A Highly Reliable LAN Protocol

As a research project for NASA\´s Langley Research Center, we developed a variation on MIL-STD-1553B (the military standard for avionics buses) whose goal was increased fault tolerance. The resulting protocol, called implicit token passing (ITP), replaces an explicit token with brief "soundoff" messages from all nodes participating on the LAN. Since every node participates on every "token cycle," bus silence is an error indication and initiates recovery action. By encoding state information in the headers of the transmitted words, nodes are continuously aware of the global state of the network. This local knowledge of the global network state allows the system to continue operation in spite of nodal failures. A station which fails but then recovers can quickly assess the global network state and then safely rejoin the active nodes. ITP features high throughput and bounded message delay, and achieves high reliability through tolerance of failed nodes and automatic resynchronization when failed nodes are revived. The protocol is ideally suited for a bus topology and fiber optic media.