Data Communications

Circuit Switching

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

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 technologiesCircuit switchingPacket switching

Simple Switched Network

Circuit SwitchingDedicated communication path between

two stationsThree phases

EstablishTransferDisconnect

Must have switching capacity and channel capacity to establish connection

Must have intelligence to work out routing

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

Telecomm ComponentsSubscriber

Devices attached to network

Local LoopSubscriber loopConnection to network

ExchangeSwitching centersEnd office - supports subscribers

TrunksBranches between exchangesMultiplexed

Circuit Switch Elements

Circuit Switching ConceptsDigital Switch

Provide transparent signal path between devices

Network InterfaceControl Unit

Establish connectionsGenerally on demandHandle and acknowledge requestsDetermine if destination is freeconstruct path

Maintain connectionDisconnect

Blocking or Non-blockingBlocking

A network is unable to connect stations because all paths are in use

A blocking network allows thisUsed on voice systems

Short duration calls

Non-blockingPermits all stations to connect (in pairs) at onceUsed for some data connections

Space Division SwitchingDeveloped for analog environmentSeparate physical pathsCrossbar switch

Number of crosspoints grows as square of number of stations

Loss of crosspoint prevents connectionInefficient use of crosspoints

All 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

Time Division SwitchingPartition low speed bit stream into pieces

that share higher speed streame.g. TDM bus switching

based on synchronous time division multiplexingEach station connects through controlled gates

to high speed busTime slot allows small amount of data onto busAnother line’s gate is enabled for output at the

same time

RoutingMany connections will need paths through

more than one switchNeed to find a route

EfficiencyResilience

Public telephone switches are a tree structureStatic routing uses the same approach all the time

Dynamic routing allows for changes in routing depending on trafficUses a peer structure for nodes

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 can not be completed indicationCall ended indicationSignal to ring phoneBilling infoEquipment and trunk status infoDiagnostic infoControl of specialist equipment

Control Signal SequenceBoth phones on hookSubscriber lifts receiver (off hook)End office switch signaledSwitch responds with dial toneCaller dials numberIf target not busy, send ringer signal to target subscriberFeedback to caller

Ringing tone, engaged tone, unobtainable

Target accepts call by lifting receiverSwitch terminates ringing signal and ringing toneSwitch establishes connectionConnection release when Source subscriber hangs up

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 readyOriginating switch sends address

Control Signals

Location of SignalingSubscriber to network

Depends on subscriber device and switch

Within networkManagement of subscriber calls and networkore complex

In Channel SignalingUse same channel for signaling and call

Requires no additional transmission facilities

InbandUses same frequencies as voice signalCan go anywhere a voice signal canImpossible to set up a call on a faulty speech path

Out of bandVoice signals do not use full 4kHz bandwidthNarrow signal band within 4kHz used for controlCan be sent whether or not voice signals are presentNeed extra electronicsSlower signal rate (narrow bandwidth)

Drawbacks of In Channel SignalingLimited transfer rateDelay between entering address (dialing)

and connectionOvercome by use of common channel

signaling

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 ModeAdditional nodes (signal transfer points)Effectively two separate networks

Common v. In Channel Signaling

Signaling Modes

Signaling System Number 7SS7Most widely used common channel signaling

schemeInternationally standardized and general

purpose

SS7SS7 network and protocol used for:

Basic call setup, management, tear downWireless services such as PCS, roaming,

authenticationToll free and toll (900) wireline servicesEnhanced features such as call forwarding, caller

ID, 3-way callingEfficient and secure worldwide

telecommunications

SS7 messages are exchanged between network elements, or signaling points, over 56 K or 64 Kbps signaling links.

SS7 Signaling Network ElementsSignaling point (SP)

Any point in the network capable of handling SS7 control messages (the user entry point into SS7)

Signal transfer point (STP)A signaling point (packet switch) capable of

routing control messages (an SS7 signal switching point)

Control planeResponsible for establishing and managing

connections

Information planeOnce a connection is set up, info is transferred in

the information plane

Transfer Points

SS7 CharacteristicsSPs are switches that send signaling

messages to SPs to setup, manage, and release voice circuits

An SP may also send a query message to a centralized database (an SCP) to determine how to route a call (e.g. a toll-free number)

Because the SS7 network is critical to call processing, SCPs and STPs are usually deployed in mated pair configurations in separate physical locations

Links between signaling points are also in pairs

Review Questions1. What is the difference between node and

station?2. What are the advantages of a circuit

switched network? Disadvantages?3. What is a non-blocking switch?4. What is a crossbar? A multi-stage switch?

A time division switch?5. What is the difference between in

channel signaling and common channel signaling?

6. What are the basic components of SS7?