Switching Techniques

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.

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

Technologies

Switching technologies Circuit switching Packet switching Frame relay Asynchronous Transfer Mode (ATM)

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

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

Simple Switched Network

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.

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.

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

Circuit SwitchingDedicated communication path between

two stationsThree phases

Establish Transfer Disconnect

Must have switching capacity and channel capacity to establish connection

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)

Public Circuit Switched Network

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

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

Circuit Switch Elements

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

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

Crossbar Matrix

Multistage SwitchReduced number of crosspointsMore than one path through network

Increased reliability

More complex controlMay be blocking

Three Stage Switch

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

Alternate RoutingPossible routes between end offices,

predefinedOriginating switch selects appropriate

routeRoutes listed in preference orderDifferent sets of routes may be used at

different times

Alternate Routing Diagram

Control Signaling FunctionsAudible communication with subscriberTransmission of dialed numberCall ended indicationSignal to ring phoneBilling infoEquipment and trunk status infoDiagnostic infoControl of specialist equipment

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

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

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

Signaling Modes

Associated ModeCommon channel closely track inter-switch trunks

Disassociated ModeAdditional nodes (signal transfer points), effectively two separate networks

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

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

Transfer Points

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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