12                                    _____________________

6.3.1.   ...................................................................... 501

   .................................. 501

      ... 503

6.3.2.     Switched 56 ............... 506

6.3.3. ISDN     ................................................ 507

     ISDN.................................. 507

  ISDN ............................................... 509

     ISDN... 511
   ISDN........................................................................ 513

     ISDN........................................ 515

  ISDN    ................. 516

 ........................................................................................................................... 518

6.4.      ............................ 519

6.4.1.      

 ................................................................................... 520

6.4.2.  .25 ............................................................................................................ 523

    .25................................................... 523

   .25 .......................................................................... 526

   .25 ................................................................... 526

6.4.3.  Frame Relay............................................................................................. 530

   ................................................ 530

  frame relay................................................................. 532

   .................................................. 534

  frame relay ........................................................ 539

6.4.4.  .............................................................................................. 540

   ............................................ 542

  ............................................................................ 548

  AAL ........................................................................ 549

  ..........................................................................................551

      ...... 554

  IP   .............................................. 559

    
  ....................................................................................561

   ....................................................... 564

 ........................................................................................................................... 566

6.5.  ....................................................................................................... 567

6.5.1.       ........ 567

   ..................................................... 568

   ............................................................. 569

   ........................................... 570

6.5.2.     ............................................................................. 571

 ......................................................................................... 572

     ........................... 573

............................................................................................................. 574

6.5.3.      .................................... 575

   .......................................... 575