Chapter 3 Crossbar Switching

School of Information Science and Engineering, Shandong UniversityAssociate Prof., Deqiang Wang

Outline

Principles of Common ControlTouch tone dial telephonePrinciples of Crossbar SwitchingCrossbar Switch ConfigurationsCrosspoint TechnologyCrossbar Exchange Organisation

Introduction

Disadvantages of Strawger SwitchingDependence on moving parts and contacts that are subject to wear and tear;Difficulties in maintenance and adjustments.

Switching systems requiring less maintenance and adjustment after installation are expected.

Introduction

Development of Crossbar SwitchingThe first patent, USA, 1915.The Second patent, Sweden, 1919.The first Crossbar Switching system, AT&T, 1938.Common Control subsystems were first introduced in crossbar exchanges.

3.1 Principles of Common Control

Uniform numberingThe same number is dialed, no matter from which exchange the call originates.It is convenient for customer to make phone call to a specific party any where.It is difficult to implement uniform numbering in direct control switching system unless a ‘Director’ is available.

A Multi-exchange Network

A Multi-exchange NetworkPartly connected network (direct control)

A level is reserved in each strawger exchange for outgoing calls.The corresponding outlets are connected to a specific neighbor exchange.

A call from A to FCalled subscriber: 1457Route 1: A-B-C-J-Fdialing sequence: 01-04-03-01-1457Route 2: A-I-H-G-Fdialing sequence: 02-05-01-02-1457

A Multi-exchange Network

Difficulties faced by direct controlIdentification number of a subscriber is route dependent;A user must have knowledge of the topology of the network and the outlet assignments in each exchange.Depending on from which exchange the call originates, the number and its size vary for the same called subscriber.

A Multi-exchange Network

How to overcome these difficulties in a Strawger exchange?

The routing is done by the exchange instead of dialling.A uniform numbering scheme is used.

Uniform Numbering schemeExchange identifier + Subscriber line identifier

Common control

Fundamental featuresA user is assigned a logical number which is independent of the physical line numberused to establish a connection to him.The logical address is translated to actual physical address for connection establishment by an address translation mechanism.Call processing takes place independent of the switching network.

Common control

Diagram of Common Control SubsystemCall processing subsystem

Digit receiver and storage registerRegister SenderInitial translatorFinal translator

Charging circuitsOperation ControlMaintenance circuitsEvent monitor

Common control

Control FunctionsEvent Monitoring

Events at line unitsEvents at Trunk junctorsEvents at interexchange signaling receiver/sender units

Common control

Control FunctionsCall Processing

Digit receiver and storage register: receive and store the dialing number from calling party.Initial translator (office code translator): determine the route for the call through the network, charging method/rate.Final translator (subscriber code translator): determine the line unit to which a call must be connected and category of the called line.Register sender: transfer the route digit and dialed digit using proper signaling.

Common control

Control FunctionsCharging

Depends on the type of subscriberDepends on the service of the subscriber

Operation and MaintenanceControlling of the switching network‘Map-in-memory’ & ‘Map-in-Network’Administration & Maintenanceexchange equipment, subscriber lines, trunks

3.2 Touch Tone Dial Telephone

Disadvantages of Rotary dial TelephoneLow dialing rateLimited usageLimited signaling capacity

Development of Touch tone dial phoneDeveloped in 1950sApplied in 1964.

Touch dial arrangementDTMF: Dual Tone Multi-Frequency

Design Considerations

Choice of codeImitation of code signal by speech and music should be difficult.

Band SeparationEasy to recognize a specific frequencyEasy to regulateReduce the probability of false response

Design Considerations

Choice of frequenciesAttenuation and delay distortion characteristics of the telephone network circuits

Choice of power levelHigh enough for reliabilityPower level should be planned according to attenuation characteristics of the channel

Signaling durationLonger duration is helpful to combat talk-off, but inefficient.Dialing habit

3.3 Principles of Crossbar Switching

Basic ideaTo provide a matrix of (n x m) sets of contacts with only (n + m) activators or less to select one of the (n x m) sets of contacts.

Crosspoint Switching matrixHorizontal & Vertical wiresContact pointsHorizontal & Vertical barsElectromagnets

Crosspoint Switching matrix

Establishment of a connectionScheme1

Step1:Energize horizontal barStep2:Energize vertical barDe-energize horizontal bar

Scheme2Step1:Energize vertical barStep2:Energize horizontal barDe-energize vertical bar

3.4 Crossbar Switch Configurations

Nonblocking Crossbar configurationN2 switching elements for N subscribers.N/2 simultaneous conversations.Crosspoint used depends on the calling ~.

3.4 Crossbar Switch Configurations

Modified Nonblocking SchemeDiagonal Crosspoint MatrixN(N-1)/2 elementsThe number of elements is the same as that of full-connected network.Connection establishment

Energize the corresponding horizontal barEnergize the corresponding vertical bar

3.4 Crossbar Switch Configurations

Comments on nonblockingschemes

Large number of switching elementsDifficult to implement in practiceNot cost-effective

Solution: blocking crossbar switching

3.4 Crossbar Switch Configurations

Blocking crossbar switchesAiming to reduce the number of crosspointswitches. Category: Single stage / MultistageMethodology:

Two subscribers share one vertical barNumber of bars reducedNumber of crosspoint switches remains the same.

All subscribers share a number of vertical barsNumbers of bars and crosspoint switches reduced

Blocking crossbar switchesScheme 1

2NK switchesN denotes the number of subscribersK is the number of simultaneous connections

Operate four bars to establish a connection (e.g. A-B).

Energize horizontal bar AEnergize free vertical bar PDe-energize horizontal bar AEnergize horizontal bar BEnergize free vertical bar P’ (associated with P)De-energize horizontal bar B

Blocking crossbar switches

Scheme 2NK switches

N denotes the number of subscribersK is the number of simultaneous connections

Operate three bars to establish a connection (e.g. A-B).

Energize horizontal bar A and BEnergize free vertical bar PDe-energize horizontal bar A and B

Transfer line supportLocally nonblocking and externally blocking

Transfer line support

Blocking both locally and externally

3.5 Crosspoint Technology

The cost of crossbar system increases in proportion to the number of crosspoints.Challenges

Reduction of the size of a crosspointReduction of the cost of a crosspointImprovement of switching time

3.5 Crosspoint Technology

Category of crosspoint technology

Reed relay crosspoint

3.6 Crossbar Exchange Organization

Basic building blocks of crossbar exchange

Link framesPrimary stage Crossbar switchesSecondary stage Crossbar switchesLinks

Control markersControl the connections between inlets and outlets

Registers

Link Frame

3.6 Crossbar Exchange Organization

Organization of a crossbar exchangeLine unit (two-way units)

Line link framesMarkers and registersOriginating or terminating calls

Group unit (unidirectional)Trunk link frame and associated hardwareHandling local, outgoing, incoming, terminating and transit calls

3.6 Crossbar Exchange Organization

Call processing (three stages)Stage 1

Preselection (performed by originating marker)

Stage 2Group selection (performed by registers, translators and senders)

Stage 3Line selection (performed by terminating marker)

Assignments

Ex. 9Ex.10