Q. Describe the time division switching?

Ans:

Space and Time Switching:

Space Switches: Connections can be made between outgoing and incoming PCM highways by means of a cross point matrix of the form displayed in Figure. Thoughdifferent channels of an incoming PCM frame may need to be switched by various cross points in order to reach different destinations. The cross point is hence a two-input AND gate. One input is connected to the incoming PCM highway and the other to a connection store that produce a pulse at the required instant. A group of cross points gates can be implemented as an integrated circuit, for instance by using a multiplexer chip.

Figure demonstrates a space switch with k incoming and m outgoing PCM highways, each one carrying n channels. Connections store for every column of cross points is a memory with an address location for every time-slot that stores number of the cross points to be operated in that time slot. This number is written into address by controlling processor in order to setup connection. Numbers are read out cyclically, in synchronism with incoming PCM frame. In every time slot, number stored at the corresponding store address is read out and decoding logic converts this into a pulse or a single lead to operate relevant cross point.

As a cross point can make a different connection in each of the n time-slots, its equivalent to n cross points in a space division network. Complete space switch is hence equivalent to n separate k x m switches in a space division switching network.