AEA PAKRATT PK-232, Technical Reference Manual

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PK-232 TECHNICAL MANUAL APPENDIX A – AX.25 LEVEL 2 PROTOCOL
PK232TM Rev. A 5/87 A-5 Page 50
A.2.13 Address-Field Encoding
The address field of all frames shall be encoded with both the destination and source amateur call
signs for the frame. Except for the Secondary Station Identifier (SSID), the address field should be
made up of upper-case alpha and numeric ASCII characters only. If level 2 amateur "repeaters"
are to be used, their call signs shall also be in the address field.
The HDLC address field is extended beyond one octet by assigning the least-significant bit of each
octet to be an "extension bit". The extension bit of each octet is set to zero, to indicate the next
octet contains more address information, or one, to indicate this is the last octet of the HDLC ad-
dress field. To make room for this extension bit, the amateur Radio call sign information is shifted
one bit left.
A.2.13.1 Nonrepeater Address-Field Encoding
If level 2 repeaters are not being used, the address field is encoded as shown in Fig. 2. The desti-
nation address is the call sign and SSID of the amateur radio station to which the frame is ad-
dressed, while the source address contains the amateur call sign and SSID of the station that sent
the frame. These call signs are the call signs of the two ends of a level 2 AX.25 link only.
Fig. 2 – Nonrepeater Address-Field Encoding
First Octet Sent
Address Field of Frame
Destination Address Source Address
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14
A1 through A14 are the fourteen octets that make up the two address subfields of the address
field. The destination subaddress is seven octets long (A1 thru A7), and is sent first. This address
sequence provides the receivers of frames time to check the destination address subfield to see if
the frame is addressed to them while the rest of the frame is being received. The source address
subfield is then sent in octets A8 through A14. Both of these subfields are encoded in the same
manner, except that the last octet of the address field has the HDLC address extension bit set.
There is an octet at the end of each address subfield that contains the Secondary Station Identifier
(SSID). The SSID subfield allows an Amateur Radio operator to have more than one packet-radio
station operating under the same call sign. This is useful when an amateur wants to put up a re-
peater in addition to a regular station, for example. The C bits (see
A.4.1.2 , below) and H bit (see
A.2.13.2 , below) are also contained in this octet, along with two bits which are reserved for future
use.
Fig. 3A shows a typical AX.25 frame in the nonrepeater mode of operation.