The analogue network or Public Switched Telephone Network (PSTN) still used by the majority of subscribers has developed over the years to include an increasing proportion of digital equipment and is a combination of digital and analogue technology. It will be referred to as analogue to distinguish it from the fully digital services which are becoming more widely used. The explanations below have been simplified and intended to give an overview which is accessible to all readers. For a more in depth explanation further reading will be required.

5.1 - Structure

5.1.1 - Exchange Network

The network has been organised on a hierarchical basis with the size and traffic handling capacity of exchanges being increased at higher levels. For purposes of illustration a three level hierarchy will be discussed here.

Figure 1 - The Three Layer Network Hierarchy

In Figure 1 a three level hierarchy is shown. The local exchanges are connected to the subscribers below them and to the larger concentrator exchanges above them. Each concentrating exchange will service many local exchanges below it and pass traffic on to the trunk exchanges above it. The trunk exchanges will then route calls to the appropriate trunk exchanges before passing the traffic back down through the levels until the local exchange makes the dialled telephone ring. The different functions may be housed in the same building, even the same physical rack. The distinction is more conceptual one than a physical one.

As the traffic is routed higher up the levels, higher capacity trunks are used (denoted by the thicker connecting lines) to carry the increased number of calls. These are now digital fibre optic links using Time Division Multiplexing (TDM) to merge many calls into a single connection. The larger the connection capacity the more calls can be carried.

The route for any given call will depend upon the distance. Local calls may take place within the local exchange and route back down to the dialled telephone if the call is nearby or may route to the concentrator and back out to a nearby local exchange to its destination. The longer the distance the call the higher up the hierarchy it will be passed.

5.1.2 - Subscriber Lines

Below the local exchanges are the subscriber lines. these will be connected to individual telephones or customer telephone systems. They are commonly twisted pair copper wire 0.5mm diameter for each line. each line can carry a single call.

5.1.3 - Party Lines

Party lines are rare today and will not be found very often. With party line connection two telephones are each connected to the same copper pair from the local exchange. When one of the telephones is taken off hook (the handset picked up) one wire of the pair is earthed and the local exchange can thus determine which of the connected devices it is. The call can then be made in the usual way. Whilst one of the telephones is using the line the other cannot make or receive calls.