Nation Calling Dialling System (NCD)

Nation Wide Dialling System (NWD) Points : nation wide dialling system (nwd), Nationwide Calling System Nation Wide dialling System (NWD was introduced in Pak among a few cities in 1976. This system has now extensive to above 200 cities in country. The advantage of NWD system is that it is a multi-choice system. If one way is busy, it directs call during some other alternative mule. In this system dissimilar cities are linked jointly by microwave links. Two frequencies are use for communication, one for talking and the other for listen. The microwaves use is of 4 GHz range. Ii is the possessions of a wave that higher its frequency, the passageway of propagation becomes nearer to that of light i.e., straight. Thus the microwaves antennas of transmitters and receivers must lie in a line of sight. For connecting longer distances, repeater stations are use. Active repeaters are use to intensify usual signal and to transmit it by greater power. For altering the direction of propagation passive reflectors are used. In-spite of the fact that these reflectors absorb some of the energy but they are extensively use in microwave network.

By help of this system, any exacting town can be reach as of any other town through the similar prefix. In this system country has be dividing into Main Trunk dialling Centres (MTDC), which have been sub divided into Secondary Trunk Dialling Centres (STDC) and further into Primary Trunk Dialling Centres (PTDC). In this system, ten STDC’s are connected to every MTDC and ten PTDC’s are connected to every STDC and similarly each PTDC has ten Terminal or Local exchanges attached to it. This system works on decimal selection i.e., decimal dialling. By the help of this system, it is possible to reach any terminal or local exchange by dialling a 4-digit prefix. By dialling the digit ‘0’, one is attached to the NWD network and then follow by the trunk code (4-digit number) of the want terminal exchange. First digit of which communicate to a MTDC, the second to the STDC, the third to PTDC and the fourth digit to the terminal or local exchange. Thus in the decimal selection, each area can be divided into not more than 10 smaller areas.