For digital sources, two alternative technologies have evolved for multiplexing. These are Time Division Multiplexing (TDM) and Code division Multiplexing (CDM). TDM provides a way to merge data from several sources into a single channel for communication over telephone lines, a microwave system or a satellite system. TDM can be implemented in two ways. These are synchronous TDM and asynchronous TDM. Asynchronous TDM is popularly known as Statistical TDM.
In synchronous TDM, a single channel is divided into time slots and each transmitting device is assigned at least one of the time slots for its transmission
Time slots are assigned in such a way that each transmitting device gets its required share of the available bandwidth. 'Because of this time-bandwidth multiplexing technique, TDMs are protocol insensitive and are capable of combining various protocols onto a single high-speed transmission link. In other words, we can say that multiplexer allocates exactly-the same time slot to each device at all times whether the device is active or idle.
Some devices such as voice and video systems may require more slots to ensure that data arrives at the distant link-end without becoming distorted from slower data rates. These different time slots are grouped into frames. A frame consists of one complete cycle of time slots.
Alternatively, Figure explains more clearly the concept of TDM in a data communication environment where three PCs are sharing the common circuit. The packets generated by each PC are multiplexed on the common line as A1, B1, and C1 and so on.
It is more flexible than the FDM unlike FDM. the whole bandwidth for a certain amount of time is provided to the user. All the users are using the same frequency but at a different time. This time allotment may be varied as per the requirement and priority of the users' services. In the Figure spaces between different time slots are shown, these are known as guard spaces in time dimension. These are used to eliminate co-channel interference.
The main disadvantage of this scheme is that a precise synchronization between different senders is necessary to avoid co-channel interference.
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