Space Switch vs Time Switch-difference between Space Switch and Time Switch
This page on Space Switch vs Time Switch describes difference between Space Switch and Time Switch.
As we know architecture of digital switch consists of two main elements viz. time and space for performing time division switching (TDM) and Space Division Switching (SDM).
Other than the space switch and time switch, there exists time space-time switch, space time-space switch etc. which combines functionality of both of these types.
The figure-1 depicts concept of time switch. As shown it is basically a time slot interchanger (ISI). A time slot in PCM consists of 8 bits and represents 1 voice channel. The time slot is repeated 8000 times in a second. DS1 has 24 time slots in a frame and E1 has 32 time slots in a frame. Refer T1 frame vs E1 frame for more information.
As shown in the figure, connectivity is established between user-C on slot-C to user-G on Slot-G.
In order to perform time switch operation following steps are needed:
• Memory to store time slot speech data to write and to read at the output for changing the position.
• Memory to store control action to be performed on the stored data.
• Time slot counter or processor.
Figure-2 depicts time division space switch. It consists of cross point matrix made up of logic gates which allow switching of time slots in spatial domain. There will be input horizontals and output verticals with logic gate at each of the crosspoints of the space switch.
A tandem switching centre, or the route switch of a local exchange must be able to connect any channel on one of its incoming PCM highways to any channel of an outgoing PCM highway. The incoming and outgoing highways are spatially separate. Hence connection here need space switch.
In general, a connection will occupy different time slots on the incoming and outgoing highways. Hence switching network must be able to receive PCM samples from one time slot and re-transmit them in a different time slot. This is referred as time slot interchange or simple time switching.
Figure mentions the data transmitted on time slot no. 1 and 2 are later transmitted on time slot no. 24 of stream A and stream B at the output. Consequently, switching network of a tandem exchange or route switch of a local exchange must perform both time switching as well as space switching.
Space switch is like matrix of N x M physical lines. Time switch is like memory having data at its different consecutive address locations.
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