Bulletins from the Pacific Packet Radio Society - page 070

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TAPR PROTOCOL

The Tucson Amateur Packet Radio group will be using a protocol based on standard segmentation of networking tasks into levels of protocols with well-defined interfaces between levels.

PHYSICAL LEVEL

The physical medium includes medium access procedures, specifically using carrier-sensed multiple access on a channel which may have, but does not require, a simplex digital repeater. The two tones used by the Bell 202 standards will be used for mark and space, and NRZI encoding will provide for synchronization of the incoming packet to the demodulator.

LINK LEVEL

The data link layer operates on the physical channel, controlling access and utilization. Specifically, the link layer handles:

control of data transfer error checking and recovery information coding
information transparency optimization of channel utilization maintenance of synchronization
communication facility transparency remote node bootstrapping capability

A "sub-set" of the IEEE-802 Local Area Network Standard is used, a combination of the Logical Link Control and Medium Access Control. Medium Access (MAC) fits within the LLC packet, and may be used to handle a large number of medium specific problems which simply cannot be handled by the point-to-point and multi-drop bit-oriented protocols currently in use. Multiple control stations, nodes located in multiple local area network domains and other problems of this ilk can be controlled by expanding header bytes beyond the control byte.

The control patterns used in the control byte are defined by LLC, and are identical to those of HDLC, SDLC, ADCCP and others. There are two address bytes, those of the sender and receiver, allowing non-repeater and repeater operation. The addresses are given to nodes at the time the node enters the net by the first station on the channel. This station, designated the Net Control or Station Node, is identical to all other nodes on frequency, except that it parcels out addresses for nodes checking in, and distributes to all nodes current addressing information. By default then, any stations leaving a net immediately lose their ID, and are given a new ID upon arriving at any new network. If no net exists on frequency, that node becomes the net control. No multiple address problems are possible in this type of addressing scheme; if a system failure occurs, however, the net control simply reassigns ID's to

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