Which techniques are utilized to gain on the number pairs and also explain how it helps in connecting number of subscribers.

In rural areas, subscribers are usually dispersed. This is both unessential and expensive to give a dedicated pair for each subscriber. Three techniques are utilized to gain on the number pairs are as following:

1. Parity lines

2. Concentrators

3. Carrier Systems.

In first technique, two or more subscribers are linked to the one line that is known as parity line. This scheme is not utilized commonly as this has a number of drawbacks. Only one subscriber at a time can utilize the line. Selective ringing is not easy and privacy is not preserved. Dialing in between two subscribers on similar line is not possible.

In second technique, a concentrator expander (CE) is utilized near the cluster of users and other one at the exchange end as demonstrated in figure.

Only some junction lines are run among the CEs that have switching ability. Classically, a ratio of 1:10 in between the subscriber lines and the junction lines is used. The CE at the exchange end remotely powers and controls the concentrator expander at the subscriber end.

FIG - Concentrator-expander connection for dispersed subscribers.