S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju) 1 Switching...

S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju) 1

Switching Technology• Circuit switching• Structure and interfaces of telephone exchanges• Call control • Maintenance functions and fault-tolerance• The principle of circuit switching• Packet switching and Challenges in switching

technology


ITU-T’s definition of switching:

The establishing, on-demand, of an individual connection from a desired inlet to a desired outlet within a set of inlets and outlets for as long as is required for the transfer of information.

An inlet represents a line, such as a channel (or a flow line,

entering an SDL/GR macro call).Source: (Blue book Fascicle I.3 Definitions).


By setting up connections on demand - networks are utilized efficiently

Subscriber/user/client- speech- data- video

Subscriber/user/client- speech- data- video

Without switching technology all subscriberswould need permanent connections to all others.


Subscriber lines and/or subscribers must have unique addresses in networks

• PSTN/ISDN/GSM - E.164 conformant phone numbers– fixed length or variable length numbers

• IP -addresses in the Internet– IPv4 addresses --> IPv6 - addresses

• In ATM - networks: NSAP - addresses


There are many communication modes

communication mode

connection-oriented Connection less

Circuit switching Packet switching Packet switching

PSTN ISDN

GSM

X.25Internet

ATM ?

Global addresses+ logical channel id’s on each hop

Global addresses


Connection set-up is needed in connection-oriented communication, so that the network can

change from global addresses to local ones

Set-up message [ B’s global address]

A -subscriber

B -subscriber

Response [ hop specific logical link/channel id]

Connection state

E.g. speech state Unique on a hop or linktime slot (PSTN…)- logical channel (x.25)- ...

Control messaging which enables setting up and tearing down connections is called signalingin telecommunications networks.


Connections can be set up also with network management commands

• Telephone networks are used to build up leased links• Frame relay is a switching technology for variable

length packets that replaces leased lines - e.g. linking LANs

• Also ATM can be used for establishing semi-permanent virtual connections.

Nodes of the network need not support signalingin this operation model.


Life cycles of product generations

1950 1960 199019801970 2000 20202010

SPC, analog fieldSPC, analog field

Digital exchangeDigital exchange

Broadband switch ?Broadband switch ?

Optic switching ?Optic switching ?

Coordinate exchange Coordinate exchange

Direct controlledDirect controlled

IP + Servers ?IP + Servers ?

?


Basic structure of a digital exchange

Subscriberinterface

Switching Matrix

Junction-line-

interface

Control system

Subscriber lines/or radio links junction lines


Directly connectedto an exchange - analog and - ISDN subscriber lines

SwitchingMatrix

subscribermodule

Local Exchange

Junc

tion

line

inte

rfac

e

Types of subscriber access in a local exchange

Access network

2/8/34MV2subscriber multiplexer

channel associated subscriber signaling

(1:1) V2inter-face

length of subscriberlines usually< 1km

2/8/34MV5.1message based subscriber signaling

(1:1) V5.1inter-face

message based subscriber signaling

V5.2 (concentration: N:1) V5.2inter-face

RSS proprietary signaling


Functions of the subscriber interface module in a fixed network

• power feed of subscriber lines

• call detection (on-hook/off-hook)

• receiving dialed digits

• A/D conversion

• traffic concentration

• subscriber line testing and line statusmanagement


The function of the switching matrix is to connect the incoming and outgoing time slots

incomingtime slots

Incoming lines Outgoing linesSwitching Matrix

Line = 2Mbit/s linkTime slot = 64kbit/s link

incoming pcm,incoming tsl

outgoing pcm,outgoing tsl


The switching matrix is based on time switches and space switches

• A switching matrix can be extended without disturbing existing connections.

• Extension step can be e.g. 64 PCMs.

• Matrixes can be single-stage or multistage.

• Due to advances in technology compared to single connection bandwidth, the trend is towards single-stage matrixes.


Time switch - serial writes - addressed reads

1 2 3 n 1 2 3 n123

n

Switchingmemory

Time slotcounter

123(x)

n

Controlmemory

rotatingwrite addresses

readaddresses

incoming frame buffer outgoing frame buffer

ClockClock

x


Space switch - an example

1 2 n1

2

m

Control memory

inputoutput

&

&

&


Most often, control systems in Exchanges are distributed

Control can be

Centralized

Non-hierarchicalHierarchical

Single-processor

Multi-processor

Single-processor

Multi-processor

DistributedDistributed

Some functions centralized

Allfunctions distributed

Pragmaticdistribution


An example of pragmatic distribution is the Finnish DX200 -exchange

Signaling system specificcontrol processor

Signaling system specificcontrol processor

Databases:subscriber- and routing

information

Databases:subscriber- and routing

information

Statistics andcharging

Statistics andcharging

Matrixcontrol unit

Matrixcontrol unit

operation andmaintenance

Division of loadby connections/linesand the signaling system type.

Grouping and division by functions, replicated functions if necessary.

Centralizedfunction

Internal bus (or ATM switch)


Service switching(SSF)

Main functions of the control system are call control and charging

incoming signaling outgoing signaling

Outgoing callcontrol

Outgoing callcontrol

Incoming call control

Incoming call control

Number analysis gathering charging information

gathering charging information

Resource management


Principles of Functional distribution in DX200 systems

• The load is partitioned by dividing the incoming and outgoing line-space either statically or dynamically– e.g. replaceable 2N or N+ 1 -replication concerning signaling

and call control functions

• Partitioning the load dynamically for a task at a time– N+1 load partitioning

• Replicating stateless services to several processors


Exchanges are fault tolerant

Equipmentand

softwareSupervision

Alarmhandling

Faultlocation

Recovery faulteffect

elimination

• Maintenance software manages the states of devices and programs in the exchange

• The critical equipment has been doubled (2N replication)

• Switchovers from active to spare without disturbing calls

• Fault tolerance must be taken into account always in allsoftware development

• Unavailability requirement < 2…3 min/year

Active Spare

TestingSeparated

(out-of-use)


80% of exchange design is software development

• The size of a software load is between 3 … 10 million code lines.

• A country adaptation for PSTN typically demands a work effort of ca.50 man-years.

• Challenges in software design are generated by:– real-time call control

– variety and nation specificity in signaling, services and features

– fault tolerance

– maintainability and extensibility of the software and the system


The junction line interface of an exchange

• Main functions are: – adaptation of internal and external transmission– supervision of the condition of junction lines

• E.g. external format is 2M PCM, internal 4Mbit/s TTL-logic format (in DX200)

• direct SDH interfaces (e.g. 155Mbit/s ) coming to digital exchanges (available in some?)


Circuit switched networks are utilized also in packet transmission

PSTN/ISDN GSM

POP

Mod

empo

le

Internet

• long connection set-up time

• inefficient use of resources (the call is on whether or not any packetsare transmitted)

• Restricted speed (33,6 kbit/s …)

• Internet calls are longer than normalcalls. Due to this fact, dimensioning ofexchanges is not the best possible.

• Because of increasing traffic switchingmatrixes and routes must be enlarged. POP - Point of presence of an Internet Service

Provider


ATM-switch

virtual connections

virtual paths

incoming ATM linesOutgoing ATM lines

Incoming line,

Incomingpath,

Incomingconnection

Outgoingline,

Outgoingpath,

Outgoingconnection

ATM cell

VC

IV

PI

ATM switching matrix


Structure of a Packet Switch

Incomingbuffer

Outgoingbuffer

Outgoingbuffer

Outgoingbuffer

Outgoingbuffer

Incoming lines Outgoing lines

Controller

A problem: Constructing fast enough controllers is difficult!An example: IP-routers.


Combined router+ATM-switch

Incomingports

Outgoingport

Outgoingport

Outgoingport

Outgoingport

incoming lines Outgoing lines

ATM-switching

matrix

Controller

• The controller includes functions of routers and ATM-switch controllers.• Packet Forwarding functions are in the Incoming ports -block.


The latest development of switching technology

• Integration of router and ATM-switch.Avoiding Layer-3 per-packet forwarding operations. – loose connection-orientation (flows) in routers

– setting up flows adaptively based on traffic = without an explicit set-up request

– topology-based label connections

• Gigabit and Terabit routers (Hw packet switching)• IP voice ==> All Services IP networks(?)


Challenges of Switching Technology

• Packet switching– IP calls and their interworking and Interoperability

with PSTN– Quality of Service in packet networks (IntServ,

DiffServ, Multi Protocol Label Switching)

• Third generation mobile telecommunications systems

• More open software development environment

S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju) 1 Switching...

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