Multiplexing

Whenever available bandwidth is divided into smaller portions for simultaneous transmission of different signals (in either direction, or both upstream and downstream), this is referred to as Multiplexing.  Multiplexing is used in both digital and analog transmission.  There are three fundamental multiplexing architectures:

• Space-division multiplexing (SDM)
• Frequency-division multiplexing (FDM)
• Time-division multiplexing (TDM)

SDM

In Space Division Multiplexing, as the name implies, different signals are segregated to different places.  Individual signals are imposed on different wires, fibers or other discrete media, and those media are bundled into trunks.  An example of already described is the hierarchic trunking of twisted wire pairs used in telephony.

FDM

Frequency Division Multiplexing combines several signals into one medium by sending signals over several frequencies over that medium. One of the most common FDM applications is cable television, only one cable is connected to the home but multiple channels can be transmitted.

TDM

Time-division multiplexing is an elementary form of digital technology. It involves slicing groups of a few bits or bytes from each individual input stream one after the other in such a way that they can be associated with the appropriate receiver. If this is done sufficiently quickly, the receiving devices will not realize or care that some of the circuit time was used to serve another logical communication path.

Compression

Compression is a mathematical technique used for increasing the data that can be modulated on a specific amount of bandwidth. Compression algorithms are applied (in advance or dynamically) to an information stream being transmitted. The removal of redundant information reduces the total number of bytes transmitted.

            The simplest form of compression identifies and records repeating characters into fewer characters. Faxing provides a commonly used example of this compression. Imagine a sliver of a text line that is only one pixel wide.  Such a line would appear as a string of dots and spaces, or a line of binary data that can be reduced to a mathematical algorithm.  In turn this algorithm is represented by a preprogrammed three-bit signal that the receiving device recognizes in the same way.  Each scan line received translates back into the original series of dots and spaces, which the receiving machine reassembles as a facsimile of the entire message.

            A more sophisticated form of compression detects patterns rather than just repeating characters. Again, special characters are designated, but in this case to a detected pattern. For example, a white background with a blue box on a computer screen repeats an identical data stream thousands of times. The savings of bandwidth is considerable when one character to represents 20 successive occurrences of a two-byte sequence.

            A third, intensive approach to compression has been designed for transmission of digitized video images. Video signals consist of still pictures that are shown in a sequence in a fast sequence (in motion pictures, this rate is 32 frames each second). Once the first image is entirely transmitted, unique algorithms examine which pixels have changed, and transmit a data stream that represents only those pixels changed since the previous image. This data stream is reassembled into what viewers at the receiving end recognize as moving images.