Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: May 2015

Signal Multiplexing

Signal multiplexing (or muxing) is a way of sending multiple signals or streams of information over a communications link at the same time in the form of a single, complex signal; the receiver recovers the separate signals, a process called signal demultiplexing (or demuxing). Networks use multiplexing for two reasons:

• To make it possible for any network device to talk to any other network device without having to dedicate a connection for each pair. This requires shared media.
• To make a scarce or expensive resource stretch further

FDM

1. Frequency-division multiplexing (FDM) is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium.
2. In FDM the signals are electrical signals. One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television.
3. Only one cable reaches a customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the appropriate frequency (channel) to access the desired signal.
4. A variant technology, called wavelength-division multiplexing (WDM) is used in optical communications.

TDM

1. Time-division multiplexing (TDM) is a digital (or in rare cases, analog) technology which uses time, instead of space or frequency, to separate the different data streams.
2. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver.
3. If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.
4. Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400 baud, so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed. Some web proxy servers use TDM in HTTP pipelining of multiple HTTP transactions onto the same TCP/IP connection.