Business Data Communications 6e

Chapter 12: Circuit Switchingand Packet SwitchingBusiness Data Communications, 6e12Switching TechniquesData transmitted through a network of intermediate switching nodes, which are not concerned with contentEnd devices receiving data are stations; switching devices are nodesA collection of nodes is a communication networkA switched communication network routes data from one station to another through nodes2Simple Switching Network3

4Switched Network CharacteristicsSome nodes connect only to other nodes for switching of data; other nodes have one or more stations attached as well.Node-station links are generally dedicated point-to-point links; ode-node links are usually multiplexed linksUsually, the network is not fully connected; however, it is desirable to have more than one possible path through the network for each pair of stations to enhance reliability4Circuit-Switching NetworksDominant technology for voice technologySynchronous communication requires a dedicated pathInvolves 3 phases-Circuit establishment-Data transfer-point-to-point from endpoints to node-internal switching/multiplexing among nodes-Circuit disconnect56Circuit EstablishmentStation requests connection from nodeNode determines best route, sends message to next linkEach subsequent node continues the establishment of a pathOnce nodes have established connection, test message is sent to determine if receiver is ready/able to accept message67Data TransferPoint-to-point transfer from source to nodeInternal switching and multiplexed transfer from node to nodePoint-to-point transfer from node to receiverUsually a full-duplex connection throughout78Circuit DisconnectWhen transfer is complete, one station initiates terminationSignals must be propagated to all nodes used in transit in order to free up resources89Circuit Switching CharacteristicsChannel capacity is dedicated for the duration of a connection, even if no data are being transferredOnce the circuit is established, the network is effectively transparent to the users, resulting in negligible delaysDeveloped to handle voice traffic but is now also used for data traffic910Circuit Switching ApplicationsPublic Telephone Network (PSTN)Private Branch Exchanges (PBX)Private Wide Area Networks (often used to interconnect PBXs in a single organization)Data Switch1011Public Switched Telephone Network (PSTN)SubscribersSubscriber Line (local loop) Connects subscriber to local telco exchangeExchanges (end office)Telco switching centers>19,000 in USTrunksConnections between exchangesCarry multiple voice circuits using FDM or synchronous TDMManaged by IXCs (inter-exchange carriers)11Circuit Establishment12

13Control SignalingManage the establishment, maintenance, and termination of signal pathsIncludes signaling from subscriber to network, and signals within networkFor a large public telecommunications network, a relatively complex control signaling scheme is required1314 Signaling FunctionsAudible communication with the subscriberTransmission of the number dialedInformation between switches that a call cannot be completedInformation between switches that a call has ended and the path can be disconnected

Telephone ring signalTransmission of billing informationTransmission of equipment and trunk status informationTransmission of system failure diagnostic informationControl of special equipment (e.g. satellite channel equipment)1415Types of Control SignalsSupervisoryAddressCall InformationNetwork Management1516Supervisory SignalsBinary character (true/false; on/off)Deal with the availability of the called subscriber and of the needed network resourcesUsed to determine if a needed resource is available and, if so, to seize it. Also used to communicate the status of requested resources.1617Address SignalsIdentify a subscriberInitially generated by a calling subscriber when dialing a telephone numberResulting address may be propagated through the network to support the routing function and to locate and ring the called subscriber's phone1718Call Information SignalsProvide information to the subscriber about the status of a callIn contrast to internal signals (which are analog or digital electrical messages), these are audible tones that can be heard by the caller or an operator with the proper phone set1819Network Management SignalsUsed for the maintenance, troubleshooting, and overall operation of the networkThese signals cover a broad scope, and it is this category that will expand most with the increasing complexity of switched networks1920In-Channel SignalingTraditionally, control signals were carried on the same channel as the call to which the control signals relateDrawbacksInformation transfer rate limitedDelay between entering a number and establishing a connection20Business Data Communications, 5e21Common-Channel SignalingControl signals are carried over paths completely independent of the voice channelsOne independent control signal path can carry the signals for a number of subscriber channels (i.e. is a common control channel for these channels)2122Softswitch ArchitectureA general-purpose computer running specialized software that turns it into a smart phone switchCost significantly less and can provide more functionalityCan convert digitized voice bits into packets, opening transmission options (e.g. voice over IP)Physical switching function: media gateway (MG)Call processing logic: media gateway controller (MGC)22PBX ComponentsControl Processor: runs the software that operates system features.Modules: House interface cards that provide endpoint interfaces to the switch.Inter-module switching: allows the interconnection of ports in diffreent modules, using circuit switching.2324Softswitch ArchitectureTraditional and IP PBX

2425Packet-Switching NetworksDeveloped in 1970s for long-distance data transmission due to circuit switching limitationsIn user/host data connection the line is often idle, so circuit-switching is inefficientCircuit-switching requires both devices to transmit and receive at the same data rate, limiting interconnection options2526Packet Switching OperationData is broken into packets, each of which can be routed separatelyAdvantages: better line efficiency, signals can always be routed, prioritization optionDisadvantages: transmission delay in nodes, variable delays can cause jitter, extra overhead for packet addresses 2627Packet Switching Illustration

2728Packet-Switching TechniquesDatagrameach packet treated independently and referred to as a datagrampackets may take different routes, arrive out of sequenceVirtual Circuitpreplanned route established for all packetssimilar to circuit switching, but the circuit is not dedicated2829Packet-Switched RoutingAdaptive routing changes based on network conditionsFactors influencing routing are failure and congestionNodes must exchange information on network statusTradeoff between quality and amount of overhead2930Packet-Switched Congestion ControlWhen line utilization is >80%, queue length grows too quicklyCongestion control limits queue length to avoid througput problemsStatus information exchanged among nodesControl signals regulate data flow using interface protocols (usually X.25)3031WANs for VoiceRequires very small and nonvariable delays for natural conversation--difficult to provide this with packet-switchingAs a result, the preferred method for voice transmission is circuit-switchingMost businesses use public telephone networks, but some have implemented private voice networksVoIP uses packet transmission over Internets and intranets; it is enjoying gradually growing acceptance as an alternative3132WANs for DataPublic packet-switched networks Private packet-switched networksPrivate leased lines Public circuit-switched networksPrivate circuit-switched networks (interconnected digital PBXs)ISDN (integrate packet and circuit switching)3233WAN ConsiderationsNature of trafficstream generally works best with dedicated circuitsbursty better suited to packet-switchingStrategic and growth control--limited with public networksReliability--greater with packet-switchingSecurity--greater with private networks33Wide Area Network Features34FeatureDedicated(Leased Lines)Public PacketPrivate PacketStrategic ControlNetwork design, service, and maintenance can be given priority and controlled by user.Service limited to that which suits average customer.Network design, service, and maintenance can be given priority and controlled by user.Growth control and operation controlNot integrated; decentralized fault detection may be expensive.Provided by service supplier to satisfy average requirements.Integrated into all equipment; centralized fault isolation and detection.ReliabilityManual and user-visible recovery from failure.Transparent and automatic recovery from failure.Transparent and automatic recovery from failure.SecurityPrivate users only.Public users, network access control.Private users only, network access control.