User:Graham.Fountain/Liner Token Passing Bus

AS4074, the Liner Token Passing Bus is a data bus protocol for a 100 megabit per second, shared media physical layer. It was primarily intended for use with fibre optic media, e.g. using a star coupler. As a protocol for a shared medium, only one transmitter may be active at any one time. Hence a logical ring is imposed on the physical architecture. Access control for this logical ring is co-operative, through the use of a token passed between transmitters. Only possession of this token grants the right for a connected terminal to transmit.

While there was some interest in the bus as a potential upgrade for military avionic systems, to replace MIL-STD-1553B buses, there is no known implementation of it. It's main 'flaw' in this context was that, if the token were to be lost in passing from one terminal to the next, the time taken to re-start token passing would have been excessive. Other than this flaw, although complicated, it was possible to calculate transmission delays (maximum time taken to pass the token around the logical ring, plus the maximum time to transmit all data not of a lower priority). The complication in this being that there were multiple priorities of data and each terminal could have different constants to control the maximum duration it could transmit at a given priority. A security related issue is that in implementations of MIL-STD-1553B buses, SEAFAC validated terminals can be relied on to only receive and transmit as commanded by the BC. Hence the reliability of transfers between any two RTs is dependent only on their performance and that of the BC, which can be relied on, and is normally implemented as a redundant function. Whereas, with AS4074 the reliability of transfers between two terminals depends on the action of other terminals.