clocks and the quality levels of clocks are listed

below in an order from high to low:

1. Primary reference clock: compatible with G.811 specifications, as the timing

reference of the whole network

2. Transit exchange clock: compatible with G.812 specifications, as the vice clock

for transit exchange

3. Terminal exchange clock: compatible with G.812 specifications, as the vice

clock for terminal exchange (local office)

4. SDH NE clock: compatible with G.813 specifications, as the internal clock of

SDH equipment

If the NE works in the normal working mode, the performance of various clocks

transmitted to corresponding exchanges is mainly determined by the performance of

the synchronous transmission link and the timing extraction circuit. If the NE works in

the protection mode or the free-run mode, the performance of various clocks mainly

relies on the performance of the clock sources generating the clocks (the clock sources

located at various NE nodes accordingly). Therefore, high-level clocks must adopt

high-performance clock sources.

87


6.5.1.2 Precautions when Transmitting Clock Reference in Digital Network

1. There should be no loop during synchronous clock transmission.

In Fig. 6.5-1, if NE2 tracks the clock of NE1, NE3 tracks the clock of NE2, and

NE1 tracks the clock of NE3, the transmission link of the synchronous clock

will form a loop, and when the clock of one NE deteriorates, the synchronous

performance of all the NEs in the whole loop will deteriorate as a domino

effect.

NE1

NE2 NE3

Fig. 6.5-1 Network diagram

2. Reduce the length of the timing transmission link as much as possible to avoid

the influence on the quality of the transmitted clock signal due to distance.

3. The clock of the sub-station should acquire its reference from the equipment of

higher level or of the same level.

4. The primary/standby clock references should be acquired through distributed

routes, to prevent losing the clock reference when the primary clock

transmission link is interrupted.

5. Choose the transmission system with high usability to transmit clock reference.

6.5.2 Instance

An application instance of SSM is shown in Fig. 6.5-2.

88


A

D

C

B

Unavailable

PRC

PRC

PRC

PRC

PRC

PRC

Synchronous path (in use)

Synchronous path (not in use)PRC ｝Synchronization status

message

( a )

Unavailable

Unavailable

Unavailable

A

D

C

B

PRC

PRC

PRC

PRC

PRC

PRC ｝

PRC

( b )

Unavailable

Synchronous path (in use)Synchronous path (not in use)

Synchronization statusmessageUnavailable

Unavailable

Unavailable

A

D

C

B

SETS

SETS

SETS

SETS

SETS

SETS ｝( c )

Unavailable

Synchronous path (in use)

Synchronous path (not in use)Synchronization statusmessageUnavailable

Unavailable

Unavailable

Fig. 6.5-2 Instance of SSM application

In Fig. 6.5-2, each NE has two synchronous clock sources to choose from. The

configurations of the synchronous sources of each NE are listed in Table 6.5-1.

89


Table 6.5-1 Settings of NE synchronous sources

NE Clock Source List

NE A External clock source and internal clock source

NE B Line clock 1 and line clock 2

NE C Line clock 1 and line clock 2

NE D Line clock 1 and line clock 2

During normal operation, the available synchronous source of NE A includes the

external access clock of Primary Reference Clock (PRC) and internal clock source.

According to rule 1, NE A will automatically choose the external clock source PRC and

send its synchronous quality level information to other NEs. The available synchronous

sources of NE B are the A-B line clock and A-D-C-B line clock. According to rule 2,

NE B will automatically choose the A-B line clock as its synchronous source.

Similarly, NE D will automatically choose the A-D line clock as its synchronous

source. NE C may choose the A-B-C line clock or A-D-C line clock. In Fig. 6.5-2 (a),

NE C chooses the A-B-C line clock. Each NE will send the “unavailable” status

message to its upstream NE according to rule 3.

In case of line interruption, as shown in Fig. 6.5-2 (b), when the line between NE B and

C is broken, NE C will choose the A-D-C line clock and send the “unavailable” status

message to its upstream NE D.

If there is no external clock source, as shown in Fig. 6.5-2 (c) where the external clock

source of NE A is interrupted, NE A will enter the clock holdover mode, and then enter

the free-oscillation mode after the time of the holdover mode is over. At this time, each

NE is still synchronous with NE A, the clock source level will degrade to the

equipment clock SETS of the NE.

Summary

This chapter describes the synchronization methods and structure of SDH synchronous

network, various working modes of NE clock sources, and the SDH network

synchronization when the external clock changes.

Exercises

1. What are the working modes of a clock source?

90


2. What kinds of clock sources are commonly used for SDH NE?

3. Give the guidelines used in the process of synchronization of an NE.

4. What is meant by Free Running Clock and why it is not recommended for use?

5. What do you understand by the term tracing the clock?

6. Where Pseudo Synchronization is used?

7. Which byte is used for clock quality information exchange purposes?

8. What clock message is conveyed by a downstream NE to an upstream NE?

9. Why line clock tracing is better than tributary clock tracing?

10. What is meant by frequency offset and phase offset?

91

7 Optical Interfaces

Key points

Types of optical interfaces

Parameters of optical interfaces

Optical interfaces are the most characteristic part of synchronous optical cable digital

line system. Since they are standardized, they can directly connect different NEs

through optical lines; thus saves unnecessary optical/electrical conversion, avoids

signal impairment (such as pulse distortion) brought by the O/E conversion, and saves

network operation cost.

7.1 Optical Interface Types

Optical interfaces can be classified into three types according to different applications:

optical interface for intra-office communications, optical interface for short-haul inter-

office communications, and optical interface for long-haul inter-office

communications. The optical interfaces of different applications have different

identifiers, as shown in Table 7.1-1.

Table 7.1-1 Optical interface identifiers

Application Intra-officeInter-office

Short-haul Long-haul

Operating Wavelength (nm) 1310 1310 1550 1310 1550

Optical Fiber Type G.652 G.652 G.652 G.652 G.652 G.653

Transmission Distance (km) ≤2 ~15 ~40 ~80

STM-1 I-1 S-1.1 S-1.2 L-1.1 L-1.2 L-1.3

STM-4 I-4 S-4.1 S-4.2 L-4.1 L-4.2 L-4.3

STM-16 I-16 S-16.1 S-16.2 L-16.1 L-16.2 L-16.3

The first character of the identifier indicates the application:

· I represents for the intra-office communications

· S represents for short-haul inter-office communications

93

· L represents for long-haul inter-office communications

The first digit following the dash after the characters represents the STM rate: e.g. 1

represents for STM-1, 16 represents for STM-16.

The second digit following the dash after the characters represents the working

wavelength window and optical fiber type:

· 1 and blank indicates that the working wavelength is 1310 nm, and the optical

fiber type is G.652

· 2 indicates that the working wavelength is 1550 nm, and the optical fiber type is

G.652 or G.654

· 3 indicates that the working wavelength is 1550 nm, and the optical fiber type is

G.653

7.2 Optical Interface Parameters

The locations of optical interfaces in SDH network system is shown in Fig. 7.2-1.

Tra

nsm

it

CTX

S

Plug

Optical cable facilities

Rec

eive

CRX

R

Plug

Fig. 7.2-1 Locations of optical interfaces in SDH network

In Fig. 7.2-1, point S is the reference point on the optical fiber just after the transmitter

optical connector (CTX) of the transmitter (TX), and point R is the reference point on

the optical fiber just before the receiver optical connector (CRX) of the receiver (RX).

Parameters of optical interfaces can be classified into three categories: optical

parameters of the transmitter at reference point S, optical parameters of the receiver at

reference point R, and optical parameters between point S and point R.

All values specified are worst-case values, i.e. the bit error ratio of each regenerator

section (optical cable section) should be no more than 1×10-10 for the extreme (worst)

case of optical path attenuation and dispersion conditions.

94


7.2.1 Optical Line Code Pattern

There are abundant overhead bytes for system OAM functions in the frame structure of

SDH system. The line code pattern of SDH system adopts the scrambled NRZ code,

and the line signal rate equals to the standard STM-N signal rate. ITU-T G.707

specified the scrambling method for NRZ code, which is the standard 7-level

scrambler, with the scramble generation polynomial of 1+X6+X7, and the scramble

sequence length of 27-1=127 (bits). The advantages of this method are: the code pattern

is simple and does not add the line signal rate; there is no optical power cost or

requirement of coding; the transmitter only needs one scrambler, and the receiver can

receive services from the transmitter by simply adopting the same standard decoder, so

that the optical lines of equipment from different manufacturers can connect with each

other. The adoption of scrambler aims to prevent too many continuous “0” or “1” of

signals during transmission, and to make it easy for the receiver to extract the timing

information (done by the SPI functional block) from signals. In addition, when the

pseudo random sequence generated by the scrambler is long enough, i.e. when the

relevancies of scrambled signals are little, the relevancies of regenerators’ jitters can be

reduced quite a bit.

7.2.2 S Point Specifications-Specifications of Optical Transmitter

1. Maximum -20 dB width

Since the main power of Single-Longitudinal Mode (SLM) laser concentrates

on the peak mode, the spectral width of SLM laser is specified by the maximum

full width of the central wavelength peak, measured 20 dB down from the

maximum amplitude of the central wavelength under standard operating

conditions. The spectral characteristics of SLM laser are shown in Fig. 7.2-2.

Fig. 7.2-2 Spectral characteristics of SLM laser

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2. Minimum Side Mode Suppression Ratio (SMSR)

The minimum SMSR is specified as the minimum ratio of the mean optical

power (P1) of the peak longitudinal mode to the mean optical power (P2) of the

most distinguished side mode, measured under full-modulated and worst

reflection conditions.

SMSR=10 log (P1/P2)

G.957 specifies that the value of SMSR should be no less than 30 dB.

3. Mean launched power

The mean launched power at reference point S is the average optical power of a

pseudo random signal sequence transmitted by the transmitter.

4. Extinction ratio (EX)

The extinction ratio is defined as the minimum ratio of the average optical

power (P1) of the logical “1” (Mark) to the average optical power (P0) of the

logical “0” (Space).

EX=10 log(P1/P0)

ITU-T specifies the extinction ratio to be 10 dB for long-haul transmission

except for L-16.2, and to be 8.2 dB for other cases.

7.2.3 R Point Specifications-Specifications of Optical Receiver

1. Receiver sensitivity

Receiver sensitivity is defined as the minimum acceptable value of average

received power at point R to achieve a 1×1010 BER. Typical margins between

a beginning-of-life, nominal temperature receiver and its end-of-life, worst-case

counterpart is in the 2 to 4 dB range. The actual measured receiver sensitivity is

usually 3 dB (sensitivity floating value) greater than the specified minimum

value (worst-case value).

2. Receiver overload

Receiver overload is the maximum acceptable value of the received average

power at point R to achieve a 1×1010 BER. When the received optical power is

greater than the receiver sensitivity, the improvement of signal-to-noise ratio

reduces the BER; but with the continuous increase of received optical power,

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the receiver will enter the non-linear working area, thus causing the BER to get

worse, as shown in Fig. 7.2-3.

BER

光接收功率

1×10¯10

A B Received optical power

Fig. 7.2-3 BER graph

In the figure, the optical power at point A is the receiver sensitivity, the optical

power at point B is the receiver overload, the range between A and B is the

dynamic range where the receiver can work normally.

Summary

This chapter describes types and parameters of optical interfaces in SDH system.

Exercise

1. Where in the SDH equipment, optical interfaces are located?

2. What are optical interface types?

3. Decode the meaning of “S-4.2”.

4. What type of scrambled line code pattern is adopted by SDH?

5. What is receiver sensitivity?

6. What is mean by receiver overload?

7. What is SMSR?

8. What is the purpose of using optical line code patterns?

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9. What is the disadvantage of frequent optical to electrical conversions?

98

8 Transmission Performance

Key points

Concepts of bit error, jitter, and wander

Specifications of bit error and jitter

Specification of wander

8.1 Bit Error Characteristics

Bit error means that errors occur to certain bits of the data flow after signal receiving,

judgment, and regeneration; resulting in impairment of the transmitted information

quality.

8.1.1 Generation and Distribution of Bit Error

The influence of bit error on services is mainly determined by service type and bit error

distribution.

The ideal optical fiber transmission system has very stable transmission channels, and

is almost free from external electromagnetic interference.

1. Bit error generated internally

The bit errors generated inside the optical fiber transmission system include bit

errors caused by various noise sources; by alignment jitters; by multiplexers,

cross-connect equipment, and switches; and by inter-bits interference generated

by the optical fiber dispersion, which can be represented by long-term system

bit error performance.

2. Bit error caused by pulse interference

Bit error of this kind is generally caused by burst pulse such as electromagnetic

interference, equipment fault, and transient interference on the power supply. It

features burst and large quantity, and can be represented by short-term system

bit error performance.

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8.1.2 Measurement of Bit Error Performance

The bit error performance specified by ITU-T G.821 refers to the bit error performance

of the 64 kbit/s path in the digital reference circuit which is 27500 km in total and is

connected end to end. It is based on the bit error state. With the increase of

transmission rate, the bit error performance measurement system based on the unit of

bit is getting more limited.

Currently the bit error performance of path with high bit rate is measured based on the

unit of block (B1, B2, and B3 all monitors block error). This measurement generates a

group of parameters based on block, which are mainly used to monitor continuous

services.

Block refers to a sequence of bits related to the path.

The parameters are defined as follows.

1. Block error

It is the block in which bit error occurs during transmission.

2. Errored Second (ES) and Errored Second Ratio (ESR)

If one or more block errors are detected in one second, this second will be

considered as an Errored Second (ES). The ratio of the number of ESs to the

total available time in the stipulated test period is called Errored Second Ratio

(ESR).

3. Serious Errored Second (SES) and Serious Errored Second Ratio (SESR)

When no less than 30% block error or at least one defect is detected in one

second, this second will be considered as a Serious Errored Second (SES).

The ratio of the number of SESs to the total available time in the stipulated test

period is called Serious Errored Second Ratio (SESR).

SES is generally the burst block error caused by pulse interference. Therefore,

SESR can usually indicate the anti-interference capability of the equipment.

4. Background Block Error and Background Block Error Ratio (BBER)

Background Block Error (BBE) refers to the block error detected during the

period other than the unavailable time and SES period. The ratio of the number

of BBEs to the total number of blocks during the period other than the

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unavailable time and SES period is called BBER.

BBER obtained via long-term test can generally indicate the bit error status of

the equipment internally, which is usually related to the performance stability

of the component employed in the equipment.

5. Defects

When the abnormality occurrence density has caused finite interruption of the

ability to execute one function, it is considered that a defect occurs. The main

network defects include Loss of Signal (LOS), Loss of Frame (LOF), Loss of

Pointer (LOP), various levels of alarm indications, and Signal Label Mismatch

(SLM).

8.1.3 Bit Error Specifications Related to Digital Section

ITU-T uses hypothetical digital reference link with the total length of 27500 km to

make equivalent of digital link, and allocates maximum bit error performance

specification for each section in the link; so that when the bit errors of each section in

the main link compose one link without exceeding the specifications, the performance

can satisfy the performance requirements of the digital signal end-to-end transmission

(27500 km).

Table 8.1-1, Table 8.1-2, and Table 8.1-3 respectively lists the bit error performance

specifications for 420 km, 280 km, and 50 km.

Table 8.1-1 Bit error performance specifications of HRDS for 420 km

Rate (kbit/s) 155520 622080 2488320

ESR 9.24×10-4 To be determined To be determined

SESR 4.62×10-5 4.62×10-5 4.62×10-5

BBER 2.31×10-6 2.31×10-6 2.31×10-6


Rate (kbit/s) 155520 622080 2488320


SESR 3.08×10-5 3.08×10-5 3.08×10-5

BBER 1.54×10-6 1.54×10-6 1.54×10-6

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Rate (kbit/s) 155520 622080 2488320


SESR 5.5×10-6 5.5×10-6 5.5×10-6

BBER 2.75×10-7 2.75×10-7 2.75×10-7

8.1.4 Measures to Reduce Bit Error

· To reduce internal bit error

Currently the average bit error ratio of the regenerator section is under the order of

magnitude 10-14, and, thus, can be considered in the operating status of “no bit

error”. To improve signal-to-noise ratio is the main measure to reduce system

internal bit errors. Besides, selecting the appropriate extinction ratio for

transmitter, improving the balance characteristic of receiver, reducing alignment

jitters can all help to improve the system internal bit error performance.

· To reduce external bit error interference

The basic measure is to enhance the anti-EMI (Electro Magnetic Interference)

ability and ESD (Electro-Static Discharge) ability. For example, enhance the

grounding.

In addition, allocating enough redundancy when designing the system is a simple

and feasible measure.

8.2 Availability Parameters

· Unavailable time

If the digital signal of any transmission direction has a bit error ratio per second

worse than 10-3 for consecutive ten seconds, from the first second of the ten

seconds on, it is considered to enter the unavailable time.

· Available time

If the digital signal of any transmission direction has a bit error ratio per second

better than 10-3 for consecutive ten seconds, from the first second of the ten

seconds on, it is considered to enter the available time.

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· Availability

Availability refers to the percentage that the available time occupies in the total

time. Certain availability specifications need to be satisfied to ensure normal

working system, as shown in Table 8.2-1.

Table 8.2-1 Availability specifications of hypothetical digital section

Length (km) Availability Unavailability Unavailable Time/Year

420 99.977% 2.3×10-4 120 minutes

280 99.985% 1.5×10-4 78 minutes

50 99.99% 1×10-4 52 minutes

8.3 Jitter/Wander Performance

Jitter and wander are related to the system timing characteristics.

· Timing jitter (hereinafter referred to as jitter) refers to the short-term deviation

between the ideal instant and the specified instant (such as the optimum

sampling time) of the digital signal. The “short-term deviation” is the phase

change with the change frequency higher than 10Hz.

· Wander refers to the long-term deviation between the ideal instant and the

specified instant of the digital signal. The “long-term deviation” is the phase

change with the change frequency lower than 10Hz.

8.3.1 Generation Principles of Jitter/Wander

In the SDH network, there are the same jitter sources as the other transmission

networks, including various noise sources, unbalance of timing filter, regenerator

defects (such as inter-bits interference, threshold wander of amplitude limiter). In

addition, SDH network introduces new jitter mechanism:

1. Mapping jitter of the plesiochronous tributary

Since fixed stuffing bits and control stuffing bits are inserted when loading

tributary signals into VC, these bits needs to be removed when dropping the

tributary signals. At this time, these signals with interspaces will result in clock

gap, and will generate pulse buffing jitter which is the remained jitter.

2. Pointer justification jitter

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This kind of jitter is caused by negative/positive justification and de-justification

of pointers.

For the mapping jitter of the plesiochronous tributary, we can take measures to

reduce it to an acceptable level; while the jitter caused by pointer justification

(with the unit of byte, happens every three frames) has low frequency and large

amplitude, so it can not be filtered using ordinary measures.

Temperature change of environment is the general reason that causes wander of

SDH network. It can change the transmission characteristics of optical cable, and

result in signal wander and clock system wander.

Finally, the combination of pointer justification and network synchronization in

SDH NE also generates jitter and wander of very low frequency. However,

wanders of SDH network generally come from clocks of different levels and the

transmission system, especially the transmission system.

8.3.2 Jitter Performance Specifications

The major parameters to measure jitter performance in SDH network are listed as

follows.

· Input jitter tolerance

The input jitter tolerance includes jitter tolerances of PDH input interface

(tributary interface) and STM-N input interface (line interface).

The input jitter tolerance of the PDH input interface (tributary interface) is the

maximum input jitter value that the PDH input interface can endure without

causing bit error in the equipment. Since SDH and PDH networks coexist, in

transmission network there requirement is that PDH services can be added to SDH

NE. In order to satisfy this requirement, the tributary input interface of the SDH

NE must be able to tolerate the maximum jitter of PDH tributary signals, that is,

the jitter tolerance of this tributary interface can bear the jitter of the transmitted

PDH signal.

The input jitter tolerance of the STM-N input interface (line interface) is defined

as the sinusoidal peak-to-peak jitter value which can enable the optical equipment

to generate 1dB optical power penalty. This parameter specifies that the input jitter

tolerance of a certain level NE should be able to tolerate the output jitter tolerance

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generated by the upper-level NE when the SDH NEs are interconnected to

transmit STM-N signal.

· Output jitter tolerance

Similar to input jitter tolerance, output jitter tolerance includes tolerances of PDH

tributary interface and STM-N line interface. It is the maximum jitter of the output

interface when there is no jitter at the equipment input interface.

When dropping PDH service from SDH NE, the output jitter of the PDH tributary

interface should guarantee that the equipment receiving the PDH signal can

endure the output jitter. The output jitter of the STM-N line interface should

guarantee that the SDH network receiving the STM-N signal can endure the

output jitter.

· Mapping jitter and combined jitter

Pointer justification and mapping at the PDH/SDH network boundary will result

in special jitter that only exists in SDH. To specify this kind of jitter, mapping

jitter and combined jitter are employed together to describe it.

Mapping jitter refers to the maximum jitter of the output PDH tributary signal

from the PDH tributary interface of the SDH equipment when PDH signals with

different frequency offsets are inputted into the PDH tributary interface of the

SDH equipment and the STM-N signal has no pointer justification.

If the input at the SDH equipment line interface complies with the pointer testing

sequence signal specified in G.783, the combined jitter refers to the maximum

jitter of the output signal tested at the PDH tributary interface of the system when

the frequency offset of the input signal is properly changed after the pointer

justification occurs in the SDH equipment.

· Jitter transfer function – characteristic of jitter transfer

This function specifies the restriction capability (jitter gain) of the jitter of the

output STM-N signal on the jitter of the input STM-N signal, hence controlling

the jitter accumulation of the line system and preventing the rapid accumulation of

system jitter.

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Jitter transfer function is defined as the relation between the frequency and the

ratio of the STM-N output signal jitter to the STM-N input signal jitter, where the

frequency is the jitter frequency.

8.3.3 Measures to Reduce Jitter

1. To reduce jitter of line system

Jitter of the line system is the main jitter sources in SDH network. Taking

measures to reduce it is one of the critical factors to ensure the network

performance.

The basic measure to reduce jitter of line system is to reduce jitter (output jitter)

of one single regenerator, control characteristic of jitter transfer (improve the

restriction ability of output signal on input signal jitter), improve the jitter

accumulation method (adopt scrambler and jitter reducer to randomize

information transmitted and reduce the relevancy between system jitters generated

by regenerators, thus improving the jitter accumulation characteristic).

2. To reduce output jitter of PDH tributary interface

Since pointer justifications adopted by SDH may cause great phase jump (pointer

justification is in the unit of byte) accompanied by jitter and wander, the

desynchronizer is used at the tributary interface of SDH/PDH network boundary

to reduce jitter and wander.

Desynchronizer has the functions of buffering and phase smoothing. It is usually

implemented by the phase-locked loop with buffer. The important techniques

include self-adapting technique and bit leakage technique.

8.3.4 Notes

1. What is optical power penalty?

Jitter, wander, and optical fiber dispersion will reduce the signal-to-noise ratio and

thus increase bit errors. This can be compensated by increasing the optical power

of the transmitter. That is to say, jitter, wander, and dispersion degrade the system

performance to be worse than some certain specification; to make the system

performance reach the certain specification; we can increase the optical power of

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the transmitter. And the optical power increased is the optical power penalty

needed to satisfy the certain specification.

The optical power penalty of 1 dB is the maximum value that the system can

tolerate.

2. Hypothetical Reference Connection (HRX)

HRX is a hypothetical connection with specified structure, length, and

performance in telecommunications network. It can be used as a model for

network performance research, can be compared with the network performance

specifications, and thus export the specifications of every smaller entities.

The longest standard HRX consists of fourteen circuits connected serially, with

two terminating office having twelve sections of circuits all together. This is the

all-digital 64 kbps connection between two subscribers at the two ends of

communication, with the full length of 27500 km.

Summary

This chapter describes the bit errors and specifications of jitter and wander, which are

used to measure the transmission performance.

The key points of this chapter are system bit error measurement, meanings of the

commonly used parameters for jitter performance.

Exercises

1. What are the different error events defined by ITU for transmission networks?

2. How is “Availability” and “Unavailability” determined in a period?

3. What are the possible causes of bit errors?

4. What is Jitter?

5. What measures should be taken to reduce bit errors?

6. What do you understand by the term “Availability Parameters”?

7. Why it is necessary to reduce jitter?

8. What are the causes of jitter generation?

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9. What is Wander and how it can be minimized in a transmission line?

108

9 Test

Key points

Issues covered in SDH tests

9.1 SDH Test Method

SDH tests can be performed by dedicated test instruments, or by functions (via

overhead bytes) of SDH network management system. The differences between these

two testing methods are: the test using network management system mainly aims to the

SDH system maintenance; the tested items are not as comprehensive as the test using

dedicated test instruments; and it is generally performed by OAM personnel. The test

using dedicated test instruments covers various test items; it is mainly applied in

science research, manufacturing, installation and debugging, check and acceptance of

project.

9.2 SDH Tested Items

SDH test generally covers regular tested items owned by SDH only. The regular tested

items are similar to those of PDH, e.g. jitter test, wander test, and transfer characteristic

test. The tested items owned by SDH only can be classified into four categories:

1. Test of transmission ability: includes BER test, mapping/demapping test. It

aims to test the ability of SDH to transmit the payload.

2. Test of pointers: includes tests of timing offset and payload output jitter. It aims

to test the ability of SDH to accommodate asynchronous work.

3. Test of embedded overhead: includes tests of alarms and performance

monitoring function, protocol analysis. It aims to confirm the overhead

function.

4. Test of line interfaces: includes a series of tests for parameters of electrical

interface and optical interface. It aims to ensure the transverse compatibility of

optical path.

109

Summary

This chapter describes the testing methods and tested items of SDH.

Exercise

1. Why functional testing method is not as powerful as test equipment method?

2. What is the need of SDH test methods?

3. What type of test items can be achieved by using dedicated test instruments?

4. What are the four tested items which are owned only by SDH?

5. Which items are classified as the regular tested items?

110

10 Introduction to Network Management

Key points

Basic concepts of TMN

Basic concepts of SDH management network

Management ability of SDH

OSI model and ECC protocol stack

10.1 TMN Fundamentals

10.1.1 TMN Management Frame

To implement the integrated, unified and efficient management of telecommunications

network, ITU-T recommended the concept of Telecommunications Management

Network (TMN). The basic concept of TMN is to provide an organizational hierarchy

to realize the interworking between various operating systems (network management

systems) and the telecommunication equipment, and to use a universal hierarchy with

standard interfaces (including protocols and information specifications) to exchange

management information, thus realizing automatic and standard management of the

telecommunications network. In concept, TMN is network independent from the

telecommunications network and specializes in network management. It has some

various interfaces connected with the telecommunications network to receive

information from telecommunications network and control the operation of

telecommunications network. TMN often utilizes part of the facilities in the

telecommunications network to provide communication. Therefore, some parts of the

two networks may overlap. The relation between TMN and telecommunications

network is shown in Fig. 10.1-1.

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TMN

Operation system Operation system Operation system

Data communications network

Switch Transmission system Switch Switch

Telecommunications network

Transmission system

Workstation

Fig. 10.1-1 Relation between TMN and telecommunications network

10.1.2 Physical Structure of TMN

The physical structure of TMN mainly describes the physical entities and interfaces

inside the TMN. The simplified physical structure is shown in Fig. 10.1-2.

OS

DCN

MD

DCN

QA NE

Q3/F/XWS

TMN

Q3Q3

NEQA

Q3/F

Qx

Qx Qx

Fig. 10.1-2 Physical structure of TMN

OS in Fig. 10.1-2 is the operating system, i.e. the network management system that

executes the OSF. In fact, it is a large-scaled system program that manages the network

resources. MD is the coordinating equipment which executes MF and implements the

coordination between OS and NE. In addition, it also provides QAF and WSF, or even

OSF sometimes. MD can be realized via the hierarchical mode. QA is the Q adapter

which implements the adaptation and interconnection between the NE and non-TMN

interfaces.

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Data Communications Network (DCN) is the telecommunications network in the TMN

that supports DCF. It mainly provides the functions of the three lower layers of the OSI

reference model, but not the functions from layer 4 to layer 7. The DCN can be formed

by connecting the subnets of different types (such as X.25 or DCC).

NE consists of the telecommunication equipment (or part of it) which executes NEF

and the supporting equipment. It can contain other TMN function blocks, generally the

MF. An NE usually has one or more standard Q interfaces, and sometimes F interfaces.

The workstation (WS) is the system to perform WSF. It mainly translates information

at the f reference point to a displayable format at the g reference point, and vice versa.

10.1.3 TMN Interfaces

Standard TMN interfaces need to be specified in order to simplify the interconnections

between the equipment of different manufacturers. It is the key point for TMN. The

standard interfaces need to give a universal specification to the protocol stack and the

messages carried by the protocol.

10.1.3.1 Q Interface

Q interface generally corresponds to Qx interface. Qx interface connects MD with MD,

NE with MD, QA with MD, and NE with NE (at least one NE has MF function). In

traditional PDH system, the Qx interface usually only provides the functions at the

three lower layers of the OSI reference model. Therefore, it is suitable to connect

simple equipment such as multiplexer and line system. Either A1 or A2 protocol stack

specified in ITU-T Recommendation G.773 is applicable for the Qx interface, where

A1 is for the connection mode, and A2 is for the connectionless mode (LAN

technology). In SDH system, Qx interface generally contains the functions of all seven

layers. Its protocol stack can be the CONS1, CLNS1, or CLNS2 specified in ITU-T

Recommendations Q.811 and Q.812; where CONS1 is the interface of the X.25 packet

network, CLNS1 is the connectionless interface that employs LAN technology, and

CLNS2 is the connectionless interface that employs the interworking protocol on the

basis of the X.25 protocol.

10.1.3.2 F/G/X Interface

F interface corresponds to the f reference point. It can connect a remote workstation to

OS or MD via the DCN. G interface corresponds to the g reference point, and X

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interface corresponds to the x reference point. In general, X interface has higher

security requirement than Q interface.

10.1.4 TMN Layers Division

According to ITU-T Recommendation M.3010, the management layer model of the

TMN is divided into Network Element Layer (NEL), Element Management Layer

(EML), Network Management Layer (NML), Service Management Layer (SML) and

Business Management Layer (BML).

Fig. 10.1-3 displays the management layer division of the TMN with the highest layer,

the Service Management Layer. NE can be an SDH equipment, or any other

manageable equipment such as PDH equipment or switch.

Network Manager Layer (NML)

NMS

NE

NE

NE

NE

EMS EMS

NENE

NE

Element Management Layer (EML)

NMS

SMSService Management layer (SML)

Network Element Layer (NEL)

Fig. 10.1-3 TMN management layers

10.2 SDH Management Network (SMN)

10.2.1 SMN and TMN

SDH management network (SMN) is a subset of TMN that manages SDH NEs. SMN

can be further divided into series of SDH Management Subnets (SMS). These SMSs

consist of series of separate Embedded Control Channel (ECC) and intra-station data

communication links, and form an organic part of the whole TMN. The significant

characteristics of SMN are its intelligent network elements and embedded ECC. The

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combination of these two characteristics greatly reduces transmission time and

response time of TMN information. In addition, it can download the network

management function to the network element via the ECC, thus realizing distributed

management. The basic characteristic of SDH is its powerful and efficient network

management capability.

The relationships among TMN, SMN, and SMS are shown in Fig. 10.2-1.

TMN

SMN

SMS-1 SMS-2 SMS-n

Fig. 10.2-1 Relationships among SMS, SMN, and TMN

Unitrans ZXONM Network Management System (NMS) can be an SDH Management

Subnet (SMS), or an SDH Management Network (SMN). Its relationship with the

Telecommunications Management Network (TMN) is described below. As shown in

Fig. 10.2-1, TMN belongs to the most general management network category. SMN

consisting of multiple SMSs is a subset of TMN, and is responsible for managing SDH

NE. Because Unitrans ZXONM network management system is part of the TMN, it

can provide standard interfaces to accept management by the upper-layer network

management center.

The logic channel that transmits NMS messages in SDH system is ECC whose physical

channel is DCC. DCC employs bytes D1~D3 of SDH regenerator section overhead

(RSOH) and bytes D4~D12 of the multiplex section overhead (MSOH) to compose

channels of 192 kbit/s and 576 kbit/s, which are respectively called DCC (R) and DCC

(M). DCC (R) can access the regenerator (REG) and the terminal multiplexer (TM),

and DCC (M) is the express channel of the NMS information between the TMs.

10.2.2 SDH Management Interfaces

The major operation and running interfaces related to SDH management network are

the Qx and F interfaces. SMS communicates with the TMN via the Qx interface.

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10.3 SDH Management Functions

ITU-T specifies five major functions for network management system: Configuration

Management, Fault Management, Performance Management, Security Management,

and Accounting Management.

1. Configuration Management: To configure resources and services of the

transmission network. It includes configuration of network data, equipment

data, link channels, protection switching function, synchronous clock source

distribution strategy, orderwire equipment, line interface parameters, tributary

interfaces, and NE time; query, backup, and restoration of configuration

information; and query and statistics of path resources.

2. Fault Management: To detect, analyze and locate equipment faults. It includes

setting of alarm levels; real-time display of alarms; alarm settings of

confirmation, shielding, filtering, reversion, and sound; query of history alarms;

locating alarm; and alarm statistics and analysis.

3. Performance Management: To perform effective check and analysis of various

performances of the equipment. It includes settings of performance thresholds,

query of current and history performance data, performance data analysis.

4. Security Management: To provide security guarantee for equipment

maintenance. It includes setting of user levels, operation rights and

management areas; and management of user login and user operation log.

5. Accounting Management: To provide the basic information related to

accounting. The information includes time for circuit establishment, duration,

and quality of service (QoS).

Maintenance management is sometimes listed as an independent functional block. It

provides measures for normal equipment operation and locating fault including loop-

back control, alarm insertion, and bit error insertion.

10.4 OSI Model and ECC Protocol Stack

10.4.1 OSI Concept

The Open System Interconnection (OSI) hierarchical model is the standard computer

network functional structure specified by International Standardization Organization

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(ISO). OSI aims at enabling interconnection of different information process system. It

is a conceptual and functional structure, and does not involve detailed implementation

methods or technologies. However, it has profound and lasting effect on the new

communication field related with computer communication. Fig. 10.4-1 shows the OSI

model.

Application layer

Presentation layer

Session layer

Transport layer (TCP)

Network layer (IP)

Data link layer (ATM)

Physical layer

Fig. 10.4-1 OSI model

10.4.2 ECC Protocol Stack Description

Application layer CMISE,ROSE,ACSE

Presentation layer X.216, X.226

Session layer X.215, X.225

Transport layer ISO8073/AD2

Network layer ISO8473

Data link layer ITU-T Q.921

Physical layer SDH DCC

Fig. 10.4-2 ECC protocol stack

Summary

This chapter describes the hierarchical structure of SDH network management, the

compositions and protocols of SDH Management Network (SMN).

Exercises

1. What is TMN and why it came into being?

2. List all the parts which are included in the physical structure of TMN.

3. What is the role of Qx interface?

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4. What is the difference between Network Management Layer and Element

Management Layer?

5. What do you know about UNITRANS ZXONM network management system?

6. What are the five major functions of a network management system according to

ITU-T?

7. List all (seven) layers of ECC protocol stack.

8. What is covered by Security Management?

9. QoS is addressed in which type of SDH management?

10. What is the function of F interface?

11. What layers are included in the management layer model of TMN?

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Transmission I 03 200909 SDH Basics 126P

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