Digital Switching

Digital Switching Points : Digital Switching, Time Switching, Space Switching, Space Time Switching In the digital exchanges, the connection between two subscribers is established with the help of digital switches. With the introduction of these switches, the bulky expensive mechanical contacts points operated by an equally bulky, expensive and power consuming, electromagnets have been replaced by digital gates. A connection between two subscribers is made, simply by enabling the gate connecting the two lines. Also, logic gates are unidirectional; two paths through a switching matrix must be established to accommodate a two-way communication.

The digital techniques are equally advantageous for both signal transmission and switching. These advantages can be fully enjoyed if both the transmission systems and exchanges used digital techniques with TDM.

The connections in the digital exchanges are established by rearranging the 8- bit PCM words of different telephone signals dialed by the subscribers. The PCM words at the sampling frequency (8000 Hz) are forwarded per second per call direction. The time interval between two consecutive samples is 125 μs. A series of consecutive 125 μs periods thus results in the exchange. In this 125 μs periods each PCM word is allocated a particular time slot, which correspond to the channel time slots of the pulse frames used in the digital transmission systems.

The digital switching are grouped into the following two basic types:
a. Time Switching
b. Space Switching a. Time Switching In the lime switch, any incoming 8-bit PCM word can be forwarded to any outgoing time slot, i.e., there is a full availability and is also non-blocking. The incoming PCM words are written into the data memory of the time switch and from where they are read out either cyclically or sorted and stored.

In the figure switching method is shown, using 4 PCM words. Two rotary switches, which are cyclically controlled, are shown explains the addressing processes. The incoming PCM words are ted consecutively into the locations of the data memory with incoming time slots. The PCM words are read out from the data memory according to the information requested by the call. At the data memory output, the control addresses of the rotary switches are given by the control memory and coincide with the outgoing time slots. The rotary switch controlled by the control memory at the data memory output is as follows:

Outgoing time slot t_A1 from data memory location 3
Outgoing time slot t_A2 from data memory location 1
Outgoing time slot t_A3 from data memory location 4
Outgoing time slot t_A4 from data memory location 2

The time slots for the PCM words are rearranged by the time switch as follows:
Incoming time slot 1 to outgoing time slot 2
Incoming time slot 2 to outgoing time slot 4
Incoming time slot 3 to outgoing time slot 1
Incoming time slot 4 to outgoing time slot 3

As the sampling frequency is 8000 Hz, the PCM word time slot interchange takes place 8000 times per second for each connection. As a result of this interchange, the delays in the time switch by the PCM words are different for each connection.

b. Space Switching The space switch operates without interchanging time slots. The 8-bit PCM words retain their original time slots but can change their highways between input and output. As the original time slots during and after switching is the same, thus there is no delay. The principle of space switching is shown in the figure. Only three input and output highways with four PCM words per 125μs ,is have taken to simplify the operation. Also, cross points in .the switching network is shown as “AND gates”. The connection for each time slot of the incoming highway is made through the AND gate. The AND gate operates 8000 times per second for one connection.

The sequence of operation of the AND gate (ON or OFF) for each column in the matrix shown is given by the control memory. The control addresses in the control memory designate AND gate to be turned ON for each time slot. The control of is shown in the following table.

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.

Consider an example shown in the figure. In the figure shown there are four incoming highways. Thus the bit [ate 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.