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                             Fall/1993 Issue: 2 
                   
                   INTRODUCTION TO BASIC DIGITAL TECHNOLOGY


                            Written by: Erik Turbo
                                
                                 File: 2 of 6



2. Time Division Multiplexing (TDM)


2.1 Defining Multiplexing


Multiplexing basically combines or merges a number of signals into one
composite signal.  The most common type of multiplexer at NET is called
a TIME DIVISION MULTIPLEXER.


Time Division Multiplexing


In the telephone industry, the D Channel Bank Time Division Multiplexer
is the type most commonly used and the one you will probably use.


The D Channel Bank Time Division Mutliplexer works by taking twenty four
(24) voice channels and time division multiplexing them at the near end
terminal.  Then the signals are sent over a pair of wires to the far
end terminal.


This smae process is occuring at the far end terminal.  The signal from the
far end terminal is sent over another pair of wires to the near end terminal.
Each terminal is equipped to restore the signal recieved to its orignal
form.


Time Division Multiplexing is used to take low speed information, sample
it, and then send this information over a high speed data line.  Each
time all twenty four (24) channels have been sampled and sent, a framing
signal is sent.  This framing signal aids the far end terminal in identying
and reassembling all of the information for each channel. This process is
known as synchronizing data.


On the next page you will learn how the sampled signals are quantized and
encoded by a process called Pulse Code Modulation (PCM) before they are
transmitted via Time Division Multiplexing to a distant terminal.


2.2 Multiplexing--Pulse Code Modulation (PCM)


> At 8,000 samples per second, a single channel is sampled once per 125
  micro-seconds.  Each sample uses 5.2 microseconds of time.


> There are approximately 120 microseconds of idle time between each sample
  on a single channel Pulse Code Modulation (PCM) facility.


> To make efficient use of the facility, many samples are sent on the same
  path, which is called multiplexing.


2.3 Pulse Code Modulation (PCM) Frame


A Frame requires 125 microseconds to transmit and contains once encoded
sample (8-bit word) for each channel that is multiplexed, plus the framing
bit.


The Frame is sub-divided into Time Slots.  A Time Slot represents the time
required the send one 8-bit word.


The basic Pulse Code Modulation (PCM) bit stream contains 1,544,000 bits/sec.


2.4 DS1 Bit Rate Computation


      24 CHANNELS
     x 8 BITS/WORD
     192 BIT
     + 1 FRAMING BIT
     193 BITS/FRAME
  X 8000 TIMES/SEC
 1544000 BITS/SEC
      OR
   1.544 M BITS/SEC



2.5 Time Division Multiplex (TDM)


> This diagram shows the overall digital transmission system.


> The Analog signal is sampled 8000 times a second via a process called
  Pulse Amplitude Modulation (PAM).  The Pulse Amplitude Modulation (PAM)
  sample represents the amplitude of the signal at the time of sampling.


> Each Pulse Amplitude Modulation (PAM) sample is quanitzed and encoded
  to an 8-bit Digital signal via a process called Pulse Code Modulation
  (PCM).


> The Pulse Code Modulation (PCM) samples from all 24 channels are combined
  via a process called Time Division Multiplexing and transmitted to a 
  distant terminal over a common path.


> At the distant terminal, the Pulse Code Modulation (PCM) samples are
  decoded, demultiplexed and filtered to reconstruct the orignal Analog
  waveform.


2.6 Conclusion


Multiplexing is vital to our business because it allows us to take advanage
of the idle time between each signle channel Pulse Code Modulation (PCM)
facility.  The idle time is used efficiently via multiplexing which allows
us to send many samples on the same path.


So multiplexing maximizes efficient use of the facility and reduces idle
time by sending numerous samples over the same path.






                                NOTICE
      
       Not  for use or disclosure outside  the NYNEX Corporation 
       or any of its subsidiaries except when rightfully stolen.






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