526     6   

      (   
 PAD),       
   ,        -
.

        .25 -
   .25  ,    -
  .25.    PAD   
 .29,         -
 PAD ,  .     ,
   .25 ,  PAD  ,  -
           
 .25.

   .25

  .25     ,     
  (    )    
.       .25  16 .

 .121 CCITT     -
      .   .25  -
     .25,      
 .121.

 .121 (  International Data Numbers, IDN)  -
 ,     14  .   -
 IDN     (Data Network Identification Code,
DNIC). DNIC    ;   (3 )  , 
  ,      .25   .  ,
      10  .25.   -
  ,  10    ,   ,
     . ,    1995 
   250,   1995         251. 
     (National Terminal Numbe,
NTN).      DTE   .25.

  .25     
  ISO 7498,  .

  ISO 7498    .25     .121
    ,   36 (  
   )  37 (  
).     ,  -
  ISDN,      .25, -
        
  .

   .25

  .25  3   (. 6.23).
       .21  .21 bis 
     DSU/CSU,    -
 ,    ,   .