A3.1 Introduction

One of the major factors influencing the development of signalling systems in the past was the relationship between signalling and the control functions of nodes. This Appendix traces the evolution of signalling systems in relation to call control technologies.

A3.2 Step-by-Step Switches

Early telecommunications networks used analogue step-by-step exchanges. In such systems, the exchange constitutes a large number of discrete switches. The switches have physically moving parts that respond directly to the dialled digits from the calling customer to route the call. The control function, or decision-making, is extremely limited and is located in each discrete switch. In concept, this can be shown as the control and switch function being co-located, as illustrated in Fig. A3.1.

Figure A3.1: Step-by-Step Switching

In this type of exchange, when a call is made, the signalling and traffic follow the same path within the node. Step-by-step exchanges are invariably associated with CAS systems. Hence, the signalling and traffic also follow the same path external to the node, i.e. on the transmission link.

A3.3 Separation of Control and Switch Block

The next stage through which exchanges evolved is shown in Fig. A3.2. In this case, the discrete switches of the step-by-step exchange are replaced by an electronic switch block through which calls are routed. The control mechanism in the exchange for setting-up and releasing calls is separated from the switch block. This technique allows much more flexibility in controlling calls and it also reduces costs.

Figure A3.2: Separate Control and Switch Block

Again,...