Source field

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The source field of an AX.25 packet is a 7-byte field, with 6 bytes for the callsign and the 7th byte for the SSID and other options[1]. The callsign bytes must be 7-bit ASCII characters, as the 8th bit is used for HDLC.

Callsign

Callsigns must be be 6 characters or less.

SSID

On AX.25 networks the SSID is optional (technically 0 if one is not given), but if specified must be between 1 and 15.

For packets that originate within the APRS-IS the SSID can be any 2-digit alphanumeric identifier, but these cannot be propagated to RF via IGates.

References