Time division Multiplexing (TDM)

Definition: "Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern".

TDM –PAM

Frame synchronization in TDM

Statistical TDM

Statistical TDM (frame format )

Statistical TDM (frame format)

Synchronization in TDM system

#Advantages of TDM:

•Full available channel bandwidth can be utilized for each channel .

•Inter-modulation distortion is absent .

•TDM circuitry is not very complex .

•The problem of crosstalk is not severe .

#Disadvantages of TDM:

•Synchronization is essential for proper operation .

•Due to slow narrowband fading , all the TDM channels may get wiped out .

#Applications of TDM:

•For multiplexing of digital signal .

•For multiplexing of digitized speech signal ( PCM-TDM TI multiplexing) .

•Statistical multiplexing .

#Difference Between TDM(Time division Multiplexing) and FDM(Frequency division Multiplexing):

Sr no.	FDM	TDM
1.	The signals which are to be multiplexed are added in the time domain . But they occupy different slots in the frequency domain .	The signals which are to be multiplexed can occupy the entire bandwidth in the time domain .
2.	FDM is usually preferred for the analog signals .	TDM is preferred for the digital signals .
3.	Synchronization is not required .	Synchronization is required .
4.	The FDM requires a complex circuitry at Tx and Rx .	TDM circuitry is not very complex .
5.	FDM suffers from the problem of crosstalk due to imperfect BPF .	In TDM the problem of crosstalk is not severe .
6.	Due to bandwidth fading in the Tx medium , all the FDM channels are affected .	Due to fading only a few TDM channels will be affected .
7.	Due to slow narrowband fading taking place in the transmission channel may be affected in FDM .	Due to slow narrowband fading all the TDM channels may get wiped out .

Comparison Between FDM Synchronous TDM Statistical TDM:

Sr no.	Parameter	FDM	Synchronous TDM	Statistical TDM
1.	Line utilization efficiency	Poor	Good	Very good
2.	Flexibility	Poor	Good	Very good
3.	Channel capacity	Poor	Good	Excellent
4.	Error control	Not possible	Not possible	Possible
5.	Multi-drop capacity	Very good	Difficult to achieve	Possible
6.	Transmission delay	Does not exist	Low	Random
7.	Cost	High	Low	Moderate

M&MA

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