Orwell Model Simplifications for Network Level Simulation

Rings form one of the three basic types of L.A.N. topology (the others being star and bus) and three basic types of protocol have been developed for use with them. By far the most popular of these are token based protocols, whereby the node holding a token is given exclusive access to the ring. Register insertion is another alternative; messages can be inserted onto the ring, delaying any existing traffic by passing it through a shift register. The third, but nowadays less favoured, approach is to use a slotted ring protocol: the ring is divided into slots which circulate around the ring; a node wishing to transmit a message waits until an unfilled slot is found, changes the header and transmits the message in the body of the slot. Slotted ring protocols were unpopular for several reasons: a monitor node is required to ensure that slots that become corrupted can be identified and regenerated (correct behaviour of the ring is critically dependent on correct behaviour of the monitor); to get a reasonable number of slots onto the ring delays have to be inserted at each node and one node, normally the monitor, has to be able to adjust its delay so that there are an integral number of slots; and the -efficiency of slotted rings is generally poor since the ratio of header to body is normally high. Its greatest advantage over token-based protocols, however, is that more than one node can be transmitting information at a time, using different slots on the ring. Acknowledgement of delivery is normally made by releasing the slot at the source (correct receipt there is taken to imply correct delivery at the destination); the node may not refill a slot that it has just released, ensuring that the slot is passed to the next node and thereby ensures fair access to all nodes on the ring. A typical implementation of a slotted ring is the Cambridge Ring protocol (British Standard BS6531). Examination of existing protocols has indicated that those based on a slotted ring are probably the best suited for carrying delay-sensitive speech, but simulation studies of high-bandwidth Cambridge Rings have indicated that there are still significant limitations when operated under high load and, further, load control is difficult since there is no relevant parameter that can easily be extracted from the ring. The Orwell protocol was developed after making a detailed study of the limitations of the Cambridge Ring protocol: it was found that by introducing destination release of slots, and by adding a novel, distributed, load control mechanism to bound access delays, a viable level of performance could be obtained. For higher capacity networks multiple, synchronized, rings can be used and such a network is known as an Orwell Torus.