Chapter_9
description
Transcript of Chapter_9
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William StallingsData and Computer Communications
Chapter 9Circuit Switching
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Switching NetworksLong distance transmission is typically
done over a network of switched nodesNodes not concerned with content of dataEnd devices are stations
Computer, terminal, phone, etc.A collection of nodes and connections is a
communications networkData routed by being switched from node
to node
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NodesNodes may connect to other nodes only,
or to stations and other nodesNode to node links usually multiplexedNetwork is usually partially connected
Some redundant connections are desirable for reliability
Two different switching technologies Circuit switching Packet switching
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Simple Switched Network
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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 - ApplicationsInefficient
Channel capacity dedicated for duration of connection
If no data, capacity wastedSet up (connection) takes timeOnce connected, transfer is transparentDeveloped for voice traffic (phone)
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Public Circuit Switched Network
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Telecomms ComponentsSubscriber
Devices attached to networkLocal Loop
Subscriber loop Connection to network
Exchange Switching centers End office - supports subscribers
Trunks Branches between exchanges Multiplexed
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Circuit Switch Elements
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Circuit Switching ConceptsDigital Switch
Provide transparent signal path between devicesNetwork InterfaceControl Unit
Establish connectionsGenerally on demandHandle and acknowledge requestsDetermine if destination is freeconstruct path
Maintain connection Disconnect
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Blocking or Non-blockingBlocking
A network is unable to connect stations because all paths are in use
A blocking network allows this Used on voice systems
Short duration callsNon-blocking
Permits all stations to connect (in pairs) at once
Used for some data connections
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Space Division SwitchingDeveloped for analog environmentSeparate physical pathsCrossbar 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
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Multistage SwitchReduced number of crosspointsMore than one path through network
Increased reliabilityMore 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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Comutare Diviziunea Timpului Se imparte fluxul de intrare în bucăţele mici care
împart un flux de mare viteză e.g. TDM bus switching
1 2 3 4 … 1 2 3 43x
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RoutingMany connections will need paths through
more than one switchNeed to find a route
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 Uses a peer structure for nodes
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Alternate RoutingPossible routes between end offices
predefinedOriginating switch selects appropriate
routeRoutes listed in preference orderDifferent sets of routes may be used at
different times
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Alternate Routing Diagram
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Control Signaling FunctionsAudible communication with subscriberTransmission of dialed numberCall can not be completed indicationCall ended indicationSignal to ring phoneBilling infoEquipment and trunk status infoDiagnostic infoControl of specialist 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 not busy, send ringer signal to target subscriber Feedback to caller
Ringing tone, engaged tone, unobtainable Target accepts call by lifting receiver Switch terminates ringing signal and ringing tone Switch establishes connection Connection release when Source subscriber hangs up
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Switch to Switch SignalingSubscribers connected to different
switchesOriginating switch seizes interswitch trunkSend off hook signal on trunk, requesting
digit register at target switch (for address)Terminating switch sends off hook
followed by on hook (wink) to show register ready
Originating switch sends address
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Control Signals
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Location of SignalingSubscriber to network
Depends on subscriber device and switchWithin network
Management of subscriber calls and network ore complex
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In Channel SignalingUse same channel for signaling and call
Requires no additional transmission facilitiesInband
Uses same frequencies as voice signal Can go anywhere a voice signal can Impossible to set up a call on a faulty speech 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 Need extra electronics Slower signal rate (narrow bandwidth)
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Drawbacks of In Channel SignalingLimited transfer rateDelay between entering address (dialing)
and connectionOvercome by use of common channel
signaling
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Common Channel SignalingControl signals carried over paths independent
of voice channelOne control signal channel can carry signals for
a number of subscriber channelsCommon control channel for these subscriber
linesAssociated Mode
Common channel closely tracks interswitch 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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Signaling Modes
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Signaling System Number 7SS7Common channel signaling schemeISDNOptimized for 64k digital channel networkCall control, remote control, management
and maintenanceReliable means of transfer of info in
sequenceWill operate over analog and below 64kPoint to point terrestrial and satellite links
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SS7 Signaling Network ElementsSignaling point (SP)
Any point in the network capable of handling SS7 control message
Signal transfer point (STP) A signaling point capable of routing control messages
Control plane Responsible for establishing and managing
connectionsInformation 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
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Required ReadingStallings chapter 9ITU-T web siteTelephone company web sites (not much
technical info - mostly marketing)