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MODEMS
COMMUNICATIONS   SYSTEMS

Fire Controlman Volume 06-Digital Communications
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Figure  1-10.—A  time-division  multiplexer  (TDM)  system. the decoder. The decoder reassembles the data into a be designated in a single modulation change. If two form  compatible  with  the  receiving  system.  In  the receiver section, the incoming signal is often fed to the receiver timing logic to control the receiver timing circuitry. MULTIPLEXING One requirement of a data communications system is  for  it  to  transmit  as  many  intelligent  signals  as possible   in   a   fixed   period   using   a   single- communications  channel. The  rate  of  data transmission  is  measured  in  the  number  of  bits  per second   (bps)   transmitted.   The   bps   rate   is   often confused with the baud rate. Baud refers to the rate at which   a   modulated   signal   between   two   devices changes  in  1  second. For  example,  if  the  signal between two modems changes frequency or phase at a rate of 2,400 times per second, the baud is 2,400. If you  are  using  a  modulation  method  in  which  a  single modulation change carries one bit, the 2,400 baud is also 2,400 bits per second. Using more sophisticated modulation methods, several bits of information can bits  of  data  are  transmitted  with  each  modulation change, the data transfer rate is 4,800 bits per second at  2,400  baud. The data signals being transmitted are normally multiplexed to increase the transmission rate of data over the communications channel or to increase the efficiency of the channel by allowing multiple users of  the  same  channel.  The  two  methods  commonly used   to   multiplex   communications   channels   are time-division multiplexing and frequency-division multiplexing. Time-Division  Multiplexing Time-division  multiplexing  (TDM)  grants  each user full channel capacity, but assigns time slots to each user. Each user is connected to a time-division multiplexer. Data signals from the user are fed to the time-division multiplexer buffer, and the time slots are  rotated  among  the  users  and  scanned  for  data. Figure 1-10 illustrates the typical construction of a 1-9






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