AEA PK-232 Technical Reference Manual page 48

PK-232 TECHNICAL MANUAL
A.2.1 Flag Field
The flag field is one octet long. Since the flag is used to delimit frames, it occurs at both the begin-
ning and end of each frame. Two frames may share one flag, which would denote the end of the
first frame, and the start of the next frame. A flag consists of a zero followed by six ones followed
by another zero, or 01111110 (7E hex). As a result of bit stuffing (see A.2.6, below), this sequence
is not allowed to occur anywhere else inside a complete frame.
A.2.2 Address Field
The address field is used to identify both the source of the frame and its destination. In addition,
the address field contains the command/response information and facilities for level 2 repeater op-
eration. The encoding of the address field is described in A.2.13.
A.2.3 Control Field
The control field is used to identify the type of frame being passed and control several attributes
of the level 2 connection. It is one octet in length, and its encoding is discussed in A.3.2.1, below.
A.2.4 PID Field
The Protocol Identifier (PID) field shall appear in information frames (I and UI) only. It identifies
what kind of layer 3 protocol, if any, is in use.
The PID itself is not included as part of the octet count of the information field. The encoding of
the PID is as follows:
M L
HEX S S
B B
0x01 00000001
0x06 00000110
00000111
0x07
0x08 00001000
yy01yyyy
**
** yy10yyyy
0xC3 11000011
0xC4 11000100
0xCA 11001010
0xCB 11001011
0xCC 11001100
11001101
0xCD
0xCE 11001110
11001111
0xCF
0xF0 11110000
0xFF 11111111
Where:
A indicates all combinations used.
y
PK232TM Rev. A 5/87
ISO 8208/CCITT X.25 PLP
Compressed TCP/IP packet. Van Jacobson (RFC 1144)
Uncompressed TCP/IP packet. Van Jacobson (RFC 1144)
Segmentation fragment
AX.25 layer 3 implemented.
AX.25 layer 3 implemented.
TEXNET datagram protocol
Link Quality Protocol
Appletalk
Appletalk ARP
ARPA Internet Protocol
ARPA Address resolution
FlexNet
NET/ROM
No layer 3 protocol implemented.
Escape character. Next octet contains more Level 3 protocol information.
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
Translation
A-3 Page 48