Chapter 3 Crossbar Switching. School of Information Science and Engineering, Shandong University Associate Prof., Deqiang Wang

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1 Chapter 3 Crossbar Switching School of Information Science and Engineering, Shandong University Associate Prof., Deqiang Wang

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

3 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.

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

5 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.

6 A Multi-exchange Network

7 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: Route 2: A-I-H-G-F dialing sequence:

8 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.

9 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

10 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.

11 Common control Diagram of Common Control Subsystem Call processing subsystem Digit receiver and storage register Register Sender Initial translator Final translator Charging circuits Operation Control Maintenance circuits Event monitor

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

14 Common control Control Functions Call 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.

15 Common control Control Functions Charging Depends on the type of subscriber Depends on the service of the subscriber Operation and Maintenance Controlling of the switching network Map-in-memory & Map-in-Network Administration & Maintenance exchange equipment, subscriber lines, trunks

16 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 Touch dial arrangement DTMF: Dual Tone Multi-Frequency

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

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

22 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

24 Crosspoint Switching matrix Establishment of a connection Scheme1 Step1:Energize horizontal bar Step2:Energize vertical bar De-energize horizontal bar Scheme2 Step1:Energize vertical bar Step2:Energize horizontal bar De-energize vertical bar

25 3.4 Crossbar Switch Configurations Nonblocking Crossbar configuration N 2 switching elements for N subscribers. N/2 simultaneous conversations. Crosspoint used depends on the calling ~.

27 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 Energize the corresponding horizontal bar Energize the corresponding vertical bar

29 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

30 3.4 Crossbar Switch Configurations Blocking crossbar switches Aiming to reduce the number of crosspoint switches. Category: Single stage / Multistage Methodology: Two subscribers share one vertical bar Number of bars reduced Number of crosspoint switches remains the same. All subscribers share a number of vertical bars Numbers of bars and crosspoint switches reduced

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

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

35 Transfer line support Locally nonblocking and externally blocking

36 Transfer line support Blocking both locally and externally

37 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

38 3.5 Crosspoint Technology Category of crosspoint technology

39 Reed relay crosspoint

40 3.6 Crossbar Exchange Organization Basic building blocks of crossbar exchange Link frames Primary stage Crossbar switches Secondary stage Crossbar switches Links Control markers Control the connections between inlets and outlets Registers

41 Link Frame

42 3.6 Crossbar Exchange Organization Organization of a crossbar exchange Line unit (two-way units) Line link frames Markers and registers Originating or terminating calls Group unit (unidirectional) Trunk link frame and associated hardware Handling local, outgoing, incoming, terminating and transit calls

44 3.6 Crossbar Exchange Organization Call processing (three stages) Stage 1 Preselection (performed by originating marker) Stage 2 Group selection (performed by registers, translators and senders) Stage 3 Line selection (performed by terminating marker)

45 Assignments Ex. 9 Ex.10

AE64 TELECOMMUNICATION SWITCHING SYSTEMS JUNE Q2 (a) Write short notes on (a) uniselector and (b) two motion selector.

AE64 TELECOMMUNICATION SWITCHING SYSTEMS JUNE Q2 (a) Write short notes on (a) uniselector and (b) two motion selector. Q2 (a) Write short notes on (a) uniselector and (b) two motion selector. Uniselectors. A uniselector is a one which has a single rotary switch with a bank of contacts. Depending upon the number of switching