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Switching Techniques
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SwitchingDef; The controlling or routing of signals
in circuits to execute logical or arithmetic operations or to transmit data between specific points in a network.
Note: Switching may be performed by electronic, optical, or electromechanical devices.
* Routing: The process of determine and prescribe the path or method to be used for establishing connections or forwarding messages.
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Switching TechniquesDes;In large networks there might be multiple
paths linking sender and receiver. Information may be switched as it travels
through various communication channels. There are many kind of switching
techniques
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Technologies
Switching technologies Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)
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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 network
Data routed by being switched from node to node
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Switching NodesNodes may connect to other nodes only,
or to end stations.Node to node links usually multiplexedNetwork is usually partially connected
Some redundant connections are desirable for reliability
Two main switching technologies Circuit switching Packet switching
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Simple Switched Network
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Circuit Switching Circuit switching is a technique that directly
connects the sender and the receiver in an unbroken path.
Telephone switching equipment, eg; establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection.
once a connection is established, a dedicated path exists between both ends until the connection is terminated.
Routing decisions must be made when the circuit is first established, but there are no decisions made after that time.
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Circuit Switching
Circuit switching in a network operates almost the same way as the telephone system works.
A complete end-to-end path must exist before communication can take place.
The computer initiating the data transfer must ask for a connection to the destination.
Once the connection has been initiated and completed to the destination device, the destination device must acknowledge that it is ready and willing to carry on a transfer.
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Circuit SwitchingDedicated communications path
established for the duration of the conversation
Des; A complete end-to-end path must exist before communication can take place
e.g. telephone 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
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Circuit Switching - ApplicationsInefficient
Channel capacity dedicated for duration of connection
If 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
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Telecomms 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 ElementsDigital Switch
Provide transparent signal path between devices
Network InterfacesControl Unit
Establish connectionsGenerally on demandHandle and acknowledge requestsDetermine if destination is freeconstruct path
Maintain connection Disconnect
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Circuit Switch Elements
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Blocking or Non-blocking circuitsBlocking
A network is unable to connect stations because all paths are in use
A blocking network allows this Used on voice systems
Short duration calls
Non-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
Inefficient use of crosspointsAll 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 reliability
More complex controlMay be blocking
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Three Stage Switch
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RoutingMany connections will need paths through
more than one switchNeed to find a route
Efficiency Resilience/flexibility
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 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 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
Target accepts call by lifting receiver Switch terminates ringing signal and ringing tone Switch establishes connection Connection release when Source subscriber hangs-off
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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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Common Channel Signaling (CSS)Common channel signaling (CCS) is
signaling in which a group of voice-and-data channels share a separate channel that is used only for control signals.
One control signal channel can carry signals for a number of subscriber channels
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Signaling Modes
Associated ModeCommon channel closely track inter-switch trunks
Disassociated ModeAdditional nodes (signal transfer points), effectively two separate networks
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Signaling System Number 7SS7Common channel signaling schemeISDN “Integrated Services Digital
Network”Optimized for 64k digital channel networkCall control, remote control, management
and maintenanceReliable, transfer of info in sequencePoint 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
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
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end