Data Communications Networking Devices - 4th Ed.pdf

636 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTFigure 6.7 a) Time division interleaving bit-by-bit.The ®rst bit from each channel ispacked into a frame for transmission.b) Time division multiplexing character-by-character.One or more complete characters are grouped with a synchronization character into a framefor transmissionon a character-by-character basis, the multiplexer assembles a full character intoone frame and then transmits the entire character, as shown in Figure 6.7b).Although a frame containing only one character of information is illustrated inFigure 6.7, to increase transmission ef®ciency most multiplexers transmit longframes containing a large number of data characters to reduce the synchronizationoverhead associated with each frame. Thus, while a frame containing one characterof information has a synchronization overhead of 50%, a frame containing four datacharacters has its overhead reduced to 20%, and a frame containing nine datacharacters has a synchronization overhead of only 10%, assuming constant slot sizesfor all characters.When TDMs were ®rst developed, the larger buffer area required for characterby-charactermultiplexing made this technique more expensive. Today the primarydifference between the two techniques is in the area of latency or delay. Bitmultiplexing results in a minimal delay and is very suitable for supporting highspeedsynchronous data streams. In comparison, character-by-character multiplexingenables the multiplexer to perform a variety of operations on data, such asstripping start, stop, and parity bits from asynchronous characters and compressinggroups of characters ®rst stored in a buffer. However, such operations add a degreeof latency which may not be suitable for high-speed data sources.TDM applicationsThe most commonly used TDM con®guration is the point-to-point system, whichis shown in Figure 6.8. This type of system, which is also called a two-pointmultiplex system, is shown linking a mixture of terminal devices to a centrallylocated multiplexer. As shown terminal devices can be connected to the multiplexerin a variety of ways: by a leased line running from the terminal's location to themultiplexer, by a direct connection if the user's terminal device is within the samebuilding as the multiplexer and a cable can be laid to connect the two, or via the