Teletraffic Studies of the TXE2 Electronic Exchange

POEEJ Vol 67 Pt 2 July 1974 txe2.pdf

Introduction

The TXE2 electronic switching system was developed under the auspices of the British Joint Electronic Research Committee formed in 1956 to coordinate electronic exchange development between the British Post Office (B.P.O.) and the five principal British manufacturers of telephone switching equipment at that time. It was designed as a local exchange for approximately 200-2,000 subscribers' lines, offering the full range of subscribers' facilities.

Limited traffic studies were carried out during the original development stage and, although these indicated that the traffic-carrying capacity of the links in the subscribers' traffic

concentrating stage could possibly be higher, a limit of 7·5 erlangs per 25 A-B links was accepted. When more extensive computer facilities and manpower became available, a more detailed teletraffic study was initiated, including the investigation of proposals for enlarged-capacity TXE2 exchanges as well as a further study of the traffic loading of the A-B links.

For a developed switching system, the main objectives of a teletraffic study, as carried out by the Teletraffic Division of the B.P.O. Telecommunications Headquarters, are to evaluate

the traffic-handling capabilities of the system, establish dimensioning standards and procedures for exchange design and management, and, by highlighting those parts of the

system which govern its traffic capacity, to offer advice on ways of improving its traffic characteristics.

Teletraffic studies mainly rely on the application of two techniques, namely, mathematical analysis and simulation.

To a large extent, they are complementary, but, in some areas of the work, the choice of technique used is determined by such factors as the time available for the study, or the degree of accuracy that is acceptable in a numerical solution. These techniques have recently been discussed more fully in an earlier issue of this Journal.1

TXE2 SYSTEM DESCRIPTION

The TXE2 system is a sn1all electronic exchangc,2 which employs a multistage switching network of co-ordinate rcedrelay switches. Jnfcrmation relating to a particular call is stored in a register during the setting up of a connexion, and all registers have access to a common-control unit which supervises all connexions made in the switching area.

Switching Area

In the TXE2 switching area, subscribers are connected to three A-switches which have access to five B-switches, three A-switches with their associated B-switches being known as a major. The originating route supervisory units (outgoing or own-exchange), which handle calls from subscribers, are connected to the C-switches as shown in Fig, 1. When a subscriber originates a call, the first path allocated to him through the switching area is via the A- B- and C-switches to either a first-choice or a second-choice supervisory ullit. The first-choice supervisory units are connected to the circuits of the most extensively used route, either outgoing or own exchange. If the own-exchange route is not the first-choice, route, it becon1cs the second-choice, but, if it is the first-choice route, the most highly used outgoing route is made the second choice route.

The selected first- or second-choice supervisory unit has access to the register which has been allocated to that subscriber. When the register has sufficient information, it requests the control to carry out a route-change if the dialled digits indicate that the subscriber requires a routing other than the one which has been allocated. If an own-exchange call is required, the subscriber is routed to an Own exchange supervisory unit whose outlet is connected to a D-switch, and, when the wanted-subscriber's number has been received by the register, the call continues as if it were an incoming call.

An incoming call arrives on an incoming supervisory unit, connected directly to a D-switch, Which has five C-D links to five C-switches. When the register allocated to the call has received the wanted-subscriber's number, the control can test the paths available and then establish a path through the switching area to the called subscriber.

Fig. 2 - Block Diagram of the TXE2 exchange control Area

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