Chapter 25. Multiplexing
Multiplexing is the transmission of multiple data communication sessions over a common wire or medium. Multiplexing reduces the number of wires or cable required to connect multiple sessions. A session is considered to be data communication between two devices: computer to computer, terminal to computer, etc.
Individual lines running from 3 terminals to one mainframe is not a problem. However, when the number of terminals increases to 10 and up it becomes a problem. Imagine a mainframe computer with 1200 terminals connected, and each terminal running its own wire to the mainframe. If each wire was 1/4" in diameter (typical Cat 5 cable), you would have a wiring bundle going into the computer that is roughly 2 feet in diameter.
A multiplexer allows sharing of a common line to transmit the many terminal communications (as in the above example). The connection between the multiplexer and the mainframe is normally a high speed data link, and is not usually divided into separate lines.
The operation of multiplexers (abbreviated MUXs) is transparent to the sending and receiving computers. Transparent means that, as far as everyone is concerned, they appear to be directly connected to the mainframe with individual wires. The multiplexer does not interfere with the normal flow of data and it can allow a significant reduction in the overall cost of connecting to remote sites (through the reduced cost of cable and telephone line charges).
Multiplexers are used to connect terminals located throughout a building to a central mainframe. They are also used to connect terminals located at remote locations to a central mainframe through the phone lines.
There are 3 basic techniques that are used for multiplexing:
Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
Statistical Time Division Multiplexing (STDM)
FDM - Frequency Division Multiplexing
Frequency Division Multiplexing (FDM) is an analog technique where each communications channel is assigned a carrier frequency. To separate the channels, a guard-band would be used. This is to ensure that the channels do not interfere with each other.
For example, if we had our 3 terminals, each requiring a bandwidth of 3 kHz and a 300 Hz guard-band, Terminal 1 would be assigned the lowest frequency channel 0 - 3 kHz, Terminal 2 would be assigned the next frequency channel 3.3 kHz - 6.3 kHz and Terminal 3 would be assigned the final frequency channel 6.6 kHz - 9.6 kHz.
The frequencies are stacked on top of each other and many frequencies can be sent at once. The downside is that the overall line bandwidth increases. Individual terminal requirement were 3 kHz bandwidth each, in the above example: the bandwidth to transmit all 3 terminals is now 9.6 kHz.
FDM does not require all channels to terminate at a single location. Channels can be extracted using a multi-drop technique, terminals can be stationed at different locations within a building or a city.
FDM is an analog--and slightly historical--multiplexing technique. It is prone to noise problems, and has been overtaken by Time Division Multiplexing (better suited for digital data).
TDM - Time Division Multiplexing
Time Division Multiplexing is a technique where a short time sample of each channel is inserted into the multiplexed data stream. Each channel is sampled in turn, and then the sequence is repeated. The sample period has to be fast enough to sample each channel according to the Nyquist Theory (2x highest frequency), and to be able to sample all the other channels within that same time period. It can be thought of as a very fast mechanical switch: selecting each channel for a very short time, then going on to the next channel.
Each channel has a time slice assigned to it (whether the terminal is being used or not). Again, to the send and receiving stations, it appears as if there is a single line connecting them. All lines originate in one location and end in one location. TDM is more efficient, easier to operate, less complex and less expensive than FDM.
STDM - Statistical Time Division Multiplexing
Statistical Time Division Multiplexing uses intelligent devices that are capable of identifying when a terminal is idle. They allocate time only to lines when required. This means that more lines can be connected to a transmission medium because this device statistically compensates for normal idle time (in data communication lines). Newer STDM units provide additional capabilities: data compression, line priority, mixed speed lines, host port sharing, network port control, automatic speed detection and much more.
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