MULTIMEDIA UNIVERSITY OF KENYA
ETI 2506 - TELECOMMUNICATION SYSTEMS

CROSSBAR SWITCHING

CONTENTS

1.0 Historical Background

2.0 Introduction to Cross-bar switching

3.0 Cross-bar Switch Configuration

3.1.1 Non-blocking cross-bar exchange

3.1.2 Blocking cross-bar exchange

4.0 Organisation of Cross-bar Telephone Exchange

5.0 Videos

6.0 Recommended References

 


1.0 HISTRORICAL BACKGROUND

A telephony crossbar switch is an electromechanical device for switching telephone calls. The first design of what is now called a crossbar switch was the Bell company Western Electric's "coordinate selector" of 1915. To save money on control systems, this system was organized on the stepping switch or selector principle rather than the link principle. It was little used in America, but the Televerket Swedish governmental agency manufactured an own design (the Gotthilf Betulander design from 1919, inspired by the Western Electric system), and used it in Sweden from 1926 until the digitalization in the 1980s in small and medium sized A204 model switches. The system design used in AT&T's 1XB crossbar exchanges, which entered revenue service from 1938, developed by Bell Telephone Labs, was inspired by the Swedish design but was based on the rediscovered link principle. In 1945, a similar design by Swedish Televerket was installed in Sweden, making it possible to increase the capacity of the A204 model switch.

 Delayed by the Second World War, several millions of urban 1XB lines were installed from the 1950s in the United States. In 1950, the Ericsson Swedish company developed their own versions of the 1XB and A204 systems for the international market. In the early 1960s, the company's sales of crossbar switches exceeded those of their rotating 500-switching system, as measured in the number of lines. Crossbar switching quickly spread to the rest of the world, replacing most earlier designs like the Strowger (step-by-step) and Panel systems in larger installations in the U.S. Graduating from entirely electromechanical control on introduction, they were gradually elaborated to have full electronic control and a variety of calling features including short-code and speed-dialing. In the UK the Plessey Company produced a range of TXK crossbar exchanges, but their widespread rollout by the British Post Office began later than in other countries, and then was inhibited by the parallel development of TXE reed relay and electronic exchange systems, so they never achieved a large number of customer connections although they did find some success as tandem switch exchanges. 

 


2.0 INTRODUCTION

 

In telecommunications, a crossbar switch (also known as cross-point switch, crosspoint switch, or matrix switch) is a switch connecting multiple inputs to multiple outputs in a matrix manner.  Originally the term was used literally, for a matrix switch controlled by a grid of crossing metal bars, and later was broadened to matrix switches in general. It is one of the principal switch architectures, together with a rotating switchmemory switch and a crossover switch.

The basic idea of crossbar switching is to provide a matrix of nx m sets of contacts with only n x m  activators or less to select one of the nxm sets of contacts as shown in Figure 1. This form of switching is also known as coordinate switching as the switching contacts are arranged in a x-y-plane.

Figure 1. Cross-bar switch as an nxm matrix

 

A diagrammatic representation of a practical crosspoint switching matrix is shown in Figure 2. There is an array of horizontal and vertical wires shown by solid lines. A set of vertical and horizontal contact points are connected to these wires. The contact points form pairs, each pair consisting of a bank of three or four horizontal and a corresponding bank of vertical contact points. A contact point pair acts as a cross-point switch and remains separated or open when not in use. The contact points are mechanically mounted (and electrically insulated) on a set of horizontal and vertical bars shown as dotted lines. The bars, in turn, are attached to a set of electromagnets.

Figure 2. Implementation of cross-bar switching matrix


When an electromagnet, say in the horizontal direction, is energised, the bar attached to it slightly rotates in such a way that the contact points attached to the bar move closer to its facing contact points but do not actually make any contact. Now, if an electromagnet in the vertical direction is energised, the corresponding bar rotates causing the contact points at the intersection of the two bars to close. This happens because the contact points move towards each other. As an example, if electromagnets M2 and M3 are energised, a contact is established at the crosspoint 6 such that the subscriber B is connected to the subscriber C.

 In order to fully understand the working of the crossbar switching, let us consider a 6 X 6 crossbar schematic shown in Figure 3.

Figure 3. Cross-bar swith with six inlets and six outlets

 

The schematic shows six subscribers with the horizontal bars representing the inlets and the vertical bars the outlets. Now consider the  establishment of the following connections in sequence: A to C and B to E. First the horizontal bar A is energised. Then the vertical bar C is energised. The crosspoint AC is latched and the conversation between A and C can now proceed. Suppose we now energise the horizontal bar of B to establish the connection B-E, the crosspoint BC may latch and B will be brought into the circuit of A-C. This is prevented by introducing an energising sequence for latching the crosspoints. A crosspoint latches only if the horizontal bar is energised first and then the vertical bar. (The sequence may well be that the vertical bar is energised first and then the horizontal bar). Hence the crosspoint BC will not latch even though the vertical bar C is energised as the proper sequence is not maintained. In order to establish the connection B-E, the vertical bar E needs to be energised after the horizontal bar is energised. In this case, the crosspoint AE may latch as the horizontal bar A has already been energised for establishing the connection A-C. This case should also be avoided and is done by de-energising the horizontal bar A after the crosspoint is latched and making a suitable arrangement such that the latch is maintained even though the energisation in the horizontal direction is withdrpm. The crosspoint remains latched as long as the vertical bar E remains energised. As the horizontal barA is de-energised immediately after the crosspoint AC is latched, the crosspoint AE does not latch when the vertical bar E is energised.


3.0. CROSSBAR SWITCH CONFIGURATIONS

 

3.1 Non Blocking Cross-bar Switch

The switching matices discussed in the preceeding ections are referred to as non-blocking. In a non-blocking crossbar configuration, there are N2 switching elements for N subscribers. When all the subscribers are engaged, only N/2 switches are actually used to establish connections.

 

Table 1 shows the values of different design parameters for four non-blocking switches. Unit cost is assumed for each cross-point switching element. Providing N2 cross-points even for moderate number of users leads to impractical complex circuitry. A 1000-subscriber exchange would require 1 million cross-point switches. Therefore ways and means have to be found to reduce the number of switch contacts for a given number of subscribers.

 

Table 1. Non-blocking cross-bar switch design parameters

3.1.1 The Diagonal Cross-point Matrix

 

It may be observed in the switch matrix of Figure 2 that different switch points are used to establish a connection between two given subscribers, depending upon who initiates the call. For example, when the subscriber C wishes to call subscriber B, crosspoint CB is energised. On the other hand, when B initiates the call to contact C, the switch BC is used. By designing a suitable control mechanism, only one switch may be used to establish a connection between two subscribers, irrespective of which one of them initiates the call. In this case, the crosspoint matrix reduces to a diagonal matrix with N2/2 switches.  A diagonal connection matrix using this principle for 4 subscribers is shown in Figure 4.

Figure 4. Diagonal Cross-point Matrix

The cross-points in the diagonal connect the inlets and the outlet of the same subscriber. This is not relevant. Hence, these are eliminated. The number of crosspoints then reduces to N(N-1)/2. It may be recalled that the quantity N(N-1)/2 represents the number of liiks in a fully connected network. So also, the diagonal crosspoint matrix is fully connected. The call establishment procedure here is dependent on the source and destination subscribers. When subscriber D initiates a call, his horizontal bar is energised first and then the appropriate vertical bar. If subscriber A initiates a call, the horizontal bar of the called party is activated first and then the vertical bar of A.

A diagonal cross-point matrix is a non-blocking configuration. Even N(N-1)/2 cross-point switches can be a very large number to handle practically. The number of cross-point switches can be reduced significantly by designing blocking configurations. These configurations may be single stage or multistage switching networks.

3.1.2 Use of  Double-Swing Bars

 

The crossbar hardware may be reduced by connecting two subscribers to a single bar and letting the bar turn both in the clockwise and the anticlockwise directions, and thus closing two different cross-point contacts. With such an arrangement the number of crossbars reduces, but the number of crosspoint switches remains the same.

Figure 5. Photograph of a cross-bar switch using double swing horizontal bars.

3.2 Blocking Crossbar Switches

 

In blocking crossbar switches, the number of vertical bars is less than the number of subscribers and determines the number of simultaneous calls that can be put through the switch. Consider the 8 X 3 switch shown in Figure 6.

Figure 6. Blocking crossbar switch (8 - 3)

Let a connection be required to be established between the subscribers A and B. First the horizontal bar A is energised. Then one of the free vertical bars, say E: is energised. The crosspoint AP latches. Now if we energise the horizontal bar B, BP will not be latched, as the P vertical is energised before B was energised. In order to be able to connect A to B, we need another vertical crossbar which should electrically'correspond to the vertical bar P. In this case, the bar P' is associated with the same electrical wire as the bar P. When P' is energised after B, the crosspoint BP' is latched and a connection between A and B is established. The sequence to be followed in establishing the A-B circuit may be summarised as:

 

(a) Energise horizontal   A

(b) Energise free vertical   P


4.0 ORGANIZATION OF CROSS-BAR EXCHANGE

 

The basic building blocks of a crossbar exchange are link frames, control markers and registers. Link frames consist of a number of crossbar switches arranged in two stages called primary and secondary with links between them as shown in Figure 7. The two-stage arrangement with links has the effect of increasing the number of outlets for a given number of inlets, thereby providing greater selectivity. The switch, in this case, is said to be expanding. Markers control the connections between the inlets and the outlets via the primary section, links and the secondary section.

Figure 7. Primary and Secondary Links

A simplified organization of a crossbar exchange is shown in Figure 8. The line link frames along with the associated markers and registers are known as line unit, and the trunk link frame with its associated hardware as group unit. The trunk link frame may be subdivided into two or three link frames like local office link frame, incoming link frame, etc. Line units are two-way units, that is to say, they can be used for originating as well as terminating calls. It may be noted that this is a significant departure from the Strowger exchange designs where the originating and terminating units are separate and independent. Because of its two-way capability, the secondary section in the line link frame is sometimes called the terminal section. The subscriber lines are terminated on the outlets of the terminal section frames. The group unit is a unidirectional device. It receives the calls from the line unit or from distant exchanges. It routes the calls to the unit of the same exchange or to distant exchanges. It is capable of handling local, outgoing, incoming, terminating and transit calls.

Figure 8. Organisation of a cross-bar exchange

 

In a crossbar exchange, the call processing progresses in three stages, i.e

(a)  preselection,

(b) group selection, and

(c) line selection.

Preselection, which is performed by the originating marker, starts from the moment the subscriber lifts the handset of the telephone and ends when the dial tone is sent out to him by a register.

In group selection stage, the call is switched through to the desired direction. The direction is decided in accordance with the code given by the translator.

In the line selection stage, the calling subscriber is connected to the called subscriber by the terminating marker. The line of the called party is then monitoredby the terminating marker until either party (calling or called) hungs up.

RECOMENDED REFERENCES (ONLINE)

[1]  Structure of a Switch, Data Communications and Networks, Fourouzan,  Page 227 - 232

[2] Principles of Cross-bar Switching

 


                                                                                                                                                        © 
Prof. Ambani Kulubi May-Aug 2015