Call Routing is defined as a trunk group to carry traffic to a certain destination exchange, or a route is one of the alternative ways to reach another exchange. Thus routing deals with the evaluation of the digits dialed by the calling subscriber, the determination of the direction the call has to be routed, and the selection of the outgoing path, that is connected to the distant exchange. The routing also provides the signaling procedure to take care of both the exchanges, so that they recognize the same signaling language.
If the calling and called parties are not connected to the same exchange, the calling exchange must take part in the correct routing of the call. For this purpose first the called number is analyzed and then the available paths through and outside the exchange are examined to route the call.
The local exchange set up the first choice of trunk group to which the call is routed and which of these is free. If all are busy, then the call is routed to the second-choice trunks, and so on.
The rule used for alternative routing is, "If no direct route is available between a particular center and a destination center, this particular center should first test (in far-to-near order) any possible routes via centers in the next hierarchical level above that of the destination". In case all the trunks are busy, then an appropriate signal (plant-engaged tone) must be sent to the calling party. The same process is done in each exchange when there are a number of exchanges.
Consider an example, in which a subscriber at a terminal exchange (TX 1) 581 wants to dial for TX2 7394. By dialing "0" the subscriber will get access to the NWD. In the PTDC, a route selector stage is arranged to check for routes. In our case, there is a choice of four routes, as given below. The basic network scheme is shown in Figure No.3.13.
As shown in Figure No.1, There is a direct route from PTDC 581 to PTDC 7394. From the destination PTDC 739, the connection is extended to the terminal exchange and then to the required local exchange. In this example, the last digit "4" of the trunk code 7394 is required. While the remaining digits 739 are suppressed in PTDC 581.
- If all trunks of the direct route are busy. Then the connection is established through STDC 73. In this route, the third digit "9" of the trunk code is required to be retransmitted. The PTDC group selector of STDC 73 uses it.
- If all trunks of this route are also busy. Then another alternative route is selected, i.e., the connection is established over the STDC group selector of the MTDC 7. In this case, the last three digits of "394" are required to be retransmitted.
- If all the trunks discussed above are busy, then there is a fourth and last possibility of routing. The PTDC extends the connection to STDC 58. This STDC extends the connection over a direct route to PTDC 739 or over the overflow routes through STDC 73 or MTDC 7. This point represents a second and a last choice alternate routing, i.e., the backbone route over MTDC 5. In this case, all four digits of the trunk code have to be transmitted.
When the called party is in another part of the world. Then the wanted telephone number must be identified in a unique way so that the international telephone network selects that particular number. For this purpose, each country has a numbering scheme with unique area codes. The CCITT has allocated codes to different countries of the world. For example. Pakistan has the country code 92, Australia 61, America 1, etc. If a subscriber in Pakistan wants to make a call to his counterpart outside Pakistan, then he would dial as follows:
The access digits are to tell the NWD network that the call is international. The country code shows the country to which the call is to be routed. The rest of the number i.e., city codes and subscriber's number is the same as in the NWD system.