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A switch or a telephone exchange has a basic task to perform - switch communication paths. Now, the switch could be one that has subscribers connected to it. You must be saying, all switches have subscribers connected to them. The point is, there are switches in any telephone network, that do not have subscribers connected to them. Instead they have other switches connected to them. So, in a sense, this switch is 'a switch of switches', performing switching for various other switches which may or may not be directly connected to each other. So why is this done ? The answer to this is an extension of our justification for the need for switching in the first place. It goes like this.
If there are 50 switches in an area of 2,500 square kilometres, it is un-economical to connect every switch to every other switch through direct trunks, just like it is un-economical to connect every subscriber to every other subscriber. So, in such a case, all the switches are connected to one switch somewhere roughly in the middle of the area, and this switch has only other switches as subscribers. This switch is called a tandem or a transit switch. It is also sometimes called a TAX, which stands for trunk automatic exchange. This tandem switch connects two switches which are not directly connected to each other. So in a metropolitan area like Madras, there will be one tandem exchange connecting all subscriber switches ( end-offices ) to each other. In fact, there might be more than one tandem exchange for resilience / redundancy. This tandem might also have trunks from a similar tandem in Bombay and Calcutta, thus making it the point where the Madras network is connected to the rest of the country. So this tandem is both an intra-city and inter-city TAX. All its incoming and outgoing ports will be trunks to other exchanges, and never subscriber loops. This means that the exchange will not have software for subscriber related functions running, simply because there are no normal subscribers. Instead, the switch will run software customised for its operation as a tandem. So the same switch with the same hardware can perform either as an end-office ( serving conventional phone subscribers ) or as a transit switch ( connecting other switches ). Depending on which switches the tandem connects, it is classified as an intra-city, or an inter-city or an international tandem. One tandem could simultaneously do all above three functions too.
But if there were two exchanges in the area having a constant stream of traffic between them ( say two exchanges in the business district ), this fact could warrant the use of direct trunks between the two switches, bypassing the tandem. In fact, in such as case, the links between the two exchanges and the tandem would serve as backup links to be used if the direct link failed. In Madras, many pairs of exchanges have direct trunks between them, but are all connected to the tandem. In fact, there are, to my knowledge, 4 tandem exchanges in Madras. And these tandem exchanges are only intra-city and inter-city tandems. International calls are routed by these tandems to the VSNL international gateway tandem in Madras, which has direct trunks with counterpart tandems in many places over the world.
So, the different switch configurations are.....
manisridhar at hotmail dot com
