Subject : Principle of Communication Engineering

Topic : PCM and Multiplexing

Difficulty : Medium

Signal multiplexing is a process in which multiple signals can be transmitted together over the same communication medium simultaneously.

Time division multiplexing

• Time division multiplexing is a technique of separating the signals in time domain.
• In TDM the transmission from multiple sources take place on the same medium but not at the same time.
• The transmissions from various sources are interleaved in time domain. In other words, the data from the various sources is arranged in non contiguous manner by dividing the data into small chunks, which also makes the system efficient.
• Pulse code modulation is the most common encoding technique used for TDM digital signals.
• PCM system used in North America is a 24-channel system with the sampling rate of 8000 samples per second, 8 bits per sample and a pulse width of 0.625 μs.
• We can calculate that sampling interval is 1/8000 = 125 μs, and period required for each pulse group is 8 x 0.625 = 5 μs.
• If we transmit only one channel without using the multiplexing technique, then the transmission will contain 8000 frames per second, which will consist of the activity only during the first 5 μs and nothing at all during the rest 120 μs.
• Thus will be wasteful and employs complicated method for encoding single channel. Therefore
• TDM technique is used so that each 125 μs frame is used to provide 24 adjacent channel time slots with the twenty-fifth time slot for synchronization.

• Fig1 shows the time division multiplexing of the data from the various channels of the PCM system. TDM finds application in the transmission of SDH and SONET system, GSM telephone system etc.

  

  Fig.1. Time division multiplexing