Better to know some
... than all
Multiplexing is a technique which makes it possible to cram a number of logical channels (each capable of supporting an independent connection) into the same physical channel or line. The objective of multiplexing should be obvious: to reduce costs by better utilizing the capacity of a line. There are three basic multiplexing methods; these are separately described below.
Space Division Multiplexing (SDM)
SDM is the simplest (and crudest) form of multiplexing. It involves grouping manyseparate wires into a common cable enclosure. A cable that has, for example, 50 twisted pairs inside it can support 50 channels. There is therefore a one-to-one correspondence between physical and logical channels.
SDM has the unique advantage of not requiring any multiplexing equipment. It isusually combined with other multiplexing techniques to better utilize the individual physical channels.
Frequency Division Multiplexing (FDM)
In FDM, the frequency bandwidth of the line is divided into a number of partitions, each of which is used as a separate logical channel. Radio and TV broadcasting represent the oldest examples of FDM. To avoid neighboring channels from interfering with one another, the extreme ends of the channel frequencies are left unused to provide a gap. For example, a line that has a bandwidth of 30 kHz can be divided into 3 times 10 kHz channels, each of which consists of 8 kHz of bandwidth for data and two gaps of 1 kHz on either side. FDM requires special multiplexing/demultiplexing hardware (MUX) at either end of the line.
Time Division Multiplexing (TDM)
In TDM, each logical channel is allocated a time slot to transmit over a shared physical channel. For example, each logical channel may be given a 5 millisecond time slot to transmit, during which time it will have the entire bandwidth of the line to itself.
Like FDM, TDM requires special multiplexing/demultiplexing hardware (MUX) at either end of the line. Because the channels are spread across time, some means of initial synchronization is also needed. Basically, the receiving end needs to know which time slot belongs to the first channel when the connection is established, and can work everything else out from this reference point.
In multiplexing, a predetermined bandwidth is reserved for each of the logical channels, the sum of which for all the logical channels equates the bandwidth of the line. In practice, none of the logical channels is fully utilized at all times by the equipment attached to them. Consequently, if the bandwidth of each of the channels could be dynamically adjusted according to its traffic, then some cost savings could be achieved by using a lower capacity line. For example, a 9600 bps line could be used to serve 10 times 2400 bps channels, assuming that no more than 4 channels are used at any one time.
This is made possible by a variation of TDM called concentration, where each channel is allocated a time slot only when it has data to transmit. Since, in this case time slots do not occur in a predetermined order, some means of indicating to which channel a time slot belongs is needed. This is easily achieved by having each time slot to contain two fields: the address of the channel to which it belongs, and the channel data.
Concentration is a popular method for connecting a set of character-based terminals to a central computer. Line capacity requirements are greatly reduced due to the fact that terminals tend to be idle for most of their operating period.