Routing of calls is performed by the network in response
to dialling signals from the subscriber telephones connected
to it. The workings of the telephone are described in the
section on the Simple Telephone and methods of dialling are
described in the following paragraphs.
Each connected telephone on the network has a unique address
or telephone number. The number is used to determine the route
the call must take to reach the called party. Figure 2 illustrates
Figure 2 - Routing A Call
For clarity a simplified network has been represented. The
caller dials the number 0617-208-994. The leading 0 tells
the local exchange the call is to go out of the area. the
617 identifies the trunk exchange for the call. The 208 tells
the trunk exchange which of its local exchanges is to get
the call and the final part 994 identifies the destination
telephone. If not busy, ringing is sent to the destination
telephone and ring tone returned to the caller.
In practice the call may cross several layers of the hierarchy
before emerging at its destination. Older exchanges worked
using the stepping process as described above, modern exchanges
wait for all the number to be dialled and then route the call
using the whole number in a single step. This makes call set
up much faster.
Dialling can be either pulse or tone the difference between
these two types of signalling are described in the following
paragraphs, along with ringing and call progress tones.
This is the oldest form of dialling. Being used from the advent
of automatic exchanges at the end of the 1800s it is still
in use today. Most modern equipment can produce or recognise
pulse (or loop disconnect) dialling, however the new tone
dialling system offers speed, flexibility, additional functionality
and service access.
It is a series of breaks in the telephone line circuit which
the exchange recognises as digits. In the original exchanges
the breaks would be created by the stepping of the dial on
the telephone and used to step the electromechanical switches
at the exchange to the select the correct destination line.
In modern equipment a semiconductor would control the breaks
in response to the keypad digits pressed and the exchange
will collect all the digits before selecting the route electronically.
Each number is represented by the same number of pulses. each
pulse lasting 100ms. This 100ms is divided into a 60ms connection
and a 40ms break. Dialling 1 would send one pulse, 2 would
send two pulses and so on to 0 which sends 10 pulses. Between
each digit is an interdigit time of approximately 700ms. Therefore
the time to send digits depends upon the digits dialled. 1
taking 100ms to 0 taking 1sec. Added to this is the interdigit
timer between each digit.
This continual making and breaking of the circuit gives the
characteristic clicking in the earpiece of the dialling telephone.