1

9. Circuit Switching9. Circuit Switching

2Prof. Sang-Jo Yoo

ContentsContents

Switching Networks Circuit-Switching Networks Switching Concepts Routing in Circuit-Switching Networks Control Signaling

3Prof. Sang-Jo Yoo

Switching NetworksSwitching Networks

Long distance transmission is typically done over a network of switched nodes

Nodes not concerned with content of data End devices are stations

Computer, terminal, phone, etc.

A collection of nodes and connections is a communications network

Data routed by being switched from node to node

4Prof. Sang-Jo Yoo

NodesNodes

Nodes may connect to other nodes only, or to stations and other nodes

Node to node links usually multiplexed Network is usually partially connected

Some redundant connections are desirable for reliability Two different switching technologies

Circuit switching Packet switching

5Prof. Sang-Jo Yoo

Simple Switched NetworkSimple Switched Network

6Prof. Sang-Jo Yoo

Circuit SwitchingCircuit Switching

Dedicated communication path between two stations Three phases

Establish Transfer Disconnect

Must have switching capacity and channel capacity to establish connection

Must have intelligence to work out routing

7Prof. Sang-Jo Yoo

Circuit Switching - ApplicationsCircuit Switching - Applications

Inefficient Channel capacity dedicated for duration of connection If no data, capacity wasted

Set up (connection) takes time Once connected, transfer is transparent Developed for voice traffic (phone)

8Prof. Sang-Jo Yoo

Circuit-Switching NetworksCircuit-Switching Networks

Public telecommunications network Subscribers

Devices that attach to the network

Local loop Link between the subscriber and the network Also called subscriber loop

Exchanges Switching centers in the network End office, Long-distance office

Trunk Branches between exchanges Using FDM or synchronous TDM

9Prof. Sang-Jo Yoo

Public Circuit Switched NetworkPublic Circuit Switched Network

10Prof. Sang-Jo Yoo

Circuit-Switching NetworksCircuit-Switching Networks

11Prof. Sang-Jo Yoo

Switching ConceptsSwitching Concepts

Single circuit-switched node A collection of stations attached to a

central switching node Digital switch

Establishes a dedicated path between any two devices

Network-interface Control unit

Establishes, maintain, and tear down the connection

12Prof. Sang-Jo Yoo

Switching ConceptsSwitching Concepts

Blocking A network is unable to connect stations because all paths are in

use A blocking network allows this Used on voice systems

Short duration calls

Non-blocking Permits all stations to connect (in pairs) at once Used for some data connections

13Prof. Sang-Jo Yoo

Switching ConceptsSwitching Concepts

Space-division Switching Signal paths between pairs of devices are divided in space

Metallic crosspoint or semiconductor gate.

Crossbar switch High cost : O(n2) Inefficiency Non-blocking

14Prof. Sang-Jo Yoo

Switching ConceptsSwitching Concepts

Multistage network

Reduced numbers of crosspoints

Blocking example

15Prof. Sang-Jo Yoo

Switching ConceptsSwitching Concepts

Time-division Switching Partition low speed bit stream

into pieces that share higher speed stream

e.g. TDM bus switching based on synchronous time

division multiplexing Each station connects through

controlled gates to high speed bus Time slot allows small amount of

data onto bus (input) Another line’s gate is enabled for

output at the same time (output)

16Prof. Sang-Jo Yoo

Switching ConceptsSwitching Concepts

Control of a TDM bus switch

17Prof. Sang-Jo Yoo

Routing in Circuit-Switched NetworksRouting in Circuit-Switched Networks

Reasons for routing in circuit switching connections require a path through more than one switch

Requirements of network architecture for routing efficiency, resilience

Efficiency busy-hour traffic load : the average load expected over the course

of the busiest hour of use during the course of a day. should be able to handle busy-hour traffic load with minimum

equipment

Resilience should be able to control the network in the case of traffic

congestion and failures in switch and trunk.

18Prof. Sang-Jo Yoo

Routing in Circuit-Switched Network Routing in Circuit-Switched Network

Traditional routing technique static approach- quite simple organized into a tree structures, or hierarchy

cannot adapt to changing conditions

Dynamic approach is needed alternate routing adaptive routing

Calling subscriberCalled subscriber

19Prof. Sang-Jo Yoo

Alternate RoutingAlternate Routing

Possible routes are predefined routing decision is based on current traffic status and

historical traffic patterns

X Y

I J

K

End Office

IntermediateSwitching node

Route A

Route B

Route C

Route D

20Prof. Sang-Jo Yoo

Control SignalingControl Signaling

Network management, call establishment, call maintenance, call termination

signaling functions audible communication : dial tone, ringing tone, busy transmission of the dial number transmission of information

call connection, call ending, call disconnection billing purpose status of equipment and trunk diagnosing and isolating system failures

supervisory,address,call-information, network management

21Prof. Sang-Jo Yoo

Categories of Control SignalingsCategories of Control Signalings Supervisory

binary character(true/false, on/off) request for service, answer, alerting, return to idle deal with availability of the called subscriber and of the needed

network resources

Address identify a subscriber phone number

call-information provides information to the subscriber about the status of a call audible tones

Network management maintenance, trouble-shooting

22Prof. Sang-Jo Yoo

Control Signaling : exampleControl Signaling : exampleon hook on hook

hang up

switch

dial tone

called address

ringing signalringing tone

off hook

ringing signaltermination

ringing tonetermination

connection establishment

connection released

hang up

off hook

23Prof. Sang-Jo Yoo

Inchannel SignalingInchannel Signaling

The same channel is used to carry control signals no additional transmission facilities are needed Inband signaling

uses the same frequency band as voice signals that are carried no limit on the use of inband signaling anywhere in the network channel for control signal is only available when no voice signals

Out-of-band signaling not full use of 4k-Hz bandwidth by the voice signal a separate narrow signaling band within the 4k-Hz is used for cont

rol signaling very narrow bandwidth -> slower signaling rate

24Prof. Sang-Jo Yoo

Inchannel SignalingInchannel Signaling

Office A Office B

SIG

SIG

SIG

SIG

SIG

SIG

Trunks

Inchannel signaling

Office A Office B

Trunks

CC-SIG CC-SIG

Processor Processor

Common Channel signaling

25Prof. Sang-Jo Yoo

Common Channel Signaling Common Channel Signaling

Control signals’ path is independent of voice signal call-setup time is reduced

signaling protocols and network control are more complex associate mode

common channel closely tracks along its entire length the interswitch trunk groups

switching point

signaling link

speech

26Prof. Sang-Jo Yoo

Common Channel SignalingCommon Channel Signaling

Non associate mode network is augmented by additional nodes(signal transfer points) control requests can be processed with a more global view of the

network conditions

switching point(speech)

signaling network

speech network

switching point(signal transfer point)

27Prof. Sang-Jo Yoo

Signaling System Number 7Signaling System Number 7

SS7 Common channel signaling scheme ISDN Optimized for 64k digital channel network Call control, remote control, management and maintenance Reliable means of transfer of info in sequence Will operate over analog and below 64k Point to point terrestrial and satellite links