Multiplexing and Techniques Using Multiplexing-Digital Signal Processing-Lecture Slides, Slides of Digital Signal Processing

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Multiplexing &

techniques using

multiplexing.

What is Multiplexing:

Multiplexing is sending the multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end.

 Frequency-division multiplexing (FDM) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is assigned a different frequency (sub channel) within the main channel

 Possible with large bandwidth

 Multiple signals carried simultaneously

 Each signal modulated onto different Carrier frequency

 Carrier frequencies must be sufficiently Separated

 Bandwidths should not overlap

 In microwave transmission

 In AM or FM radio broadcast

 In satellite system

 In cellular radio system

 Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing

 TDM can be implemented in two ways: synchronous TDM and asynchronous TDM.  In synchronous time-division multiplexing, the term synchronous means that the multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit.  Frames

Time slots are grouped into frames. A frame consists of a one complete cycle of time slots, including one or more slots dedicated to each sending device.

Framing Bits

Because the time slot order in a synchronous TDM system doest no vary from frame to frame, very little overhead information needs to be included in each frame. However, one or more synchronization bits are usually added to the beginning of each frame. These bits, called framing bits, allows the demultiplexer to synchronize with the incoming stream so that it can separate the time slot accurately.

Framing bits

Figure 6.11 Prisms in wavelength-division multiplexing and demultiplexing

One application of WDM is the SONET network in which multiple optical fiber lines are multiplexed and demultiplexed. We discuss SONET in Chapter 17. A new method, called dense WDM (DWDM), can multiplex a very large number of channels by spacing channels very close to one another. It achieves even greater efficiency.

Using this technique, a multiplexer can be made to combine several input beams of light, each containing a narrow band of frequencies, into one output beam of a wider band of frequencies. A demultiplexer can also be made to reverse the process. Figure 6.11 shows the concept.