6.3.1 Circuit switching

The public telephone network is officially known as the public switched telephone network (PSTN). The function of the network is simply to connect the wires of two telephones (or compatible devices such as fax machines or modems), so that sounds coming from one end are transmitted to the other.

This is called a ‘circuit-switched’ (or more simply a ‘switched’) network architecture. The way in which this is done has changed over time – human switchboard operators were replaced by mechanical processes and later by computerised switching equipment; meanwhile, optical (glass) fibre has replaced much of the copper wiring.

Figure 8: circuit-switched (or ‘switched’) network architecture

While this system is very reliable – just think how rarely the system fails to connect you when you have dialled correctly – it is also extremely inefficient and expensive because the connection is made at the beginning of a conversation, fax transmission or modem session, and is maintained until the connection is terminated. This means that a certain portion of the network is reserved exclusively for that conversation whether or not communication is actually taking place at any given moment. If one party puts down the phone handset (i.e. without actually hanging up and breaking the connection) or is silent, or neither computer sends or receives data for a period of time (as is the case when using the internet), the circuit and the ports on the phone switches between the two devices are unavailable for other activity even though they are not being used at that particular moment. Since it is estimated that up to 50 per cent of a typical voice conversation is in fact silence, clearly a tremendous amount of network capacity is wasted. Put another way, a company must build double the network it really needs for a given number of simultaneous calls, at double the cost.