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.
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
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.
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.
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.