APPENDIX
Protocol Data Mode
98
2.10
•
19.01.2007
5.1.6 LAP Information Elements
A LAP frame consists in a control byte and an optional information field.
LAP frame
CONTR
Information
There are different frame types, information frames (I-frames) and supervisory frames (RR-,
RNR-, REJ-, SABM- and UA-frames).
CONTR:
The 1-Byte wide control field defines the frame type and, depending on the frame
type, other information element. The detailed definition of its structure is shown below.
Frame Type
I Command
N(R)
P
N(S)
0
RR Command / Response
N(R)
P/F
0
0
0
1
RNR Command / Response
N(R)
P/F
0
1
0
1
REJ Command / Response
N(R)
P/F
1
0
0
1
SABM Command
0
0
1
P
1
1
1
1
UA Response
0
1
1
F
0
0
1
1
N(S):
The 3-bit send sequence number is used by the sender to identify an I-frame.
N(R):
The 3-bit receive sequence number is used by the receiver to acknowledge I-frames.
P:
The poll bit is set by the sender in order to request flow control information from the
receiver.
F:
The final bit is set by the receiver in response to a command with P bit set.
Information:
This variable length field (0 to 26 bytes) contains higher layer information to be
transported through a LAP channel. It is only present in I-frames.