Data Communications Networking Devices - 4th Ed.pdf

6.1 MULTIPLEXERS _______________________________________________________________ 635Depending upon the type of TDM system, the buffer area in each adapter willaccommodate either bits or characters.The central logic of the TDM contains controlling, monitoring, and timingcircuitry which facilitates the passage of individual terminal data to and from thehigh-speed transmission medium. The central logic will generate a synchronizingpattern which is used by a scanner circuit to interrogate each of the channel adapterbuffer areas in a predetermined sequence, blocking the bits of characters from eachbuffer into a continuous, synchronous data stream which is then passed to acomposite adapter. The composite adapter contains a buffer and functions similarto the I/O channel adapters. It now, however, compensates for the difference inspeed between the high-speed transmission medium and the internal speed of themultiplexer.The multiplexing intervalWhen operating, the multiplexer transmits and receives a continuous data streamknown as a message train, regardless of the activity of connected terminal devices.The message train is formed from a continuous series of message frames whichrepresents the packing of a series of input data streams. Each message framecontains one or more synchronization characters followed by a number of basicmultiplexing intervals whose number is dependent upon the model and manufacturerof the device. The basic multiplexing interval can be viewed as the ®rstlevel of time subdivision which is established by determining the number of equalsections per second required by a particular application. Then, the multiplexinginterval is the time duration of one section of the message frame.When TDMs were ®rst introduced the section rate was established at 30 sectionsper second, which then produced a basic multiplexing interval of 0.033 seconds or33 ms. Setting the interval to 33 ms made the multiplexer directly compatible with a300-baud asynchronous channel which transmits data at up to 30 characters persecond cps). With this interval, the multiplexer was also compatible with 150-baud15-cps) and 110-baud 10-cps) data channels, since the basic multiplexing intervalwas a multiple of those asynchronous data rates. Later TDMs had a section rate of120 sections per second, which then made the multiplexer capable of servicing arange of asynchronous data streams up to 1200 bps. Modern TDMs have a sectionrate of at least 19 200 sections per second, providing support for asynchronous datastreams up to 19 200 bps.TDM techniquesThe two TDM techniques available are bit interleaving and character interleaving.Bit interleaving is generally used in systems which service synchronous terminaldevices, whereas character interleaving is generally used to service asynchronousterminal devices.When interleaving is accomplished on a bit-by-bit basis, the multiplexer takesone bit from each channel adapter and then combines them as a word or frame fortransmission. As shown in Figure 6.7a), this technique produces a frame containingone data element from each channel adapter. When interleaving is accomplished