MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL PACKET PROTOCOL
Page 224
PACKET RADIO PROTOCOL
Explanation of Protocol
The material in this chapter is intended to supply an overview of the packet
protocol used to transmit data by the TAPR software. The MFJ-TNC employs
the
genuine TAPR TNC 2 packet software. References are given 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 wri-
ting).
The MFJ-TNC hardware and software architecture is organized in accordance
with the International Standards Organization (ISO) layered network model.
The model describes seven levels and is officially known as the ISO Reference
Model of Open Systems Interconnection, or simply the ISO Model. 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-TNC 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. It is concerned only with how each bit is
physically transmitted, i.e., voltages on a hardwire line or modem tones on phone
or RF links.
The physical layer of the MFJ-TNC is described in Chapter 7, Hardware. It is
compatible with the various TNCs currently available to radio Amateurs. The
actual modem interface is compatible with the Bell 202 standard which is similar
to the CCITT V.23 standard. Any other hardware device which is compatible