Question

Through two block diagrams explain the difference between Space division and time division switching.

Answer 1

 Space and Time Switching:

Space Switches: Connections can be made between incoming and outgoing PCM highways by means of a cross point matrix of the form shown in Fig. However, different channels of an incoming PCM frame may need to be switched by different cross points in order to reach different destinations. The cross point is therefore 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 example by using a multiplexer chip.  

Fig. shows a space switch with k incoming and m outgoing PCM highways, Each carrying n channels. The connections store for each column of cross points is a memory with an address location for each time-slot, which stores the number of the cross points to be operated in that time slot. This number is written into the address by the controlling processor in order to setup the connection. The numbers are read out cyclically, in synchronism with the incoming PCM frame. In each time slot, the number stored at the corresponding store address is read out and decoding logic converts this into a pulse or a single lead to operate the relevant cross point. Since a cross point can make a different connection in each of the n time-slots, it is equivalent to n cross points in a space division network. The complete space switch is thus equivalent to n separate k x m switches in a space division switching network.

Time Switches: The principle of a time switch is shown in Fig. It connects an incoming n channel PCM highway to an outgoing n channel PCM highway. Since any incoming channel can be connected to any outgoing channel, it is equivalent to space division cross point matrix with n incoming and n outgoing trunks, as shown in Fig. Time-slot interchange is carried out by means of two stores, each having a storage address for every channel of the PCM frame. The speech store contains the data of each of the incoming time-slots (i.e. its speech sample) at a corresponding address. Each address of the connection store corresponds to a time slot on the outgoing highway. It contains the number of the time-slot on the incoming highways whose sample is to be transmitted in that outgoing time-slots. Information is read into the speech store cyclically in synchronism with the incoming PCM systems; however, random access read out is used .The connection store has cyclic read out, but writing in is non cyclic. To establish a connection, the number (X of the time-slot of an incoming channel is written into the connection store at the address corresponding to the selected outgoing channel (Y ).During each cyclic scan of the speech store ,the incoming PCM sample from channel X is written into address X. During each cyclic scan of the connection store, the number X is read out at the beginning of time-slot Y. This is decoded to select address X of the speech store, whose contents are read out and sent over the outgoing highway.