6.2.            479

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. ,       
64 /    ,   
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    1.       PDH
   ,   -
       -
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 (back hauling).   1     ,
  672  ,      
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     2,    
   64 /.    1  -
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 1. ,     
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     .

         -
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         ,  
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  PDH       
 139 /.

         ,
      Synchronous Digital Hierarchy,
SDH.

    SONET/SDH

       -
 Bellcore       Synchronous Optical
NETs, SONET.      i984 .   -
    Tl ANSI.  
      -
  (ETSI)  CCITT   ANSI   -