Switching NetworksSwitching Networks

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

Nodes not concerned with content of dataNodes not concerned with content of data

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 Data routed by being switched from node to nodenode

Switching NodesSwitching Nodes

Nodes may connect to other nodes only Nodes may connect to other nodes only (internal), or to stations and other nodes(internal), or to stations and other nodes

Node-to-node links usually multiplexedNode-to-node links usually multiplexed FDM or TDMFDM or TDM

Network is usually not fully connectedNetwork is usually not fully connected Partially connectedPartially connected Some redundant connections desirable for reliabilitySome redundant connections desirable for reliability

Two different switching technologiesTwo different switching technologies Circuit switchingCircuit switching Packet switchingPacket switching

A Simple Switched NetworkA Simple Switched Network

Circuit SwitchingCircuit Switching

Basic ConceptsBasic Concepts

Circuit SwitchingCircuit Switching

Dedicated communication path provided between two stationsDedicated communication path provided between two stations

Three phasesThree phases Circuit establishmentCircuit establishment

Free channel must be allocated for each leg in the routeFree channel must be allocated for each leg in the route Data transferData transfer

Data may be analog or digital, depending upon network typeData may be analog or digital, depending upon network type

Digital transmission for voice and data becoming dominantDigital transmission for voice and data becoming dominant

Typically full-duplexTypically full-duplex Circuit disconnectCircuit disconnect

As requested by one of two stations involvedAs requested by one of two stations involved

Action propagated to deallocate the dedicated resourcesAction propagated to deallocate the dedicated resources

Must have switching capacity and channel capacity between each Must have switching capacity and channel capacity between each pair of switching nodes on path to establish connectionpair of switching nodes on path to establish connection

Switches must have intelligence to make allocations and device Switches must have intelligence to make allocations and device route through networkroute through network

Circuit Switching - ApplicationsCircuit Switching - Applications

InefficientInefficient Channel capacity dedicated for duration of connectionChannel capacity dedicated for duration of connection When no data, capacity wastedWhen no data, capacity wasted

Setup (connection) takes timeSetup (connection) takes time But once connected, transfer is transparent!But once connected, transfer is transparent!

Developed for voice traffic (phone)Developed for voice traffic (phone) Now widely used for data trafficNow widely used for data traffic Best-known example is public telephone networkBest-known example is public telephone network

Substantial data traffic from modemsSubstantial data traffic from modemsGradually being converted to digital networkGradually being converted to digital network

Another example is private branch exchange (PBX)Another example is private branch exchange (PBX)Interconnect phones within a building or officeInterconnect phones within a building or office

Public Circuit-Switched NetworkPublic Circuit-Switched Network

Telecomm Network ComponentsTelecomm Network Components

SubscriberSubscriber Devices attached to networkDevices attached to network

Local LoopLocal Loop a.k.a. subscriber loop or local loopa.k.a. subscriber loop or local loop Connection to networkConnection to network

ExchangeExchange Switching centers within the networkSwitching centers within the network End office – switching center supporting subscribersEnd office – switching center supporting subscribers Intermediate switching nodes in-betweenIntermediate switching nodes in-between

TrunksTrunks Branches between exchangesBranches between exchanges Carry multiple voice-frequency circuits via FDM or Carry multiple voice-frequency circuits via FDM or

synchronous TDMsynchronous TDM

Circuit EstablishmentCircuit Establishment

Elements of a Circuit-Switching NodeElements of a Circuit-Switching Node

Digital SwitchDigital Switch Provide transparent signal path Provide transparent signal path

between devicesbetween devices

Network interfaceNetwork interface Functions and hardware to connect Functions and hardware to connect

digital devices (computers, digital digital devices (computers, digital telephones) to networktelephones) to network

Analog phones can also be attached if Analog phones can also be attached if interface logic includes converterinterface logic includes converter

Control logicControl logic Establishes connections on demand of Establishes connections on demand of

attached deviceattached deviceHandle and acknowledge requestsHandle and acknowledge requests

Must determine if destination free and Must determine if destination free and construct path through switchconstruct path through switch

Maintains connectionsMaintains connections Terminates connections at request of Terminates connections at request of

attached device or for its own reasonsattached device or for its own reasons

Blocking vs. Non-blockingBlocking vs. Non-blocking

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

paths are in usepaths are in use A blocking network allows this situation to occurA blocking network allows this situation to occur Used on voice systemsUsed on voice systems

Tolerance when assuming short-duration callsTolerance when assuming short-duration calls

Non-blockingNon-blocking Permits all stations to connect (in pairs) at oncePermits all stations to connect (in pairs) at once Used in some data applicationsUsed in some data applications

Space-Division SwitchingSpace-Division Switching

BackgroundBackground Originally developed for analog environment, but carried over Originally developed for analog environment, but carried over

into digital realminto digital realmPrinciples same whether switch carries analog or digital signalsPrinciples same whether switch carries analog or digital signals

Separate physical paths (divided in “space”)Separate physical paths (divided in “space”) Each connection requires establishment of physical path through Each connection requires establishment of physical path through

switch dedicated to data transfer between 2 endpointsswitch dedicated to data transfer between 2 endpoints Basic building block is crossbar switchBasic building block is crossbar switch

Crossbar switchCrossbar switch Number of crosspoints grows as square of number of stationsNumber of crosspoints grows as square of number of stations Loss of crosspoint prevents connectionLoss of crosspoint prevents connection Inefficient use of crosspointsInefficient use of crosspoints

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

Crossbar SwitchCrossbar Switch

Multistage SwitchMultistage Switch

Designed to overcome crossbar limitations Designed to overcome crossbar limitations ScalabilityScalability Resilience to failed crosspointsResilience to failed crosspoints Utilization of crosspointsUtilization of crosspoints

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

Increased reliabilityIncreased reliability But, more complex control scheme requiredBut, more complex control scheme required May be blockingMay be blocking Can be made non-blocking by increasing number Can be made non-blocking by increasing number

and/or size of intermediate switchesand/or size of intermediate switchesAt increased costAt increased cost

Three-Stage SwitchThree-Stage Switch

NOTE: 48 crosspoints instead of 100 in crossbar switch

Time Division SwitchingTime Division Switching

modern digital systems use intelligent modern digital systems use intelligent control of space & time division elementscontrol of space & time division elements

use digital time division techniques to set use digital time division techniques to set up and maintain virtual circuitsup and maintain virtual circuits

partition low speed bit stream into pieces partition low speed bit stream into pieces that share higher speed streamthat share higher speed stream

individual pieces manipulated by control individual pieces manipulated by control logic to flow from input to outputlogic to flow from input to output

Softswitch ArchitectureSoftswitch ArchitectureRecent trend in circuit-switching technology is SoftswitchRecent trend in circuit-switching technology is Softswitch

General-purpose computer running software to make it a smart General-purpose computer running software to make it a smart phone switchphone switch

Lower costs and greater functionality in addition to Lower costs and greater functionality in addition to handling traditional circuit-switching functionshandling traditional circuit-switching functions

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

Most complex part of telephone network switch is Most complex part of telephone network switch is software that controls call processingsoftware that controls call processing

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

Difference with SoftswitchDifference with Softswitch Separate call processing from h/w switching function of switchSeparate call processing from h/w switching function of switch Physical switching performed by Physical switching performed by media gatewaymedia gateway (MG) (MG) Call processing performed by Call processing performed by media gateway controllermedia gateway controller (MGC) (MGC) MG and MGC perhaps from different vendors; standard protocolMG and MGC perhaps from different vendors; standard protocol

Traditional Circuit SwitchingTraditional Circuit Switching

SoftswitchSoftswitch

Packet SwitchingPacket Switching

Basic ConceptsBasic Concepts

PrinciplesPrinciples

Circuit switching designed for voiceCircuit switching designed for voice Resources dedicated to a particular callResources dedicated to a particular call Fairly high utilization with talkingFairly high utilization with talking Shortcomings with data connectionsShortcomings with data connections

Much of the time the line is idleMuch of the time the line is idle Inefficient use of resourcesInefficient use of resources

Data rate is fixedData rate is fixed Both ends must operate at same rateBoth ends must operate at same rate Limits utility in interconnecting variety of host computersLimits utility in interconnecting variety of host computers

Packet switching is far better for dataPacket switching is far better for data

Basic Operation of Packet SwitchingBasic Operation of Packet Switching

Data transmitted in small packetsData transmitted in small packets e.g. packet length of 1000 octetse.g. packet length of 1000 octets Longer messages split into series of packetsLonger messages split into series of packets Each packet contains a portion of user data from Each packet contains a portion of user data from

message, plus some control infomessage, plus some control info

Control infoControl info Routing (addressing) infoRouting (addressing) info

At each switching nodeAt each switching node Each packet received, stored briefly (buffered), and Each packet received, stored briefly (buffered), and

passed on to next nodepassed on to next node Store and forwardStore and forward

Use of PacketsUse of Packets

AdvantagesAdvantages

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

packets over timepackets over 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 local node at its own speedEach station connects to local node at its own speed Nodes buffer data if required to equalize ratesNodes buffer data if required to equalize rates

Packets accepted even when network is busyPackets accepted even when network is busy Unlike blocking of calls in circuit switchingUnlike blocking of calls in circuit switching But, delivery may slow down (i.e. increased delay)But, delivery may slow down (i.e. increased delay)

Priorities can be usedPriorities can be used Useful when packets queued, as can send higher-Useful when packets queued, as can send higher-

priority packets first when output link availablepriority packets first when output link available

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 one of two approachesPackets handled in one of two approaches DatagramDatagram Virtual circuitVirtual circuit

DatagramDatagram

Basic characteristicsBasic characteristics 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

Some get through faster than othersSome get through faster than others Packets may go missingPackets may go missing

e.g. momentary crash of switching node may e.g. momentary crash of switching node may cause all queued packets on it to be lostcause all queued packets on it to be lost

Up to receiver to re-order packets and recover Up to receiver to re-order packets and recover from missing packetsfrom missing packets

Example:Example:DatagramDatagram

Virtual CircuitVirtual Circuit

Basic characteristics Basic characteristics Not a dedicated pathNot a dedicated path Preplanned route is established before any data Preplanned route is established before any data

packets sentpackets sent First, call request and call accept packets establish First, call request and call accept packets establish

connection (handshake)connection (handshake) Then, each data packet contains a virtual circuit Then, each data packet contains a virtual circuit

identifier (VCI) instead of destination addressidentifier (VCI) instead of destination addressNo routing decisions required for each packetNo routing decisions required for each packet

Clear request packet used to drop circuitClear request packet used to drop circuit

Example:Example:Virtual Virtual CircuitCircuit

Virtual Circuit vs. DatagramVirtual Circuit vs. Datagram

Virtual circuitVirtual circuit Network can provide sequencing and error controlNetwork can provide sequencing and error control

By design, packets arrive in orderBy design, packets arrive in orderRetransmission request for missing packetsRetransmission request for missing packets

Data packets forwarded more quicklyData packets forwarded more quicklyNo routing decisions to makeNo routing decisions to make

Less reliableLess reliableLoss of switching node loses all virtual circuits through that nodeLoss of switching node loses all virtual circuits through that node

DatagramDatagram No call setup phaseNo call setup phase

Better if few packetsBetter if few packets More flexibleMore flexible

Routing with each data packet permits it to avoid congested or Routing with each data packet permits it to avoid congested or failed parts of networkfailed parts of network

Packet SizePacket SizeA virtual circuit from station X A virtual circuit from station X through nodes a and b to station Ythrough nodes a and b to station Y

Each packet contains 40 octets Each packet contains 40 octets and 3 octets of control informationand 3 octets of control information

Relationship between packet size Relationship between packet size and transmission timeand transmission time

Smaller packets provide more Smaller packets provide more potential for concurrency in potential for concurrency in network when multiple hopsnetwork when multiple hops

Reduces msg. trans. time Reduces msg. trans. time However, law of diminishing However, law of diminishing

returns applies as usual returns applies as usual At some point, too small a packet At some point, too small a packet starts to increase total transmission starts to increase total transmission time again time again

Of course, other factors also in Of course, other factors also in play with change in packet sizeplay with change in packet size

e.g. smaller packets exhibit higher e.g. smaller packets exhibit higher ratio of overhead ratio of overhead

Circuit vs. Packet SwitchingCircuit vs. Packet Switching

Delay factors in performanceDelay factors in performance Propagation delayPropagation delay

As we’ve seen, speed of signal through mediumAs we’ve seen, speed of signal through medium e.g. 2x10e.g. 2x1088 meters/sec through a wire meters/sec through a wire

A function of distance and mediumA function of distance and medium Transmission timeTransmission time

As we’ve seen, time for transmitter to emit block of dataAs we’ve seen, time for transmitter to emit block of data

Clearly a function of packet size and data rateClearly a function of packet size and data rate Node delayNode delay

Processing delay of node to switch dataProcessing delay of node to switch data

A function of technology and perhaps packet sizeA function of technology and perhaps packet size

Event TimingEvent Timing

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External and Internal OperationExternal and Internal Operation

Can consider datagrams vs. virtual circuits at two levelsCan consider datagrams vs. virtual circuits at two levels Internal vs. externalInternal vs. external They need not necessarily be the same at both levelsThey need not necessarily be the same at both levels

Interface between station and network switching nodeInterface between station and network switching node Connection-oriented serviceConnection-oriented service

Station requests logical connection (virtual circuit)Station requests logical connection (virtual circuit)

All packets identified as belonging to that connection and sequentially numberedAll packets identified as belonging to that connection and sequentially numbered

Network delivers packets in sequenceNetwork delivers packets in sequence

External VC serviceExternal VC service

e.g. X.25e.g. X.25

Different from internal VC operationDifferent from internal VC operation Connectionless serviceConnectionless service

Packets handled independentlyPackets handled independently

External datagram serviceExternal datagram service

Different from internal datagram operationDifferent from internal datagram operation Basically, the logical behavior (VC or datagram) provided by service to upper Basically, the logical behavior (VC or datagram) provided by service to upper

layer (“user”) is the external service (DG vs. VC)layer (“user”) is the external service (DG vs. VC)

ExternalExternal VC and Datagram VC and Datagram

Operation inside the network fabric (i.e. cloud) is not necessarily the same as external view.

InternalInternal VC and Datagram VC and Datagram

X.25X.25

ITU-T standard for interface between host ITU-T standard for interface between host and packet switched networkand packet switched network

almost universal on packet switched almost universal on packet switched networks and packet switching in ISDNnetworks and packet switching in ISDN

defines three layersdefines three layers PhysicalPhysical LinkLink PacketPacket

X.25 - PhysicalX.25 - Physical

interface between station node linkinterface between station node linktwo ends are distinct two ends are distinct Data Terminal Equipment DTE (user Data Terminal Equipment DTE (user

equipment)equipment) Data Circuit-terminating Equipment DCE Data Circuit-terminating Equipment DCE

(node)(node)

physical layer specification is X.21physical layer specification is X.21can substitute alternative such as EIA-232can substitute alternative such as EIA-232

X.25 - LinkX.25 - Link

Link Access Protocol Balanced (LAPB)Link Access Protocol Balanced (LAPB) Subset of HDLCSubset of HDLC see chapter 7see chapter 7

provides reliable transfer of data over linkprovides reliable transfer of data over link

sending as a sequence of framessending as a sequence of frames

X.25 - PacketX.25 - Packet

provides a logical connections (virtual provides a logical connections (virtual circuit) between subscriberscircuit) between subscribers

all data in this connection form a single all data in this connection form a single stream between the end stationsstream between the end stations

established on demandestablished on demand

termed external virtual circuitstermed external virtual circuits

X.25 Use of Virtual CircuitsX.25 Use of Virtual Circuits

Virtual-Circuit ServiceVirtual-Circuit Service

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

Specific preplanned route through networkSpecific preplanned route through network Internal virtual circuitInternal virtual circuit

Typically one-to-one relationship between Typically one-to-one relationship between external and internal virtual circuitsexternal and 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

User Data and X.25 Protocol User Data and X.25 Protocol Control InformationControl Information

Issues with X.25Issues with X.25

key features include:key features include: call control packets, in band signalingcall control packets, in band signaling multiplexing of virtual circuits at layer 3multiplexing of virtual circuits at layer 3 layers 2 and 3 include flow and error controllayers 2 and 3 include flow and error control

hence have considerable overheadhence have considerable overhead

not appropriate for modern digital systems not appropriate for modern digital systems with high reliabilitywith high reliability

PSN Example: Frame RelayPSN Example: Frame Relay

Designed to be more efficient than X.25Designed to be more efficient than X.25 Developed before ATMDeveloped before ATM Larger installed base than ATMLarger installed base than ATM

Key features of X.25Key features of X.25 Call-control packets, in-band signalingCall-control packets, in-band signaling Multiplexing of virtual circuits at layer 3Multiplexing of virtual circuits at layer 3 Layer 2 and 3 include flow and error controlLayer 2 and 3 include flow and error control

WeaknessesWeaknesses Considerable overheadConsiderable overhead Not particularly appropriate for modern digital Not particularly appropriate for modern digital

communications systems with high reliability at high communications systems with high reliability at high speedspeed

Frame Relay – Key DifferencesFrame Relay – Key DifferencesStreamlines the communications processStreamlines the communications processCall-control signaling carried on separate logical Call-control signaling carried on separate logical connection from user dataconnection from user data

Intermediate nodes in fabric relieved of much work in Intermediate nodes in fabric relieved of much work in maintaining state tables and processing maintaining state tables and processing

Multiplexing and switching is at layer 2Multiplexing and switching is at layer 2 Eliminates one layer of processingEliminates one layer of processing

No hop-by-hop error or flow controlNo hop-by-hop error or flow control End-to-end flow and error control (if used) performed by higher End-to-end flow and error control (if used) performed by higher

layerlayer

Single user data frame sent from source to destination Single user data frame sent from source to destination and ACK carried backand ACK carried back

No hop-by-hop exchanges of data and ACK framesNo hop-by-hop exchanges of data and ACK frames

End-to-End vs Hop-by-hopEnd-to-End vs Hop-by-hop

Advantages and DisadvantagesAdvantages and Disadvantages

Lost link by link error and flow controlLost link by link error and flow control Increased reliability makes this less a problemIncreased reliability makes this less a problem

Streamlined communications processStreamlined communications process Lower delayLower delay Higher throughputHigher throughput

Better solution for higher data rates, Better solution for higher data rates, reliable linksreliable links

Protocol ArchitectureProtocol Architecture

Control PlaneControl Plane

Between subscriber and networkBetween subscriber and network

Separate logical channel usedSeparate logical channel used Similar to common channel signaling for circuit-Similar to common channel signaling for circuit-

switching servicesswitching services

Data link layerData link layer LAPD (Q.921)LAPD (Q.921) Reliable data link controlReliable data link control Error and flow controlError and flow control Between user (TE) and network (NT)Between user (TE) and network (NT) Used for exchange of Q.933 control signal messagesUsed for exchange of Q.933 control signal messages

User PlaneUser PlaneEnd-to-end functionalityEnd-to-end functionalityTransfer of info between endsTransfer of info between endsLAPF (Link Access Procedure for Frame Mode LAPF (Link Access Procedure for Frame Mode Bearer Services) Q.922Bearer Services) Q.922 Called “LAPF Core” for minimum-function protocolCalled “LAPF Core” for minimum-function protocol Frame delimiting, alignment and transparencyFrame delimiting, alignment and transparency Frame mux and demux using addressing fieldFrame mux and demux using addressing field Ensure frame is integral number of octets (zero bit Ensure frame is integral number of octets (zero bit

insertion/extraction)insertion/extraction) Ensure frame is neither too long nor shortEnsure frame is neither too long nor short Detection of transmission errorsDetection of transmission errors Congestion control functionsCongestion control functions LAPF CoreLAPF Core is similar to LAPB, LAPD, HDLC, etc. is similar to LAPB, LAPD, HDLC, etc.

Except no control field!Except no control field!

Frame Relay Data Link Frame Relay Data Link ConnectionsConnections

logical connection between subscriberslogical connection between subscribers

data transferred over themdata transferred over them

not protected by flow or error controlnot protected by flow or error control

uses separate connection for call controluses separate connection for call control

overall results in significantly less work in overall results in significantly less work in networknetwork

User Data TransferUser Data Transferonly have one frame type whichonly have one frame type which carries user datacarries user data

no control frames meansno control frames means no inband signalingno inband signaling no sequence numbersno sequence numbers

flag and FCS function as in HDLCflag and FCS function as in HDLCaddress field carries DLCIaddress field carries DLCIDLCI (Data Link Connection Identifier) has DLCI (Data Link Connection Identifier) has local significance onlylocal significance only

User Data TransferUser Data Transfer

The information field carries higher-layer data. The information field carries higher-layer data.

If the user selects to implement additional data If the user selects to implement additional data link control functions end to endlink control functions end to end then a data link frame can be carried in this field. then a data link frame can be carried in this field.

A common selection will be to use the full LAPF A common selection will be to use the full LAPF protocol (known as LAPF control protocol), to protocol (known as LAPF control protocol), to perform functions above the LAPF core functions. perform functions above the LAPF core functions.

Note that the protocol implemented in this fashion Note that the protocol implemented in this fashion is strictly between the end subscribers and is is strictly between the end subscribers and is transparent to the frame relay network.transparent to the frame relay network.

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LAPF-Core FormatsLAPF-Core Formats

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LAPF-Core Address FormatsLAPF-Core Address FormatsThe address field has a default length of 2 octets The address field has a default length of 2 octets

may be extended to 3 or 4 octets.may be extended to 3 or 4 octets.

It carries a data link connection identifier (DLCI) of 10, 16, or 23 It carries a data link connection identifier (DLCI) of 10, 16, or 23 bits.bits.

The DLCI serves the same function as the virtual circuit number The DLCI serves the same function as the virtual circuit number in X.25in X.25

It allows multiple logical frame relay connections to be It allows multiple logical frame relay connections to be multiplexed over a single channel.multiplexed over a single channel.

As in X.25, the connection identifier has only local significance: As in X.25, the connection identifier has only local significance: Each end of the logical connection assigns its own DLCI from the pool of locally Each end of the logical connection assigns its own DLCI from the pool of locally

unused numbers, and the network must map from one to the other.unused numbers, and the network must map from one to the other.

The alternative, using the same DLCI on both ends, would The alternative, using the same DLCI on both ends, would require some sort of global management of DLCI values.require some sort of global management of DLCI values.

LAPF-Core Address FormatsLAPF-Core Address Formats

The length of the Address field, and hence The length of the Address field, and hence of the DLCI, is determined by the Address of the DLCI, is determined by the Address field extension (EA) bits. field extension (EA) bits.

The C/R bit is application specific and not The C/R bit is application specific and not used by the standard frame relay protocol.used by the standard frame relay protocol.

The remaining bits in the address field have The remaining bits in the address field have to do with congestion controlto do with congestion control

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