AEA PAKRATT PK-232, Technical Reference Manual

Page: 49 / 101

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-4 Page 49
Note:
All forms of
yy11yyyy
and
yy00yyyy
other than those listed above are reserved at this time
for future level 3 protocols. The assignment of these formats is up to amateur agreement. It is
recommended that the creators of level 3 protocols contact the ARRL Ad Hoc Committee on Digital
Communications for suggested encodings.
A.2.5 Information Field
The information field is used to convey user data from one end of the link to the other. I fields are
allowed in only three types of frames: the I frame, the UI frame, and the FRMR frame. The I field
can be up to 256 octets long, and shall contain an integral number of octets. These constraints ap-
ply prior to the insertion of zero bits as specified in
A.2.6 , below. Any information in the I field
shall be passed along the link transparently, except for the zero-bit insertion (see A.2.6 ) necessary
to prevent flags from accidentally appearing in the I field.
A.2.6 Bit Stuffing
In order to assure that the flag bit sequence mentioned above doesn't appear accidentally any-
where else in a frame, the sending station shall monitor the bit sequence for a group of five or
more contiguous one bits. Any time five contiguous one bits are sent the sending station shall in-
sert a zero bit after the fifth one bit. During frame reception, any time five contiguous one bits are
received, a zero bit immediately following five one bits shall be discarded.
A.2.7 Frame-Check Sequence
The frame-check sequence (FCS) is a sixteen-bit number calculated by both the sender and re-
ceiver of a frame. It is used to insure that the frame was not corrupted by the medium used to get
the frame from the sender to the receiver. It shall be calculated in accordance with ISO 3309
(HDLC) Recommendations.
A.2.8 Order of Bit Transmission
With the exception of the FCS field, all fields of an AX.25 frame shall be sent with each octet's
least-significant bit first. The FCS shall be sent most-significant bit first.
A.2.9 Invalid Frames
Any frame consisting of less than 136 bits (including the opening and closing flags), not bounded
by opening and closing flags, or not octet aligned (an integral number of octets), shall be consid-
ered an invalid frame by the link layer. See also
A.4.4.4 , below.
A.2.10 Frame Abort
If a frame must be prematurely aborted, at least fifteen contiguous ones shall be sent with no bit
stuffing added.
A.2.11 Interframe Time Fill
Whenever it is necessary for a DXE to keep its transmitter on while not actually sending frames,
the time between frames should be filled with contiguous flags.
A.2.12 Link Channel States
Not applicable.