Epson MFJ-1278B manual Physical Layer, Data Link Layer

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-RadioLink-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.