AEA PAKRATT PK-232, Technical Reference Manual

Page: 52 / 101

PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-7 Page 52
4. The bit marked "C" is used as the command/response bit of an AX.25 frame, as outlined in
A.4.1.2 below.
5. The characters of the call sign should be standard seven-bit ASCII (upper case only) placed in
the leftmost seven bits of the octet to make room for the address extension bit. If the call sign
contains fewer than six characters, it should be padded with ASCII spaces between the last call
sign character and the SSID octet.
6. The 0000 SSID is reserved for the first personal AX.25 station. This establishes one standard
SSID for "normal" stations to use for the first station.
A.2.13.2 Level 2 Repeater-Address Encoding
If a frame is to go through level 2 amateur packet repeater(s), there is an additional address sub-
field appended to the end of the address field. This additional subfield contains the call sign(s) of
the repeater(s) to be used. This allows more than one repeater to share the same RF channel. If
this subfield exists, the last octet of the source subfield has its address extension bit set to zero,
indicating that more address-field data follows. The repeater-address subfield is encoded in the
same manner as the destination and source address subfields, except for the most-significant bit
in the last octet, called the "H" bit. The H bit is used to indicate whether a frame has been re-
peated or not.
In order to provide some method of indicating when a frame has been repeated, the H bit is set to
zero on frames going to a repeater. The repeater will set the H bit to one when the frame is re-
transmitted. Stations should monitor the H bit, and discard any frames going to the repeater (up-
link frames), while operating through a repeater. Fig. 4 shows how the repeater- address subfield
is encoded. Fig. 4A is an example of a complete frame after being repeated.
Fig. 4 – Repeater-address encoding
Octet ASCII Bin. Data Hex Data
A15 W 10101110 AE
A16 B 10000100 84
A17 4 01101000 68
A18 J 10010100 94
A19 F 10001100 8C
A20 I
10010010
92
A21 SSID HRRSSID1
Bit Order -->
76543210
Where
:
1. The top octet is the first octet sent, with bit 0 being sent first and bit 7 sent last of each octet.
2. As with the source and destination address subfields discussed above, bit 0 of each octet is the
HDLC address extension bit, which is set to zero on all but the last address octet, where it is
set to one.
3. The "R" bits are reserved in the same manner as in the source and destination subfields.
4. The "H" bit is the has-been-repeated bit. It is set to zero whenever a frame has not been re-
peated, and set to one by the repeater when the frame has been repeated.