Chapter 3 Crossbar Switching

School of Information Science and Engineering, Shandong University Associate Prof., Deqiang Wang Outline

† Principles of Common Control † Touch tone dial telephone † Principles of Crossbar Switching † Crossbar Switch Configurations † Crosspoint Technology † Crossbar Exchange Organisation Introduction

† Disadvantages of Strawger Switching „ Dependence 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 Switching „ The 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 numbering „ The 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 Network

† Partly 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 F „ Called subscriber: 1457 „ Route 1: A-B-C-J-F dialing sequence: 01-04-03-01-1457 „ Route 2: A-I-H-G-F dialing sequence: 02-05-01-02-1457 A Multi-exchange Network

† Difficulties faced by direct control „ Identification 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 scheme „ Exchange identifier + Subscriber line identifier Common control † Fundamental features „ A user is assigned a logical number which is independent of the physical line number used 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 Subsystem „ Call processing subsystem ¾ Digit receiver and storage register ¾ Register Sender ¾ Initial translator ¾ Final translator „ Charging circuits „ Operation Control „ Maintenance circuits „ Event monitor

Common control † Control Functions „ Event Monitoring ¾ Events at line units ¾ Events at Trunk junctors ¾ Events at interexchange signaling receiver/sender units Common control

† Control Functions „ Call Processing ¾ Digit receiver and storage register: receive and store the dialing number from calling party. ¾ Initial translator (office code translator): determine the route for the call through the network, charging method/rate. ¾ Final translator (subscriber code translator): determine the line unit to which a call must be connected and category of the called line. ¾ Register sender: transfer the route digit and dialed digit using proper signaling. Common control † Control Functions „ Charging ¾ Depends on the type of subscriber ¾ Depends on the service of the subscriber „ Operation and Maintenance ¾ Controlling of the switching network ‘Map-in-memory’ & ‘Map-in-Network’ ¾ Administration & Maintenance exchange equipment, subscriber lines, trunks 3.2 Touch Tone Dial Telephone † Disadvantages of Rotary dial Telephone „ Low dialing rate „ Limited usage „ Limited signaling capacity † Development of Touch tone dial phone „ Developed in 1950s „ Applied in 1964. † Touch dial arrangement „ DTMF: Dual Tone Multi-Frequency

Design Considerations

† Choice of code ¾ Imitation of code signal by speech and music should be difficult. † Band Separation ¾ Easy to recognize a specific frequency ¾ Easy to regulate ¾ Reduce the probability of false response Design Considerations

† Choice of frequencies ¾ Attenuation and delay distortion characteristics of the telephone network circuits † Choice of power level ¾ High enough for reliability ¾ Power level should be planned according to attenuation characteristics of the channel † Signaling duration ¾ Longer duration is helpful to combat talk-off, but inefficient. ¾ Dialing habit

3.3 Principles of Crossbar Switching † Basic idea „ To 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 matrix „ Horizontal & Vertical wires „ Contact points „ Horizontal & Vertical bars „ Electromagnets

Crosspoint Switching matrix

† Establishment of a connection „ Scheme1 ¾ Step1:Energize horizontal bar ¾ Step2:Energize vertical bar ¾ De-energize horizontal bar „ Scheme2 ¾ Step1:Energize vertical bar ¾ Step2:Energize horizontal bar ¾ De-energize vertical bar 3.4 Crossbar Switch Configurations

† Nonblocking Crossbar configuration „ N2 switching elements for N subscribers. „ N/2 simultaneous conversations. „ Crosspoint used depends on the calling ~.

3.4 Crossbar Switch Configurations

† Modified Nonblocking Scheme „ Diagonal Crosspoint Matrix „ N(N-1)/2 elements „ The number of elements is the same as that of full-connected network. „ Connection establishment ¾ Energize the corresponding horizontal bar ¾ Energize the corresponding vertical bar

3.4 Crossbar Switch Configurations † Comments on nonblocking schemes „ Large number of switching elements „ Difficult to implement in practice „ Not cost-effective † Solution: „ blocking crossbar switching 3.4 Crossbar Switch Configurations † Blocking crossbar switches „ Aiming to reduce the number of crosspoint switches. „ Category: Single stage / Multistage „ Methodology: ¾ Two subscribers share one vertical bar 9 Number of bars reduced 9 Number of crosspoint switches remains the same. ¾ All subscribers share a number of vertical bars 9 Numbers of bars and crosspoint switches reduced Blocking crossbar switches

† Scheme 1 „ 2NK switches ¾ N denotes the number of subscribers ¾ K is the number of simultaneous connections „ Operate four bars to establish a connection (e.g. A-B). ¾ Energize horizontal bar A ¾ Energize free vertical bar P ¾ De-energize horizontal bar A ¾ Energize horizontal bar B ¾ Energize free vertical bar P’ (associated with P) ¾ De-energize horizontal bar B

Blocking crossbar switches † Scheme 2 „ NK switches ¾ N denotes the number of subscribers ¾ K is the number of simultaneous connections „ Operate three bars to establish a connection (e.g. A-B). ¾ Energize horizontal bar A and B ¾ Energize free vertical bar P ¾ De-energize horizontal bar A and B

Transfer line support

† Locally 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 crosspoint „ Reduction of the cost of a crosspoint „ Improvement 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 frames ¾ Primary stage Crossbar switches ¾ Secondary stage Crossbar switches ¾ Links „ Control markers ¾ Control the connections between inlets and outlets „ Registers Link Frame 3.6 Crossbar Exchange Organization

† Organization of a crossbar exchange „ Line unit (two-way units) ¾ Line link frames ¾ Markers and registers ¾ Originating or terminating calls „ Group unit (unidirectional) ¾ Trunk link frame and associated hardware ¾ Handling local, outgoing, incoming, terminating and transit calls

3.6 Crossbar Exchange Organization † Call processing (three stages) „ Stage 1 ¾ Preselection (performed by originating marker) „ Stage 2 ¾ Group selection (performed by registers, translators and senders) „ Stage 3 ¾ Line selection (performed by terminating marker) Assignments

† Ex. 9 † Ex.10