4.8.2.1                   Keret struktúra

     A keret egy bitszervezésű szinkron keret: (lásd még a 4.16.3 fejezetet!)


                8  7  6  5  4  3  2  1
               +----------------------+
         octet |         flag         |
             1 |0  1  1  1  1  1  1  0|
               Ă----------------------´
         octet |    Address           |
             2 |(high order octet)    |
               Ă----------------------´
         octet |    Address           |
             3 |(low  order octet)    |
               Ă----------------------´
         octet |    Control           |
             4 |                      |
               Ă----------------------´
         octet |    Control           |
             5 |                      |
               Ă- - - - - - - - - - - ´
             . |    Information       |
             . |                      |
               Ă----------------------´
         octet |    FCS (first octet) |
            N-2|                      |
               Ă----------------------´
         octet |    FCS (second octet)|
            N-1|                      |
               Ă----------------------´
         octet |         flag         |
             N |0  1  1  1  1  1  1  0|
               +----------------------+

     Három alapvető kerettípus létezik:
        I - információ átviteli keret
             ebben továbbítjuk a 3. réteg üzeneteit
        S - felügyelő keret (információs blokkja nincs)
             a hibamentes átvitelt és forgalom vezérlést biztosító protokoll
             vezérlő keretei
        U - számozatlan keret (információs blokkja nincs, a vezérlő
             mező egy oktettből áll.
             a 2. réteg kapcsolat felvétel, bontás eljárásban résztvevő
             keretek.

    A címmező (Address field) formátuma:

           8   7   6   5   4   3   2   1
          +----------------------Â---Â---+
    octet |    SAPI              |C/R|EA |
        2 |                      |   | 0 |
          Ă----------------------Á---Ĺ---´
    octet |    TEI                   |EA |
        3 |                          | 1 |
          +--------------------------Á---+

    A címmező elemei:

        EA   Address field extension bit
              LAPD-ben az első oktetben 0, a második oktetben 1.

        C/R  Command/response field bit
                                           C/R
              Command  network -> user      1
              Command  user -> network      0
              Response network -> user      0
              Response user -> network      1

        SAPI Service access port identifier
              0          call control procedures
              1 - 15     reserved for future
              16         packet communication (X.25 3. layer)
              17- 31     reserved for future
              63         Layer 2 management procedures
              all others Not available

        TEI  Terminal endpoint identifier
              0 -63      non automatic TEI assignment user equipment
              64-126     automatic TEI assignment user equipment
              127        broadcast

    Vezérlő mező formátumok:

           8   7   6   5   4   3   2   1
          +--------------------------Â---+
    octet |    N(S)                  |   |  I - format
        4 |                          | 0 |  information transfer
          Ă--------------------------Ĺ---´  információátviteli keret
    octet |    N(R)                  |   |
        5 |                          | P |
          +--------------------------Á---+
 ------------------------------------------------------------------------
           8   7   6   5   4   3   2   1
          +--------------Â-------Â-------+
    octet |              |       |       |  S - format
        4 |X   X   X   X | S   S | 0   1 |  supervisory
          Ă--------------Á-------Á---Â---´  felügyelő keret
    octet |                          |   |
        5 |    N(R)                  |P/F|
          +--------------------------Á---+

          +--Â---Â---Â---Â---Â---Â---Â---+
       S  |0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |  RR -receive ready
          Ă--Á---Á---Á---Á---Á---Á---Ĺ---´      vételkész
          |    N(R)                  |P/F|
          +--------------------------Á---+
          +--Â---Â---Â---Â---Â---Â---Â---+
       S  |0 | 0 | 0 | 0 | 0 | 1 | 0 | 1 |  RNR -receive not ready
          Ă--Á---Á---Á---Á---Á---Á---Ĺ---´       nem vételkész
          |    N(R)                  |P/F|
          +--------------------------Á---+
          +--Â---Â---Â---Â---Â---Â---Â---+
       S  |0 | 0 | 0 | 0 | 1 | 0 | 0 | 1 |  REJ -reject
          Ă--Á---Á---Á---Á---Á---Á---Ĺ---´  újraküldés kérés N(R)-től
          |    N(R)                  |P/F|
          +--------------------------Á---+

  --------------------------------------------------------------------

           8   7   6   5   4   3   2   1
          +----------Â---Â-------Â-------+
    octet |          |   |       |       |  U - format
        4 |M   M   M |P/F| M   M | 1   1 |  unnumbered
          +----------Á---Á-------Á-------+  számozatlan keret

           N(S)  Transmitter send sequence number
           N(R)  Transmitter receive sequence number
           S     Supervisory function bit
           M     Modifier function bit
           P/F   Poll bit when issued as a command, final bit when issued
                 as a response
           X     Reserved and set to 0

          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |0 | 1 | 1 | P | 1 | 1 | 1 | 1 |  SABME -set asynchronous balanced
          +--Á---Á---Á---Á---Á---Á---Á---+         mode
                                                   2. layer kapcsolat felépítés
                                                   kezdeményezés
          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |0 | 0 | 0 | F | 1 | 1 | 1 | 1 |  DM    -disconnected mode
          +--Á---Á---Á---Á---Á---Á---Á---+
          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |0 | 0 | 0 | P | 0 | 0 | 1 | 1 |  UI    -unnumbered information
          +--Á---Á---Á---Á---Á---Á---Á---+
          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |0 | 1 | 0 | P | 0 | 0 | 1 | 1 |  DISC  -disconnect
          +--Á---Á---Á---Á---Á---Á---Á---+         2. layer kapcsolat bontás
          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |0 | 1 | 1 | F | 0 | 0 | 1 | 1 |  UA    -unnumbered acknowledge
          +--Á---Á---Á---Á---Á---Á---Á---+
          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |1 | 0 | 0 | F | 0 | 1 | 1 | 1 |  FRMR  -frame reject
          +--Á---Á---Á---Á---Á---Á---Á---+
          +--Â---Â---Â---Â---Â---Â---Â---+
       U  |1 | 0 | 1 |P/F| 1 | 1 | 1 | 1 |  XID   -exchange identification
          +--Á---Á---Á---Á---Á---Á---Á---+

    UA - üzemmódbeállító parancs (SABME,DISC) nyugtázása:
    UI - információtovábbítás (nem checkelt)



Go one level up

Go to Title page