520     6                        ________________

      ,   .25, frame
relay  ,      (
        .25, 
frame relay   ).       
       -
.

6.4.1.      
 

  ,       -
 ,  TCP/IP,   .

      ,   -
      ,  
.       
  (Switched Virtual Circuit, SVC)    
(Permanent Virtual Circuit, PVC).     -
       ,  -
 ,       ,
     , , 
    .

      ,   -
        
        (   
  ,    -
          ). -
        
       
(Virtual Channel Identifier, VCI).     -
 VCI     ,     
 ,   ,      -
    .   -
 ,        
      ,  
    ,      VCI.
          
     ,  
 ^  VCI  .   -
      ,   -
,           ,
   .

^         . -
   ,        -
   .    
    .      -
       14-15  , 
  8  (    20 )    . -