Trunking Principle

Trunking Principle Points : trunking principle, 100-lines exchange, 1000-lines exchange using subscriber’s uniselector:, 10,000-lines exchange using subscriber’s uniselector The various selector stages of an exchange arc represented by block diagram. The block diagram that shows different equipment in an exchange is called trunking diagram. The trunking principle for 100-lines, 1000-lines and 10,000-lines exchanges are given below: (a) 100-Lines exchange This exchange serves 100 subscribers and the number of digits are 00 to 99. As all subscribers will not use their telephones at the same time. Therefore the electors remain idle for most of the time. For the sale of saving, in practice 20 selectors are used for a group of 100 subscribers. Ten of these act as linefinders and ten as final selectors.

When a subscriber lifts his handset, his d.c loop is completed and the linefinder hunts for the line of the calling subscriber and gets itself connected to it. The subscriber hears a dial tone, which means that the line is ready for dialling. He then dials the two digit number and the final selector makes vertical and horizontal stepping, according to the digits dialled. The connection is established between the calling and called subscribers. The trunking diagram for a 100-lines exchange is shown in the figure.

(b) 1000-Lines exchange using subscriber’s uniselector This exchange serves 1000 subscribers, the number consists of three digits, i.e., 000 to 999.The numbers are divided into 10 groups as given below;
100------199 200------299 300------399 400------499 500------599 600------699 700------799 800------899 900------999 000------099 Each subscriber has a uniselector, which automatically hunts for a free group selector when the subscriber lifts his handset. The first digit selects, the appropriate 100s group while the last two digits give the particular subscriber required. Group selection is achieved by the use of two-motion selectors. A two-motion final selector is connected to each of the 100 outlets of the group selectors. Trunking diagram for a 1000-lines exchange is shown in the figure.

(c) 10,000-Lines exchange using subscriber’s uniselector The 10,000-lines exchange has 4-digits. The trunking principle is an extension of the principle used for 1000- lines exchange. The numbers of group selectors required is 2, i.e., first group selector, which select the first digit and 2nd group selector, which select the 2 digit dialled by the subscriber. The last two digits are selected by the final selector.

In the same way a 5 or 6 digits exchange can be designed. The trunking diagram for a 10,000-lines exchange is shown.