Data and Computer CommunicationsPart 3 - Wide area Networks - Concepts

Chapter 9Circuit Switching

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Switching NetworksLong distance transmission is typically

done over a network of switched nodesNodes are not concerned with content of

dataEnd devices are referred to as stations

Computer, terminal, phone, etc.

A collection of nodes and connections is a communications network

Data is routed by being switched from node to node

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Switching NetworksCommunication Network

Switching Network (used in WAN) Broadcast Network (used in LAN)

Switching Networks Circuit-switching networks Packet-switching networks

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NodesNodes may connect to other nodes only,

or to stations and other nodesNode to node links are usually multiplexedA network is usually partially connected

But some redundant connections are desirable for reliability

Two different switching technologies exist: Circuit switching Packet switching

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Simple Switched Network

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Simple switching network

Communication Network

Communication Network Node

Network Station

end node (source & destination)

provides switching facility (routing)

Communication is achieved by transmittingdata from source to destination through anetwork of switching nodes

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Switching technology

Circuit switching need a connection established between end nodes connection is maintained until one of end nodes terminates Connection is dedicated to the communication between two nodes Example : Public Switch Telephone Network (PSTN)

Packet switching data are transmitted in short messages called packets a connection between the two end-nodes is not maintained a node-to-node link can be dynamically shared by many packets Example : Public Data Network (PDN) like X.25, Frame Relay

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Key differences

Keys Circuit switching Packet switching

data dedicated single route different routes

node status both must be ready sender ready to send is enough

connection dedicated shared

utilization poor good

data rate fixed varies

Prioritization not supported supported

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Public Switching Telephone Network

end office

Long-distance office

connecting trunk

subscriber loop

Intercity trunk subscriber loop (local loop) link between subscriber and network

connecting trunk (exchanges) switching center; localized support for subscriber

Intercity trunk (trunks) branches between exchanges; carry multiple voice circuits

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PSDN

PSDN= Public Switched Data Network

LAN

LAN

LAN

PSDN

• PSDN is designed specifically for the transmission of data rather than voice• Communication is shared

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Packets

header data

packet

Messages are broken up into a series of packets

Header is used to route the packet through the network

header data header data

data

packet packet

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PSDN switching approach

Virtual Circuit network protocols establish a logical route called virtual circuit packets use the same route; data arrive in order similar to circuit switching ! but the route is not dedicated need call setup

Datagram each packet is transmitted independently network protocols route each packet as though it were a separate

message packet may not arrive in order; need protocol to ensure ordering call setup is not required

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Virtual circuit

1.3 1.2 1.1

2.3 2.2 2.1

2.32.2 2.1

1.31.2

1.1

A

B

C

A 1

2

3

4

5

B

C

virtual circuit #1

virtual circuit #2

A logical connection known as virtual circuit (VC) is set up between two stations.

Packets are labeled with a virtual circuit number and a sequence number

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Datagram

B.3 B.2 B.1

C.3 C.2 C.1

C.3 C.2 C.1

B.3B.2

B.1

A

B

C

A 1

2

3

4

5

B

C

B.3 B.2

B.1

C.3 C.2

C.1

Each packet is transmitted independently. Packets are labeled with a destination address and may arrive out of sequence

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Routing methods in packet switching

Fixed routing each packet is transmitted independently

Flooding network protocol establishes a logical route called virtual circuit packets use the same route; data arrive in order

Random routing the route taken is random

Adaptive routing the route reacts to changing conditions within the network

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Circuit-Switching Networks During communication, a dedicated communication

path exists between sender and receiver; e.g., telephone system.

Communication involves 3 phases: Circuit establishment: “Call request” propagates, hop

by hop through the network, to establish a dedicated link (channels in each component link in some path, from sender to receiver)

Data transfer: continuous transfer; either analog or digital signal

Circuit disconnect: “circuit disconnect” signal to deallocate the links

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Circuit-Switching

Definition: Communication in which a dedicated communications path is established between two devices through one or more intermediate switching nodes

Dominant in both voice and data communications today e.g. PSTN is a circuit-switched network

Relatively inefficient (100% dedication even without 100% utilization)

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Circuit SwitchingDedicated communication path between

two stationsThree phases

Establish Transfer Disconnect

Must have switching capacity and channel capacity to establish connection

Must have intelligence to work out routing

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Circuit-Switching StagesCircuit establishmentTransfer of information

point-to-point from endpoints to node internal switching/multiplexing among nodes

Circuit disconnect

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Circuit-Switching NetworksDisadvantages:

Both stations need to be simultaneously ready Inefficient use of link capacities, especially

with bursty intermittent traffic

Advantages Low delay once circuit is established

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Circuit-Switching Networks Public telephone network

Best-known example of a circuit-switching network Four generic architectural components

Subscribers - Devices that attach to the networkLocal loop - The link between the subscriber and

the network (end office). Also called subscriber loop

Exchanges - Switching centers in the network. End offices, long-distance offices, …

Trunks - Links between exchanges. Carry multiple voice channels by using FDM or synchronous TDM

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Circuit Switching - ApplicationsInefficient

Channel capacity dedicated for duration of connection

If there is no data, capacity wasted

Set up (connection) takes timeOnce connected, transfer is transparentDeveloped for voice traffic (phone)

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Public Circuit Switched Network

Connecting Trunk

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Circuit-Switching Networks

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AT&T telephone hierarchy

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Switching ConceptsSingle circuit-switched node network

A collection of stations attached to a central switching unit

The central switch establishes a dedicated path between any two devices that wish to communicate

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Switching Concepts

Digital switch Establishes a dedicated

path between any two devices

Control unit Establishes, maintains,

and tears down the connection

Network interface (circuit) switch

Blocking Nonblocking

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Telecommunication ComponentsSubscriber

Devices attached to network

Local Loop Subscriber loop Connection to network

Exchange Switching centers End office - supports subscribers

Trunks Branches between exchanges Multiplexed

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Circuit Switching Node's Elements

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Circuit Switching Node's Elements

Digital Switch Provides transparent signal path between devices

Network Interface (functions & hardware needed to connect devices to the network)

Control Unit; its function is to: Establish connections

Generally on demandHandle and acknowledge requestsDetermine if destination is freeconstruct path

Maintain connection Disconnect

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Circuit Switching Node:Digital SwitchProvides transparent signal path between

any pair of attached devicesTypically full-duplex

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Circuit-Switching Node:Network InterfaceProvides hardware and functions to

connect digital devices to switchAnalog devices can be connected if

interface includes CODEC functionsTypically full-duplex

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Circuit-Switching Node:Control UnitEstablishes on-demand connectionsMaintains connection while neededBreaks down connection on completion

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Blocking or Non-blocking An important characteristic of a circuit-switching

device is whether it is blocking or non-blocking.

Blocking A network is unable to connect stations

because all paths are in use Used on voice systems

example: Short duration calls

Non-blocking Permits all stations to connect (in pairs) at

once Used for some data connections

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Blocking/Nonblocking NetworksBlocking: network is unable to connect

two stations because all possible paths are already in use

Nonblocking: permits all possible connection requests because any two stations can be connected

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There are 2 Types of Switching Techniques Internal to a Single Circuit-Switching Node:

1. Space-Division Switching

2. Time-Division Switching

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Switching Techniques Space-Division

Switching Developed for analog

environment, but has been carried over into digital communication

Requires separate physical paths for each signal connection

Uses metallic or semiconductor “gates”

Time-Division Switching Used in digital

transmission Utilizes multiplexing to

place all signals onto a common transmission path

Bus must have higher data rate than individual I/O lines

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Space Division SwitchDeveloped for the analog environment (but

is now used for both analog & digital signals)Separate physical paths for every channel

(signal path) Its basic device is the Crossbar switch

Number of crosspoints grows as square of number of stations

Loss of crosspoint prevents connection Inefficient use of crosspoints

All stations connected, only a few crosspoints in use

Non-blocking

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Crossbar Matrix (switch)

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Multistage SwitchReduced number of crosspointsMore than one path through network

Results in increased reliability

More complex controlMay be blocking

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Three Stage Switch

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Time Division SwitchingPartition low speed bit stream into pieces

that share higher speed streame.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 Another line’s gate is enabled for output at the

same time

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Routing in Circuit-Switched NetworksTraditional circuit-switched model is

hierarchical, sometimes supplemented with peer-to-peer trunks

Newer circuit-switched networks are dynamically routed: all nodes are peer-to-peer, making routing more complex

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Routing in Circuit-Switching NetworksMany connections will need paths through

more than one switchTherefore, the network needs to find an

appropriate route. Important factors are: Efficiency Resilience

Public telephone switches are a tree structure Static routing uses the same approach all the time Dynamic routing allows for changes in routing

depending on traffic

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Alternate RoutingPossible routes between two end offices

are predefinedOriginating switch selects the best route

for each callRouting paths can be fixed (1 route) or

dynamic (multiple routes, selected based on current and historical traffic)

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Alternate Routing Diagram

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Control SignalingControl unit manages the establishment,

maintenance, and termination of signal paths

Includes signaling from subscriber to network, and signals within network

In-channel signaling uses the same channel for control signals and calls

Common-channel signaling uses independent channels for controls (e.g. SS7)

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Control Signaling Functions The means by which the network is managed,

calls are established, maintained, and terminated Audible communication with subscriber Transmission of dialed number Call cannot be completed indication Call ended indication Signal to ring phone Billing info Equipment and trunk status info Diagnostic info Control of special equipment

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Control Signal Sequence Both phones on hook Subscriber lifts receiver (off hook) End office switch signaled Switch responds with dial tone Caller dials number If target is not busy, ringer signal is sent to target subscriber Feedback to caller

Ringing tone, engaged tone, unobtainable (disconnected line, etc.)

Target accepts call by lifting receiver Switch terminates ringing signal and ringing tone Switch establishes connection Connection is released when Source subscriber hangs up

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Switch to Switch SignalingSubscribers connected to different switchesOriginating switch seizes inter-switch trunkOff hook signal is sent on trunk, requesting

a digit register at target switch (so that address may be communicated)

Terminating switch sends off hook followed by on hook (known as wink) to show register-ready status

Originating switch sends address

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Control Signals through a Circuit-Switching Telephone Network

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Location of SignalingSubscriber to network

Depends on subscriber device and switch

Within network is concerned with management of subscriber

calls and network More complex

Two types of control signaling are used in circuit switching networks: In-channel signaling Common channel signaling

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In-Channel SignalingUse same channel for signaling and call

Requires no additional transmission facilities

Inband Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech path (because the

control signals used to set up the path have to follow same path)

Out of band Voice signals do not use full 4kHz bandwidth Narrow signal band within 4kHz used for control Can be sent whether or not voice signals are present Requires extra electronics Slower signal rate (narrow bandwidth)

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Drawbacks of In Channel SignalingLimited transfer rateDelay between entering address (dialing)

and connectionThis is overcome by use of common

channel signaling

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Common Channel Signaling Control signals carried over paths independent of

voice channel One control signal channel (path) can carry signals

for a number of subscriber channels and is therefore the common control channel for these subscriber lines (channels)

Two modes of operation are used in Common channel signaling:

Associated Mode Common channel closely tracks inter-switch trunks

Disassociated Mode Additional nodes (signal transfer points) Effectively two separate networks

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Common v. In Channel Signaling

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Common Channel Signaling Modes

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Signaling System Number 7 SS7 It is the most widely used common channel signaling

scheme Used in ISDN Optimized for 64kbps 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 Although the network being controlled is a circuit

switched network, the control signaling is implemented using packet switching technology

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SS7 Signaling Network ElementsSignaling point (SP)

Any point in the network capable of handling SS7 control messages

Signal transfer point (STP) A signaling point capable of routing control

messages

Control plane Responsible for establishing and managing

connections

Information plane Once a connection is set up, info is transferred in

the information plane

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Transfer Points

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Signaling Network StructuresSTP capacities

Number of signaling links that can be handled Message transfer time Throughput capacity

Network performance Number of SPs Signaling delays

Availability and reliability Ability of network to provide services in the

face of STP failures