Data Communications Networking Devices - 4th Ed.pdf

378 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTexpense associated with line conditioning. Due to the incorporation of microprocessorsinto modems for signal processing they were soon employed to performautomatic equalization. This resulted in most modern modems incorporatingautomatic equalization.Automatic equalizationAutomatic equalization is used on most 4800 bps and above modems designed foroperation over the switched telephone network and on all 7200 bps and abovemodems which are primarily designed to operate over dedicated lines but whichcan operate over the switched network in a fallback operational mode. Withautomatic equalization, a certain initialization time is required to adapt the modemto existing line conditions. This initialization time becomes important during andafter line outages, since long initial equalization times can extend otherwise shortdropouts unnecessarily. Recent modem developments have shortened the initialequalization time to between 15 and 25 ms, whereas only a few years ago a muchlonger time was commonly required. After the initial equalization, the modemcontinuously monitors and compensates for changing line conditions by anadaptive process. This process allows the equalizer to `track' the frequentlyoccurring line variations that occur during data transmission without interruptingthe traf®c ¯ow. On one 9600 bps modem, this adaptive process occurs 2400 times asecond, permitting the recognition of variations as they occur.SynchronizationFor synchronous communications, generally in speeds exceeding 1800 bps, thestart±stop bits characteristic of asynchronous communications can be eliminated.Bit synchronization is necessary so that the receiving modem samples the link at theexact moment that a bit occurs. The receiver clock is supplied by the modem inphase coherence with the incoming data bit stream, or more simply stated, tuned tothe exact speed of the transmitting clock. The transmitting clock can be supplied byeither the modem internal) or the terminal external).The transmission of synchronous data is generally under the control of a masterclock which is the fastest clock in the system. Any slower data clock rates requiredare derived from the master clock by digital division logic, and those clocks arereferred to as slave clocks. For instance, a master clock oscillating at a frequency of96 kHz could be used to derive 9.6 kpbs 1/10), 4.8 kbps 1/20), and 2.4 kbps 1/40)clock speeds.Multiport capabilityModems with a multiport capability offer a function similar to that provided by amultiplexer. In fact, multiport modems contain a limited function time divisionmultiplexer TDM) which provides the user with the capability of transmitting