CIRCUIT SWITCHINGCIRCUIT SWITCHING AND PACKET AND PACKET SWITCHINGSWITCHING

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Switching NetworksSwitching Networks

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

Nodes not concerned with content of dataNodes not concerned with content of data purpose is to provide a switching facility that will move the purpose is to provide a switching facility that will move the

data from node to node until they reach their destinationdata from node to node until they reach their destination

End devices are stationsEnd devices are stations• Computer, terminal, phone, etc.Computer, terminal, phone, etc.

A collection of nodes and connections is a A collection of nodes and connections is a communications networkcommunications network

Data routed by being switched from node to nodeData routed by being switched from node to node

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NodesNodes Nodes may connect to other nodes only, or to Nodes may connect to other nodes only, or to

stations and other nodesstations and other nodes Node to node links usually multiplexedNode to node links usually multiplexed

FDM or TDMFDM or TDM Network is usually partially connected (not fully Network is usually partially connected (not fully

connected)connected) Some redundant connections are desirable for reliability Some redundant connections are desirable for reliability

(more than 1 possible path for each pair)(more than 1 possible path for each pair) Two different switching technologiesTwo different switching technologies

Circuit switchingCircuit switching Packet switchingPacket switching

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Simple Switched Network

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Circuit SwitchingCircuit Switching

Dedicated communication path between two Dedicated communication path between two stationsstations

Three phasesThree phases EstablishEstablish TransferTransfer DisconnectDisconnect

Must have switching capacity and channel Must have switching capacity and channel capacity to establish connectioncapacity to establish connection

Must have intelligence to work out routingMust have intelligence to work out routing

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Circuit Switching - ApplicationsCircuit Switching - Applications

InefficientInefficient Channel capacity dedicated for duration of Channel capacity dedicated for duration of

connectionconnection If no data, If no data, capacity wastedcapacity wasted

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

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Public Circuit Switched Network(eg. public telephone network)

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Telecommunications ComponentsTelecommunications Components SubscriberSubscriber

Devices attached to network.eg. telephoneDevices attached to network.eg. telephone Subscriber line Subscriber line

Local Loop @ Subscriber loopLocal Loop @ Subscriber loop Connection to network Connection to network (link between subscriber & network)(link between subscriber & network) Few km up to few tens of kmFew km up to few tens of km

ExchangeExchange Switching centersSwitching centers Switching centre supports subscribers - End office Switching centre supports subscribers - End office

TrunksTrunks Branches between exchangesBranches between exchanges Carry multiplexed multiple voice-frequency circuitsCarry multiplexed multiple voice-frequency circuits

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Circuit Establishment

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4.1.3 Circuit Switching Concepts4.1.3 Circuit Switching Concepts Digital SwitchDigital Switch

Provide transparent signal path between devicesProvide transparent signal path between devices Network InterfaceNetwork Interface

Represents the functions and hardware needed to connect digital Represents the functions and hardware needed to connect digital devicesdevices

Control UnitControl Unit Establish connectionsEstablish connections

• Generally on demandGenerally on demand• Handle and acknowledge requestsHandle and acknowledge requests• Determine if destination is freeDetermine if destination is free• construct pathconstruct path

Maintain connectionMaintain connection DisconnectDisconnect

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Blocking or Non-blockingBlocking or Non-blocking

BlockingBlocking A network is A network is unable to connect stations because all unable to connect stations because all

paths are in usepaths are in use A blocking network allows thisA blocking network allows this Used on voice systemsUsed on voice systems

• Short duration callsShort duration calls

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

(as long as the called party is free)(as long as the called party is free) Used for some data connectionsUsed for some data connections

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Space Division SwitchingSpace Division Switching

Developed for analog environmentDeveloped for analog environment Separate physical paths (divided in space)Separate physical paths (divided in space) Crossbar switchCrossbar switch

Number of crosspoints grows as square of number Number of crosspoints grows as square of number of stations. of stations.

Loss of crosspoint prevents connectionLoss of crosspoint prevents connection Inefficient use of crosspointsInefficient use of crosspoints

• All stations connected, only a few crosspoints in useAll stations connected, only a few crosspoints in use Non-blockingNon-blocking

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Space Division Switch

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Multistage SwitchMultistage Switch

Reduced number of crosspointsReduced number of crosspoints More than one path through networkMore than one path through network

Increased reliabilityIncreased reliability More complex controlMore complex control May be blockingMay be blocking

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Time Division SwitchingTime Division Switching

Modern digital systems rely on intelligent Modern digital systems rely on intelligent control of space and time division elementscontrol of space and time division elements

Use digital time division techniques to set up Use digital time division techniques to set up and maintain virtual circuitsand maintain virtual circuits

Partition low speed bit stream into pieces that Partition low speed bit stream into pieces that share higher speed streamshare higher speed stream

Manipulated by control logic to route data from Manipulated by control logic to route data from input to output.input to output.

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4.1.4 Control Signaling Functions4.1.4 Control Signaling Functions Audible communication with subscriberAudible communication with subscriber Transmission of dialed numberTransmission of dialed number Call can not be completed indicationCall can not be completed indication Call ended indicationCall ended indication Signal to ring phoneSignal to ring phone Billing infoBilling info Equipment and trunk status infoEquipment and trunk status info Diagnostic infoDiagnostic info Control of specialist equipment – satellite channel Control of specialist equipment – satellite channel

equipmentequipment

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Control Signal SequenceControl Signal Sequence Both phones on hook (not in use)Both phones on hook (not in use) Subscriber lifts receiver (off hook)Subscriber lifts receiver (off hook) End office switch signaledEnd office switch signaled Switch responds with dial toneSwitch responds with dial tone Caller dials numberCaller dials number If target not busy, send ringer signal to target subscriberIf target not busy, send ringer signal to target subscriber Feedback to callerFeedback to caller

Ringing tone, engaged tone, unobtainableRinging tone, engaged tone, unobtainable Target accepts call by lifting receiverTarget accepts call by lifting receiver Switch terminates ringing signal and ringing toneSwitch terminates ringing signal and ringing tone Switch establishes connectionSwitch establishes connection Connection release when Source subscriber hangs upConnection release when Source subscriber hangs up

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Switch to Switch SignalingSwitch to Switch Signaling

Subscribers connected to different switchesSubscribers connected to different switches Originating switch seizes interswitch trunkOriginating switch seizes interswitch trunk Send off hook signal on trunk, requesting digit Send off hook signal on trunk, requesting digit

register at target switch (for address)register at target switch (for address) Terminating switch sends off hook followed by Terminating switch sends off hook followed by

on hook (wink) to show register readyon hook (wink) to show register ready Originating switch sends addressOriginating switch sends address

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Location of SignalingLocation of Signaling

Subscriber to networkSubscriber to network Depends on subscriber device and switchDepends on subscriber device and switch

Within networkWithin network Management of subscriber calls and networkManagement of subscriber calls and network ore complexore complex

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4.1.5 4.1.5 Softswitch ArchitectureSoftswitch Architecture General purpose computer running software to make it a smart General purpose computer running software to make it a smart

phone switchphone switch Lower costsLower costs Greater functionalityGreater functionality

Packetizing of digitized voice dataPacketizing of digitized voice data Allowing voice over IPAllowing voice over IP

Most complex part of telephone network switch is software controlling Most complex part of telephone network switch is software controlling call processcall process

Call routingCall routing Call processing logicCall processing logic Typically running on proprietary processorTypically running on proprietary processor

Separate call processing from hardware function of switchSeparate call processing from hardware function of switch Physical switching done by media gatewayPhysical switching done by media gateway Call processing done by media gateway controllerCall processing done by media gateway controller

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Traditional Circuit Switching

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Softswitch

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Summary : Circuit SwitchingSummary : Circuit Switching A circuit can be…A circuit can be…

• A physical lineA physical line• A time slot in a frame in a TDM systemA time slot in a frame in a TDM system• A carrier frequency in an FDM systemA carrier frequency in an FDM system• A wavelength in a WDM systemA wavelength in a WDM system• A code in a CDMA systemA code in a CDMA system

A connection-during its existence-uses one circuitA connection-during its existence-uses one circuit Connection setupConnection setup

• Find path through network and through swithes towards destinationFind path through network and through swithes towards destination• Establish pathEstablish path

CommunicationCommunication• Send fixed data rate into the connectionSend fixed data rate into the connection• Release connection after useRelease connection after use

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Packet Switching Packet Switching PrinciplesPrinciples

Circuit switching designed for voiceCircuit switching designed for voice Resources dedicated to a particular callResources dedicated to a particular call Much of the time a data connection is idleMuch of the time a data connection is idle Data rate is fixedData rate is fixed

• Both ends must operate at the same rateBoth ends must operate at the same rate

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Packet Switching - Packet Switching - Basic OperationBasic Operation Data transmitted in small packetsData transmitted in small packets

Typically 1000 octetsTypically 1000 octets Longer messages split into series of packetsLonger messages split into series of packets Each packet contains a portion of user data plus some Each packet contains a portion of user data plus some

control infocontrol info Control infoControl info

Routing (addressing) infoRouting (addressing) info Packets are received, stored briefly (buffered) and Packets are received, stored briefly (buffered) and

past on to the next nodepast on to the next node Store and forwardStore and forward

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Use of Packets

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AdvantagesAdvantages

Line efficiencyLine efficiency Single node to node link can be shared by many packets Single node to node link can be shared by many packets

over timeover time Packets queued and transmitted as fast as possiblePackets queued and transmitted as fast as possible

Data rate conversionData rate conversion Each station connects to the local node at Each station connects to the local node at its own speedits own speed Nodes buffer data if required to equalize ratesNodes buffer data if required to equalize rates

Packets are accepted even when network is busyPackets are accepted even when network is busy Delivery may slow downDelivery may slow down

Priorities can be usedPriorities can be used

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Packet Switching – Packet Switching – Switching TechniqueSwitching Technique

Station breaks long message into packetsStation breaks long message into packets Packets sent one at a time to the networkPackets sent one at a time to the network Packets handled in two waysPackets handled in two ways

(a)(a) DatagramDatagram

(b)(b) Virtual circuitVirtual circuit

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(a)(a) DatagramDatagram

Each packet treated independentlyEach packet treated independently Packets can take any practical routePackets can take any practical route Packets may arrive out of orderPackets may arrive out of order Packets may go missingPackets may go missing Up to receiver to re-order packets and recover Up to receiver to re-order packets and recover

from missing packetsfrom missing packets

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DatagramDiagram

• Packets belong to the same message but may go by different route

• Routed between 2 points. – Each switch creates

an entry in the routing table

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(b)(b) Virtual CircuitVirtual Circuit

Preplanned route established before any packets Preplanned route established before any packets sentsent

Call request and call accept packets establish Call request and call accept packets establish connection (handshake)connection (handshake)

Each packet contains a virtual circuit identifier Each packet contains a virtual circuit identifier instead of destination addressinstead of destination address

No routing decisions required for each packetNo routing decisions required for each packet Clear request to drop circuitClear request to drop circuit Not a dedicated pathNot a dedicated path

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VirtualCircuitDiagram

• Packets belong to the same message sent in a single route

• Packets travel one after another in the same– Each switch creates

an entry in the routing table

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Virtual Circuits v DatagramVirtual Circuits v Datagram Virtual circuitsVirtual circuits

Network can provide sequencing and error controlNetwork can provide sequencing and error control Packets are forwarded more quicklyPackets are forwarded more quickly

• No routing decisions to make (using only 1 route)No routing decisions to make (using only 1 route) Less reliableLess reliable

• Loss of a node looses all circuits through that nodeLoss of a node looses all circuits through that node

DatagramDatagram No call setup phaseNo call setup phase

• Better if few packetsBetter if few packets More flexibleMore flexible

• Routing can be used to avoid congested parts of the networkRouting can be used to avoid congested parts of the network

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X.25X.25 1976,ITU-T1976,ITU-T Standard used for packet-switching networkStandard used for packet-switching network Interface between host and packet switched networkInterface between host and packet switched network Almost universal on packet switched networks and Almost universal on packet switched networks and

packet switching in ISDNpacket switching in ISDN Defines three layersDefines three layers

PhysicalPhysical LinkLink PacketPacket

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X.25 - PhysicalX.25 - Physical Interface between attached station and link to Interface between attached station and link to

nodenode Data terminal equipment DTE (user equipment)Data terminal equipment DTE (user equipment) Data circuit terminating equipment DCE (node)Data circuit terminating equipment DCE (node) Uses physical layer specification X.21Uses physical layer specification X.21 Reliable transfer across physical linkReliable transfer across physical link Sequence of framesSequence of frames

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X.25 - LinkX.25 - Link

Link Access Protocol Balanced (LAPB)Link Access Protocol Balanced (LAPB) Subset of HDLC (high level data link level)Subset of HDLC (high level data link level)

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X.25 Use of Virtual Circuits

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Virtual Circuit ServiceVirtual Circuit Service

Logical connection between two stationsLogical connection between two stations External virtual circuitExternal virtual circuit

Specific pre-planned route through networkSpecific pre-planned route through network Internal virtual circuitInternal virtual circuit

Typically one to one relationship between external Typically one to one relationship between external and internal virtual circuitsand internal virtual circuits

Can employ X.25 with datagram style networkCan employ X.25 with datagram style network External virtual circuits require logical channel External virtual circuits require logical channel

All data considered part of streamAll data considered part of stream

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User Data and X.25 Protocol Control Information

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Packet SwitchingPacket Switching

DataData transmitted in short packettransmitted in short packet Message isMessage is broken up into a series of packetbroken up into a series of packet Each Each packet contain user data plus control informationpacket contain user data plus control information that that

required to required to route the packet through networkroute the packet through network until to the dest. until to the dest.

Circuit SwitchingCircuit Switching

Network is Network is built around a single circuit-switching nodebuilt around a single circuit-switching node consists consists of a of a collection of collection of stations(computers)stations(computers) attached to a central attached to a central switching unit.switching unit.

The The central unit establishes a dedicated pathcentral unit establishes a dedicated path bet any two bet any two devices to communicate.devices to communicate.

Circuit vs Packet Switching (Circuit vs Packet Switching (Operation)Operation)

END!!