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MFJ-1278B MULTI-MODE PACKET RADIO PROTOCOL
PACKET RADIO PROTOCOL
EXPLANATION OF PROTOCOL
The intention of this chapter is to give the user an overview of the packet protocol used to
transmit data by the TAPR software. The MFJ-1278B employs the genuine TAPR TNC 2
packet software. We give references to more detailed information required by those wishing
to implement these protocols on other hardware. The material presented below is somewhat
tutorial in nature for those who have not had previous exposure to layered network protocols,
but it presumes some knowledge of general communications hardware and software. Persons
already well versed in networking may want to skip this chapter and refer to the primary
defining document, Amateur Packet-Radio Link-Layer Protocol, AX.25 Version 2.0,
available from the ARRL, 225 Main Street, Newington, CT 06111 ($8.00 US, postpaid in
the United States as of this writing).
Organization of the MFJ-1278B hardware and software structure is according to the
International Standards Organization layered network model. The ISO model, sometimes
refered to as the ISO Reference Model of Open Systems Interconnection, describes the
seven protocol levels. The model and many other interesting topics are discussed in
Computer Networks by Andrew S. Tanenbaum.
The ISO model provides for layered processes, each supplying a set of services to a higher
level process. The MFJ-1278B currently implements the first two layers, the Physical layer
and the Data Link layer.
Physical Layer
The duty of the Physical Layer, layer one, is to provide for the transmission and reception of
data at the bit level. The only concern of the Physical Layer is how each bit is physically
transmitted, i.e., voltages on a hardwire line, modem tones on a telephone line or RF links.
Chapter 7, Hardware, gives a description on the physical layer in the MFJ-1278B. It is
compatible with the various TNCs currently available to radio Amateurs. The actual modem
interface is compatible with the Bell 202 standard that is similar to the CCITT V.23 standard.
Any other hardware device that is compatible with the Bell 202 standard should be
compatible with the MFJ-1278, at least at level one of the ISO reference model.
Data Link Layer
The duty of the Data Link layer is to supply an error-free stream of data to higher levels.
Since level one simply passes any bits received to level two and is unaware of the content or
overlying structure of the data, transmission errors are not detectable at level one. Level two
carries the responsibility of detecting and rejecting bad data, re-transmitting rejected data,
and detecting the reception of duplicate data.