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Time Division Multiplexing (TDM)

Time Division Multiplexing (TDM). TDM is a technique used for transmitting several message signals over a single communication channel by dividing the time frame into slots, one slot for each message signal. TDM-PAM: Transmitter. TDM-PAM : Receiver. time. 0. Ts. 2Ts. g 1 (t).

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Time Division Multiplexing (TDM)

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  1. Time Division Multiplexing (TDM) • TDM is a technique used for transmitting several message signals over a single communication channel by dividing the time frame into slots, one slot for each message signal

  2. TDM-PAM: Transmitter

  3. TDM-PAM : Receiver

  4. time 0 Ts 2Ts g1(t) Samples of Signal -1

  5. Ts Ts Samples of signal - 2 g2(t)

  6. 0 Ts 2Ts Multiplexing of TWO signals

  7. TDM-PAM for 3 signals 2 2 1 1 2 3 3 1 3 Time

  8. 4 4 4 1 1 1 2 2 2 3 3 3 Time TDM-PAM for 4 signals.

  9. g1(t) g2(t) g3(t) g4(t) Commutator Arrangement

  10. g1(t) g2(t) 2 1 3 4 g3(t) g4(t) Decommutator

  11. TDM-PAM • Types of TDM: • Synchronous TDM • Asynchronous TDM

  12. Same Sampling rate for all signals. • Minimum Sampling rate = twice the maximum frequency of all the signals. • Total number of samples transmitted per second is equal to N times the sampling rate, Fs plus sync pulses. • Transmission Bandwidth = N. Fs/2 Synchronous TDM

  13. Different Sampling rate for different. signals. • Sampling rate of a signal = twice the maximum frequency of that signal. • Total number of samples transmitted per second is equal to Sum of samples of all the signals plus sync pulses. Asynchronous TDM

  14. 4. Transmission Bandwidth = Half the total number of samples transmitted. • 5. Bandwidth is less for Asynchronous TDM. • 6. Design of Commutator / Decommutator is difficult. Asynchronous TDM

  15. Two low-pass signals of equal bandwidth are sampled and time division multiplexed using PAM. The TDM signal is passed through a Low-pass filter & then transmitted over a channel with a bandwidth of 10KHz. • Continued…. Problem-1

  16. Problem –1 continued • What is maximum Sampling rate for each Channel? • What is the maximum frequency content allowable for each signal?

  17. Channel Bandwidth = 10 KHz. • Number of samples that can be transmitted through the channel = 20K • Maximum Sampling rate for each channel = 10K Samples/sec. • Maximum Frequency for each Signal = 5KHz Problem –1 : Solution

  18. Problem –2 • Two signals g1(t) and g2(t) are to transmitted over a common channel by means of TDM. The highest frequency of g1(t) is 1KHz and that of g2(t) is 1.3KHz. What is the permissible sampling rate? • Ans: 2.6K samples/sec and above. • { Synchronous TDM}

  19. 24 voice signals are sampled uniformly and then time division multiplexed . The sampling operation uses the flat-top samples with 1microsec duration. The multiplexing operation includes provision for Synchronization by adding an extra pulse of sufficient amplitude and also 1micro second. • Contd… Problem-3

  20. Contd.. • Assuming a sampling rate of 8KHz, calculate the spacing between successive pulses of the multiplexed signal. Problem-3

  21. In One frame, Total number of pulses = 25. • Time duration for one time frame = Ts = 125µ seconds. • Time duration utilized by pulses = 25µ sec • Time spacing between successive pulses • = (125- 25)/25 = 4µ sec. Problem-3: Solution

  22. Three independent message signals of bandwidths 1KHz, 1KHz and 2KHz respectively are to be transmitted using TDM scheme. Determine • Commutator segment arrangement • Speed of the commutator if all the signals are sampled at its Nyquist rate. • Minimum Transmission bandwidth. Problem-4

  23. g1(t) g2(t) 2 1 g3(t) 4 3 • Commutator Segment arrangement. Problem-4: Solution

  24. 3 3 3 1 1 1 3 3 3 2 2 2 Time Problem-4 : Solution.

  25. Total number of Samples to be transmitted per second = 8K samples/sec. • Number of Commutator segments = 4 • Speed of Commutator = 2000 rotations/sec. • c) Transmission Bandwidth = 4000 Hz. Problem-4: Solution

  26. Q. Eight independent message signals are sampled and time multiplexed using PAM. Six of the message signals are having a bandwidth of 4KHz and other two have bandwidth of 12KHz. Compare the transmission bandwidth requirements of Synchronous TDM and Asynchronous TDM. Problem-5.

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