Write a short note on TDM (Time Division Multiplexing).
The Time Division Multiplexing (TDM) is a multiplexing method by which multiple number of channel containing information passes over a common transmission media or channel at different time slots. In other way TDM is a technique in which combining the digital message signal from different sources on a single data stream and transmit over a common channel at different time slot.
There are two types of TDM
Synchronous TDM:
In an synchronous TDM, data from all the sources is organized in a frame that has a fixed repetition interval. In Fig.13 shows a conceptual TDM that multiplexes the data from five sources. The multiplexer acts like a commutating switch, which sequentially connects the input source to the channel. One complete switching sequence is carried out in T second, which is called the Frame Period. Each source is connected to the channel for an equal duration. Thus data S1 from source 1 is connected to the channel for \(\frac{T}{5}\) second, then data S2 from source 2 to the channel for the next \(\frac{T}{5}\) seconds and so on.
Fig.13 Block diagram of TDM
Therefore, one frame corresponds to the time period required to transmit the entire signal once on the transmission channel.
Fig. 14 Frame representation of TDM
After the frame period T, the switch connects source 1 to the channel again. This process repeats frame after frame. The data signals S1, S2, S3, S4, etc, can be bits or ward (group of bits). If single bits from each channel are inter leaved, then it is called bit multiplexing. If ward from each channel are inter leaved, then it is known as word multiplexing. Since the digital signals are after the sampled signals, it may be appropriate to consider S1, S2, S3, S4, etc, as samples from channels 1, 2, 3, 4 and so on.
The locations of samples (bit or word) of each channel in the TDM frame are called time slots. The de-multiplexer carried out reverse function of the multiplexer. The de- multiplexer separating the signals from the transmission channel and routing them to appropriate receivers. Thus it contains another switching or routing device, which separates the signal from the common channel.
Time compression of the data bit in the samples correspondingly increases the data rate. The data rate of the multiplexed stream increases in proportion to the number of channel. If N sources with data rates R1, R2, R3, ....., RN are multiplexed then the average rate RM of multiplexed data is
\(R_M=\sum_{i=1}^{N}R_i\) bits / s
If all the source have equal rate RS then we have RM = NRS bits / s
If the frame period is Tf, then the number of bits in a frame Nb is given by
Nb=RMTf = NRSTf bits.
The bit period T of multiplexed data is
\(T=\frac{1}{R_M}=\frac{T_f}{N_b}\) second, hence the frame rate \(R_f=\frac{1}{T_f}\)
Therefore average data rate RM = NbRf bits /s
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Fig. 14(a)