The Nation Wide Dialing System ( NWD ) was introduced in Pakistan between a few cities in 1976. This system has now extended to all over the country. The advantage of NWD system is that it is a multi-choice system. If one route is busy, it-directs the call through some other alternative route.In this system different cities are connected together by microwave links. Two frequencies are used for communication, one for talking and the other for listening. The microwaves used are of 4 GHz range. It is the property of a wave that higher its frequency, the path 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 used. Active repeaters are used to amplify the received signal and to transmit it with greater power. For changing 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 used in microwave networks.
With the help of this system, any particular town can be reached from any other town by the same prefix. In this system the country has been divide into Main Trunk dialing Centers (MTDC), which have been sub divided into Secondary Trunk Dialing Centers (STDC) and further into Primary Trunk Dialing Centers (PTDC). In this system, ten STDC's are connected to every MTDC and ten PTCC's are connected to every STDC and similarly each PTDC has ten Terminal or Local exchanges connected to it. This system works on decimal selection i.e., decimal dialing.
With the help of this system, it is possible to reach any terminal or local exchange by dialing a 4-digit prefix. By dialing the digit '0', one is connected to the NWD network and then followed by the trunk code (4-digit number) of the wanted terminal exchange. The first digit of which corresponds 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. The block diagram of such a system is shown in the figure.
NWD NETWORK OPERATION
In Pakistan, in the NWD network a center or an exchange is connected with the respective higher-ranking center, which results in a star or radial connection. The MTDC's have no higher-ranking center but are inter-connected among themselves by means of a complete mesh network.
In the NWD system each area is clearly defined by a number. This number has a single digit for the main areas, while the terminal exchanges have four digit numbers, called trunk code or exchange code. By dialing the trunk code prior to the subscriber's number will connect the calling subscriber to the required local exchange. The first digit of the trunk code indicates the main area in which the required local exchange lies. The second digit indicates the secondary area, the third digit indicates the primary area and the fourth digit indicates the local exchange.
The MTDC's, STDC's and PTDC's are usually placed in the larger communities. Therefore the local networks coincide geographically with their higher-ranking centres and are called concealed centres. All others which do not coincide geographically with their higher-ranking centres are called open or non-concealed centres. This makes it possible to reduce the total number of digits in the trunk code for cities with large populations having lengthy subscriber's numbers. Thus the terminal exchange at the site of the main centre, as well as the terminal •exchange at the site of the secondary centre are reached under an abbreviated code.
There are two methods, in which the trunk codes and subscriber's numbers are li>k\i. One is called open or self-contained numbering, in which the trunk code and subscriber's numbers arc clearly separated and hence open. This means that if a person wants to make a long distance call, he dials "0" to indicate to his local exchange that the digits following are the trunk prefix, after which he dials the trunk prefix, followed by the subscriber's number.
The other is closed or linked numbering, in which only a single number consists of trunk code and the subscriber's number, is assigned. In other words, the trunk code is included in the number and has to be dialed even for local calls. The block diagram of NWD network, which is a combined mesh-star network, is shown in the figure. All the main centres are mesh connected, while all secondary, primary and terminal exchanges are star/radially connected to their respective higher-ranking centres. Wherever the traffic justifies, some of the primary and secondary centres are also connected over direct routes to reduce load on the switching equipment at their respective higher-ranking centres.