508     6   

     ,     
  4ESS    Western Electric  1976 .

  ,       CCITT,
 1980    G.705,       .
   ISDN   1984     -
 I.           -
 .          
  .  ,   ISDN   -
    80- ,    , 
    .  1988    I  -
        ,  
 .  1992  1993   ISDN   
  .     .

  ISDN       80- , 
    ,  -
      ,    -
        
  ,       ,  ,
     ,   ISDN  -
 .  ,     ISDN  --
.           ,
   .      ISDN    
   5 %      .  
   ISDN    ,   -
       .     
        -
 ,     ,   ISDN -
 -  1994 - 1995 ,        
    ISDN.    -
  ,       ISDN   -
    ,      
    .

  ISDN     (. 6.16):

   (  );

     ;

      ;

      ;

       (frame relay);

      .

    ,    ISDN -
     ,    frame
relay.  ,      , 
       
,       .  
      , -
  ,       .