MFJ-1270C/1274C MANUAL PACKET PROTOCOL
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the destination call sign and an optional list of one to eight digipeater (simplex
digital repeater) call signs.
The UI frame is used to send information bypassing the normal flow control and
acknowledgment protocol. The UI frame is not acknowledged but can be
transmitted at layer two without fear of disturbing higher layers. It is used for
beacon frames, for automatic identification packets, and for sending information
frames when the MFJ-TNC is not connected to another TNC, e.g., CQ and QST
activities.
The exact specifications for AX.25 are supplied in the ARRL publication
Amateur
Packet-Radio Link-Layer Protocol, AX.25 Version 2.0. The TAPR
implementation adheres to this standard for AX.25 version 2.0. The
implementation of version 1.0 is almost identical to the TAPR TNC 1 version of
AX.25 protocol in software releases 3.x. This provides compatibility with the
majority of Amateur packet radio stations.
The following table lists the frame types used by AX.25 and describes their
purpose. This material is provided to give a general understanding of the
protocol, and is not intended to replace the published specification. The byte
fields are given as they appear in memory after data is received, i.e., the high
order bit is at the left and the low order bit is at the right. This is also the format
of the display provided by the TRACE command. Some texts, including the
AX.25 protocol specification, list the bits in the order in which they are
transmitted, which is low order bit first.
The control bytes are presented in hex with "x" used to indicate four bits which
depend on the acknowledge functions the packet is performing. Usually "x" is a
frame number. Frame numbers fit into three bits and are used to ensure that
frames are received in order and that no frames are missed. Since only three bits
are available, the frame number is counted module 8. This is why the
MAXFRAME parameter has a ceiling of 7: no more than seven frames can be
"in flight" (transmitted but unacknowledged) at one time. A short description of
the use of the frames is given after the table.
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