5.2 - Routing

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 this process.

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.

5.2.1 - Pulse Dialling

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.