In space division switching, crosspoints are used to establish a specific connection between two subscribers. The crosspoints of multistage space switches assigned to a particular connection is dedicated to that connection for its duration. Thus the crosspoints can not be shared.
Time division switching involves the sharing of crosspoints for shorter periods of time. This paves way for the reassign of crosspoints and its associated circuits for other needed connections. Therefore, in time division switching, greater savings in crosspoints can be achieved. Hence, by using a dynamic control mechanisms, a switching element can be assigned to many inlet-outlet pairs for few microseconds. This is the principle of time division switching.
Time division switching uses time division multiplexing to achieve switching. Two popular methods that are used in time division multiplexing are (a) the time slot interchange (TSI) and (b) the TDM bus. In ordinary time division mutliplexing, the data reaches the output in the same order as they sent. But TSI changes the ordering of slots based on the desired connections. The demultiplexer separates the stots and passes them to the proper outputs. The TDM uses a control unit. The control unit opens and closes the gates according to the switching need.
The principle of time division switching can be equally applied to analog and digital signals. For interfacing sampled analog signals but not digitized, the analog time division switches are attractive. But for larger switches, there are some limitations due to noise, distortion and crosstalk which nomally occurs in PAM signals. Thus analog switching is now used only in smaller switching systems. In this section, the analog time division switching and digital time division switching are described briefly.
Fig. 5.13 shows a simple analog time division switching structure. The speech is carried as PAM analog samples or PCM digital samples, occurring at 125 |is intervals. When PAM samples are switched in a time division manner, the switching is known as analog time division switching. If PCM binary samples are switched, then the switching is known as digital time division switching. A single switching bus supports a multiple number of connections by interleaving PAM samples from receive line interfaces to transmit line interfaces. There are two cyclic control stores. The first control store controls gating of inputs onto the bus one sample at a time. The second control store operates in synchronism with the first and selects the appropriate output line for each input sample.
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