In the digital telephone the commutation b/w subscriber is establish with the help of digital switches. The commutation b/w subscriber is made simply by establish the Gate connection of the two lines. The Logical Gates and Switching matrix also establish two-way connection b/w subscriber .The connection in the Digital telephone exchange are established by re-arranging the 8-bit PCM word of the different telephone signals dialed by the subscribers. The PCM word at the Sampling frequency (8 KHz) per sec is used for each call duration. The time interval b/w two consecutive samples is125µsec. A series of consecutive 125µsec periods produced in the exchange in the result of sampling process. In this 125µsec period each PCM word is located a Particular Slot. Which corresponding to the channel (time slot) of the pulse frame used in the digital Telephone System.
The Digital Switching is divided in to two main Types.
1. Time Switching
In the Time switching any incoming 8-bit PCM ward can be forwarded to any out-going time slot, the in-coming PCM word are written into the data memory of the time switch and from where they are read-out either in cycle are stored & stored.
In the below Fig. the switching method is shown, using 4-PCM word. Two rotary switches are used, which are cicely controlled, as shown in below Fig, the In-coming PCM ward are feed continuously into the location of the data memory with in coming time slots. The PCM word are read-out from the data memory according to the information requested by the call, At the data memory o/p, the control addresses of the control memory and the out-going time slots. The rotary switches are control by the control memory at the data by the control memory at the data memory o/p is as following.
Out-going time slot tA1 from data memory location 3
Out-going time slot tA2 from data memory location 1
Out-going time slot tA3 from data memory location 4
Out-going time slot tA4 from data memory location 2
2. Space Switching
The space switching operates with out inter changing time slot. The 8-bit PCM word retains their time slot but can change their high ways b/w I/P and O/P. As the original time slot during and after switching is the same thus there is no delay of time. the principle of space switching is shown in fig: below, only 3 I/P and O/P high ways with 4-PCM word per 125µsee have taken to simplify the total operation . Also crossing points in the switching network is shown (AND GATE). The connection for each time slot of the in-coming high ways is made through the AND GATE. The and gate operates 800 times per sec for each connection.
SPACE - TIME SWITCHING
The space-time switch works on the same principles as the time switch. One way in which the operating speed and the number of simultaneous connections can be increased is the space-time switching. This is a high-speed variation of the time switch. The PCM word from one of several incoming highways can be forwarded to any time slot of one of the several outgoing highways. The incoming highways are extended to the data memory through a multiplexer. Due to this, the bit rate of the highways from multiplexer to data memory is several times higher than that of the incoming highways.
Conside r an example shown in the figure No.5.4. In the figure shown there are four incoming highways. Thus the bit rate of the highway from multiplexer to the data memory is four times greater than the bit rate of a single incoming highway. The demultiplexer operates at the original bit rate to distribute the PCM words to four outgoing highways. It has full availability without blocking.