Q. What is Time Switches?

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