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Frame Structure

Link layer packet radio transmissions are send in small blocks of data, called frames. Each frame is made up of several smaller groups, called fields.

Figure : AX.25 Frame Format

Figure 2 depicts the structure of the AX.25 frame. The frame format is as follows:

There are two differences between the AX.25 frame format and that of HDLC: the Protocol Identifier (PID) field and the address field. The PID field is used to designate the layer 3 protocol that is using the AX.25 link protocol. This would allow multiple users of the link layer protocol. The most important difference between AX.25 and HDLC is the addressing technique. In HDLC, there two possible configurations: a point-to-point link with two stations, and a multi-drop link with one primary and multiple secondaries. In either case, a single address is not sufficient in a packet-radio network for two reasons:

  1. Since the network is a peer, distributed network, both the source and destination stations should be identified; neither is unique. For flow control, error control, and sequence numbering, both addresses are needed.
  2. If repeaters are involved, these repeaters must be specified. In particular, it is the responsibility of the transmitting station to specify the repeater or repeaters that must be used to get from source to destination.

The AX.25 address field is from 14 to 70 octets long, depending on whether and how many repeaters are used between a particular source-destination pair. If the sending and receiving stations are in the same cluster (within range of each other), then it is only necessary to specify the source and destination station addresses. Each is specified using 7 octets, which contains a callsign of up to 7 characters. If a frame is to go through a repeater, an additional address subfield is appended to the end of the address field. For more comprehensive discussion about how to encode the AX.25 address field see [WB4JFI 84].

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Fri Feb 10 12:57:56 EST 1995