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)
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