![]() ![]() when the callsign is shorter then 6 characters/bytes. Unused bytes are filled with the “space” character (0x20), eg. Each address field is seven bytes long and consist of the radio amateur callsigns, plus an additional byte for the SSID. For this system to work, all address bytes have to be left shifted by one bit to make space for this bit. The end of the complete address field is indicated by a “1” in the least significant bit of the last byte of the last address. First the destination address, followed by the source address and then one or two optional repeater addresses. Length of each field are in bytes: Address FieldĪX.25 defines that the address field is composed of multiple addresses. It also defines the FCS used, the 16-bit CRC-CCITT.īellow is the structure of a AX.25 frame with all the fields and how they fit into the underlying HDLC frame. At its core it uses HDLC frames and adds to it the definition and semantics of all the fields. AX.25 is designed for use by amateur radio operators. The JavaScript source code used can be downloaded bellow.ĪX.25 is a data link layer (2nd OSI Layer) derived from the X.25 protocol, which in it’s part is derived from the HDLC standard. Changing any field, including the “AX.25 Frame” or the “HDLC Frame” updates all the others, doing encoding and decoding accordingly. The frame generator updates in real-time and also works in reverse. The result is displayed in HEX format in the “HDLC Frame” field. The generated AX.25 frame is then encoded into the complete AX.25 HDLC frame, the bit order is inverted (LSBit), the FEC is calculated (16-bit CRC-CCITT) and added to the end of the frame, bit stuffing is done, start and end flags are added. The result is displayed in HEX format in the “AX.25 Frame” field. These fields are then used to generate the AX.25 frame first, shifting the address bytes and adding the end address bit to the last byte of the address field. ![]() The payload has to be in ASCII characters and with a maximum length of 255 characters. They can be input in either decimal or HEX format. Both the PID and Control fields are also encoded/decoded. The bellow frame encoder/decoder works for simple AX.25 frames where only one destination and source address is used, each up to 6 characters long plus the SSID field. ![]()
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December 2022
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