LAPB



HDLC become derive an extensive Layer-2 protocol from him/it sub variations like LAPB or LAPD. LAPB is included the Layer 2 from the CCITT recommendation to X25.
The here presented data (representing Modulo-8) only serve as short overview over the types of LAPB-pakets. For detailierte information, the ISO 7776 only is responsible.


broadly:

Flag

AF

CF

Parameter / Daten

FCS

Flag

7Eh

7Eh



AFAddress Field - command / responce (01h/03h)
CFControl Field - Type of the package
Parameter / Datendependent on the typ
FCSFrame Check Sequence


Types:

This overview treats exclusively the most frequently use variant in Germany at the representing modulo-8.


SABM

" set asyncronous balanced mode "

Flag

c

3Fh

FCS

Flag



3FhTyp: SABM
• bit-4: poll-bit


DM

" disconnected mode "

Flag

c

1Fh

FCS

Flag



1FhTyp: DM
• bit-4: poll-bit


UA

" unnumbered acknowledgment "

Flag

r

73h

FCS

Flag



73hTyp: UA
• bit-4: poll-bit


DISC

" disconnect "

Flag

c

53h

FCS

Flag



53hTyp: DISC
• bit-4: poll-bit


FRMR

" frame reject "

Flag

c

87h

xxh

xxh

xxh

FCS

Flag



87hTyp: FRMR
• bit-4: poll-bit
xxhsee ISO 7776 for information-field


REJ

" reject "

Flag

c/r

x9h

FCS

Flag



x9hTyp: REJ
• bit-4: poll-bit
• bit-7..5 receive sequence number


RNR

" receive not ready "

Flag

c/r

x5h

FCS

Flag



x5hTyp: RNR
• bit-4: poll-bit/final-bit
• bit-7..5 receive sequence number


RR

" receive ready "

Flag

c/r

x1h

FCS

Flag



x1hTyp: RR
• bit-4: poll-bit/final-bit
• bit-7..5 receive sequence number


I-frame

" data-frame "

Flag

c

xx..xx0b

data

FCS

Flag



xx..xx0bTyp: I-frame
• bit-4: poll-bit
• bit-7..5 receive sequence number
• bit-3..1 send sequence number
• bit-0 must be zero





In the standard to LAPB, costly State-diagrams are represented to the respective transitions and mistake treatments. Getting a variation about a space-saving and simultaneously good to in the habit of state/change-mechanism is the implementation by means of a State-Table. The current status is used as line and the received Paket-Typ on that occasion as columns. The there deposited byte returns the necessary reaction and the low Nibble the new State in the upper Nibble. There sufficiently exceptions exists, however even further evaluations are around necessary about such a table, sees ISO 7776.