Chapter 14

Other

Wired

Networks

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Chapter 14: Outline

14.1 TELEPHONE NETWORKS

14.2 CABLE NETWORKS

14.3 SONET

14.4 ATM

Chapter 14: Objective

The first section discusses the telephone network. It

describes the telephone network as a voice network. It then

shows how the voice network has been used for data

transmission either as a dial-up service or DSL service.

The second section discusses the cable network. It first

briefly describes it as a video network. The section then

shows how the video network has been used for data

transmission.

The third section discusses SONET, both as a fiber-optic

technology and a network. The section shows how the

technology can be used for high-speed connection to carry

data.

Chapter 14: Objective (continued)

The fourth section discusses ATM, which can use SONET

as the carrier to create a high-speed wide area network

(WAN). ATM is a cell-relay network that uses a fixed-size

frame (cell) as the unit of transmitted data.

14.5

14-1 TELEPHONE NETWORK

The telephone network had its beginnings in the

late 1800s. The entire network was originally an

analog system using analog signals to transmit

voice. With the advent of the computer era, the

network, in the 1980s, began to carry data in

addition to voice. During the last decade, the

telephone network has undergone many technical

changes. The network is now digital as well as

analog.

14.6

14.1.1 Major Components

• The telephone network, as shown in Figure

14.1, is made of three major components:

local loops, trunks, and switching offices.

• The telephone network has several levels of

switching offices such as end offices, tandem

offices, and regional offices.

Figure 14.1

14.7

14.1.2 LATAs

• After the divestiture of 1984 (see Appendix H), the

United States was divided into more than 200 local-

access transport areas (LATAs).

• The number of LATAs has increased since then.

• A LATA can be a small or large metropolitan area.

• A small state may have a single LATA; a large state

may have several LATAs.

• A LATA boundary may overlap the boundary of a

state; part of a LATA can be in one state, part in

another state.

14.8

Figure 14. 2: Switching offices in a LATA

14.9

Figure 14. 3: Points of presence (POPs)

14.10

14.1.3 Signaling

• The telephone network, at its beginning, used a

circuit-switched network with dedicated links to

transfer voice communication.

• The operator connected the two parties by using a

wire with two plugs inserted into the corresponding

two jacks.

• Later, the signaling system became automatic.

• Rotary telephones were invented that sent a digital

signal defining each digit in a multi-digit telephone

number.

• As telephone networks evolved into a complex

network, the functionality of the signaling system

increased. The signaling system was required to

perform other tasks.

14.11

Figure 14. 4: Data transfer and signaling network

14.12

Figure 14. 5: Layers in SS7

14.13

14.1.4 Services

Telephone companies provide two types of

services: analog and digital.

• Analog services

• Analog switched service: dial-up, 800

service, 900 service

• Analog leased service: dedicated line

• Digital services

• Switched/56 service: switched, 56 kbps

• Digital data service: leased, 64 kbps

14.14

14.1.5 Dial-Up Service

Traditional telephone lines can carry frequencies

between 300 and 3300 Hz, giving them a

bandwidth of 3000 Hz.

A dial-up service uses a modem to send data

through telephone lines.

The term modem is a composite word that refers

to the two functional entities that make up the

device: a signal modulator and a signal

demodulator.

14.15

Figure 14. 6: Telephone line bandwidth

Audio frequency shift keying:

originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz;

answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz.

Phase-shift keying:

two tones for any one side of the connection are sent at similar frequencies,

but slightly out of phase.

14.16

Figure 14. 7: Modulation/demodulation

14.17

Figure 14.8: Dial-up network to provide Internet access

14.18

14.1.6 Digital Subscriber Line (DSL)

• After traditional modems reached their peak

data rate, telephone companies developed

another technology, DSL, to provide higher-

speed access to the Internet.

• Digital subscriber line (DSL) technology is

one of the most promising for supporting high-

speed digital communication over the existing

telephone.

• DSL technology is a set of technologies, each

differing in the first letter (ADSL, VDSL,

HDSL, and SDSL).

14.19

Figure 14.9: ADSL point-to-point network

14.20

14-2 CABLE NETWORK

The cable networks became popular with

people who just wanted a better signal. In

addition, cable networks enabled access to

remote broadcasting stations via microwave

connections. Cable TV also found a good

market in Internet access provision, using

some of the channels originally designed

for video.

14.21

14.2.1 Traditional Cable Networks

• Cable TV started to distribute broadcast video

signals to locations with poor or no reception

in the late 1940s.

• It was called community antenna television

(CATV) because an antenna at the top of a

tall hill or building received the signals from

the TV stations and distributed them, via

coaxial cables, to the community.

• Figure 14.10 shows a schematic diagram of a

traditional cable TV network.

14.22

Figure 14.10: Traditional cable TV network

14.23

14.24

14.2.2 HFC Network

• The second generation of cable network is

called a hybrid fiber-coaxial (HFC) network.

• The network uses a combination of fiber-

optic and coaxial cable.

• The transmission medium from the cable TV

office to a box, called the fiber node, is optical

fiber; from the fiber node through the

neighborhood and into the house is still

coaxial cable.

• Figure 14.11 shows a schematic diagram of

an HFC network.

14.25

Figure 14.11: Hybrid Fiber-Coaxial (HFC) Network

14.26

14.27

14.2.3 Cable TV for Data Transfer

• Cable companies are now competing with telephone

companies for the residential customer who wants

high-speed data transfer.

• DSL technology provides high-data-rate connections

for residential subscribers over the local loop.

• However, DSL uses the existing unshielded twisted-

pair cable, which is very susceptible to interference.

This imposes an upper limit on the data rate.

• A solution is the use of the cable TV network. In this

section, we briefly discuss this technology.

14.28

Figure 14.12: Division of coaxial cable band by CATV

14.29

Figure 14.13: Cable modem transmission system (CMTS)

14.30

14-3 SONET

In this section, we introduce a wide area

network (WAN), SONET, that is used as a

transport network to carry loads from other

WANs. We first discuss SONET as a

protocol, and we then show how SONET

networks can be constructed from the

standards defined in the protocol.

14.31

14.3.1 Architecture

Let us first introduce the architecture of a

SONET system: signals, devices, and

connections.

Table 14.1: SONET rates

14.32

Signals

ITU-T SDHANSI SONET

14.33

Figure 14.14: A simple network using SONET equipment

Devices

Connections

14.34

14.3.2 SONET Layers

The SONET standard includes four functional

layers: the photonic, the section, the line, and the

path layer. They correspond to both the physical

and the data-link layers (see Figure 14.15). The

headers added to the frame at the various layers

are discussed later in this chapter.

14.35

Figure 14.15: SONET layers compared with OSI or the Internet layers

14.36

14.3.3 SONET Frames

Each synchronous transfer signal STS-n is

composed of 8000 frames. Each frame is a two-

dimensional matrix of bytes with 9 rows by 90 ×

n columns. For example, an STS-1 frame is 9

rows by 90 columns (810 bytes), and an STS-3

is 9 rows by 270 columns (2430 bytes). Figure

14.17 shows the general format of an STS-1 and

an STS-n.

14.37

Figure 14.16: Device-Layer relationship in SONET

14.38

Figure 14.17: An STS-1 and an STS-n frame

14.39

Figure 14.18: STS-1 frames in transition

Find the data rate of an STS-1 signal.

Solution

STS-1, like other STS signals, sends 8000 frames per

second. Each STS-1 frame is made of 9 by (1 × 90) bytes.

Each byte is made of 8 bits. The data rate is

Example 14.1

14.40

Find the data rate of an STS-3 signal.

Solution

STS-3, like other STS signals, sends 8000 frames per

second. Each STS-3 frame is made of 9 by (3 × 90) bytes.

Each byte is made of 8 bits. The data rate is

Example 14.2

14.41

What is the duration of an STS-1 frame? STS-3 frame?

STS-n frame?

Solution

In SONET, 8000 frames are sent per second. This means

that the duration of an STS-1, STS-3, or STS-n frame is the

same and equal to 1/8000 s, or 125 μs.

Example 14.3

14.42

14.43

Figure 14.19: STS-1 frame overheads

14.44

Figure 14.20: STS-1 frame: section overheads

14.45

Figure 14.21: STS-1 frame: line overhead

14.46

Figure 14.22: STS-1 frame path overhead

Table 14.2: SONETs overhead

14.47

What is the user data rate of an STS-1 frame (without

considering the overheads)?

Solution

The user data part of an STS-1 frame is made of 9 rows and

86 columns. So we have

Example 14.4

14.48

14.49

Figure 14.23: Offsetting of SPE related to frame boundary

14.50

Figure 14.24: The use of H1 and H2 to show the start of SPE

What are the values of H1 and H2 if an SPE starts at byte

number 650?

Solution

The number 650 can be expressed in four hexadecimal digits

as 0x028A. This means the value of H1 is 0x02 and the

value of H2 is 0x8A.

Example 14.5

14.51

14.52

14.3.4 STS Multiplexing

In SONET, frames of lower rate can be

synchronously time-division multiplexed into a

higher-rate frame. For example, three STS-1

signals (channels) can be combined into one

STS-3 signal (channel), four STS-3s can be

multiplexed into one STS-12, and so on, as

shown in Figure 14.25.

14.53

Figure 14.25: STS multiplexing/demultiplexing

14.54

Figure 14.26: Byte interleaving

14.55

Figure 14.27: An STS-3 frame

14.56

Figure 14.28: A concatenated STS-3c signal

14.57

Figure 14.29: Dropping and adding frames in an add/drop multiplexer

14.58

14.3.5 SONET Networks

Using SONET equipment, we can create a

SONET network that can be used as a high-

speed backbone carrying loads from other

networks such as ATM (Section 14.4) or IP

(Chapter 19). We can roughly divide SONET

networks into three categories: linear, ring, and

mesh networks, as shown in Figure 14.30.

14.59

Figure 14.30: Taxonomy of SONET networks

14.60

Figure 14.31: A point-to-point SONET network

14.61

Figure 14.32: A linear SONET network

14.62

Figure 14.33: Automatic protection switching in linear networks

14.63

Figure 14.34: A unidirectional path switching ring

14.64

Figure 14.35: A bidirectional switching ring

14.65

Figure 14. 36: A combination of rings

14.66

Figure 14.37: A mesh SONET network

14.67

14.3.6 Virtual Tributaries

SONET is designed to carry broadband

payloads. Current digital hierarchy data rates

(DS-1 to DS-3), however, are lower than STS-1.

To make SONET backward-compatible with the

current hierarchy, its frame design includes a

system of virtual tributaries (VTs) (see Figure

14.38). A virtual tributary is a partial payload that

can be inserted into an STS-1 and combined

with other partial payloads to fill out the frame.

Instead of using all 86 payload columns of an

STS-1 frame for data from one source, we can

subdivide the SPE and call each component a

VT.

14.68

Figure 14.38: Virtual tributaries

14.69

Figure 14.39: Virtual tributary types

14.70

14-4 ATM

• Asynchronous Transfer Mode (ATM) is a

switched wide area network based on

the cell relay protocol designed by the

ATM forum and adopted by the ITU-T.

• The combination of ATM and SONET will

allow high-speed interconnection of all

the world’s networks.

• In fact, ATM can be thought of as the

“highway” of the information

superhighway.

14.71

14.4.1 Design Goals

Among the challenges faced by the designers of

ATM, six stand out.

1. The need for a transmission system to optimize

the use of high-data-rate.

2. The system must interface with existing

systems.

3. The design must be implemented inexpensively.

4. The new system must be able to work with and

support the existing hierarchies

5. The new system must be connection-oriented.

6. Last but not least, one objective is to move as

many of the functions to hardware as possible.

14.72

14.4.2 Problems

Before we discuss the solutions to these design

requirements, it is useful to examine some of the

problems associated with existing systems.

• Different protocols use frames of varying size

• As networks become more complex,

information that must be carried in the header

becomes more extensive

• Larger header

• Enlarge date unit size: often a waste

• Variable frame size: hard to manage

14.73

Figure 14.40: Multiplexing using different frame size

14.74

Figure 14.41: Multiplexing using cells

14.75

Figure 14.42: ATM multiplexing

14.76

14.4.3 Architecture

ATM is a cell-switched network. The user access

devices, called the endpoints, are connected

through a user-to-network interface (UNI) to the

switches inside the network. The switches are

connected through network-to-network interfaces

(NNIs). Figure 14.43 shows an example of an

ATM network.

14.77

Figure 14.43: Architecture of an ATM network

14.78

Figure 14.44: TP, VPs, and VCs

TP: Transmission Path: physical connection between an endpoint and a switch or between two switchesVP: Virtual PathVC: Virtual Circuit

14.79

Figure 14.45: Virtual connection identifiers in UNIs and NNIs

14.80

Figure 14.46: An ATM cell

14.81

Figure 14.47: Routing with a switch

14.82

Figure 14.48: ATM layers

14.83

Figure 14.49: AAL5