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Thursday, December 15, 2011

Digital Multiplexing

Digital Multiplexing

Multiplexing is a technique in which several message signals are combined into a composite signals for transmitting over a common channel. In order to transmit a number of this signals over the same channel, the signals must be kept apart so that they do not interfere with each other, and hence they can be separated easily at the receiver end.
Basically multiplexing is two types as under:
(i)Frequency division multiplexing (FDM)
(ii)Time division multiplexing (TDM)

Frequency division multiplexing (FDM): This technique permits a fixed frequency band to every user in the complete channel bandwidth. Such frequency slot is allotted continuously to that user. As an example consider  the channel bandwidth is 1Mhz. Let there be 10users, each requiring upto 100 KHz bandwidth .So each user will get 100KHz bandwidth. This technique is known as frequency division multiplexing (FDM). It is basically used for modulated signals. This is due to the fact that a modulated signal can be placed in any frequency band by just changing the carrier frequency.

Time division multiplexing: In TDM the signal to be multiplexed are transmitted sequentially one after other. Each signal occupies a short time slot. Thus, the signals are isolated from each other in time domain, but all of them occupy the same slot in the frequency spectrum, therefore the complete bandwidth of the communication channel is available to each signal to be transmitted in TDM.

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