Performance analysis of the pi-persistent protocol in unidirectional bus networks

• Authors: Rudolf Mathar;Krzysztof Pawlikowski
• Venue: Computer Communications
• Year: 1997

The p"i-persistent medium access protocol is an attractive solution for high-speed time-slotted unidirectional bus networks. This protocol, with its very simple flow control mechanism allowing station i to access empty slots with station-dependent probability p"i, has been seen as an alternative for solving fairness problems associated with the original IEEE 802.6 standard for the distributed queue dual bus (DQDB) for high-speed networks. In this paper we analyze the main aspects of the fairness provision mechanism employed in the p"i-persistent protocol. Assuming heavy traffic load, the stations' access probabilities are determined so as to allocate certain portions of the channel capacity for guaranteeing prescribed service requirements. Having determined the probability generating function of queue length as well as the two first moments of the worst-case packet delay (delay of the last packet arriving at a station within a slot time) for a single station, we investigate packet delays and fairness in a bus network with N stations. Our results show that, in a network with homogeneous Poissonian arrival streams, if the bus is able to carry all the offered load, protecting fairness of services by lowering the access probability at a station leads only to deterioration of that station's performance without affecting performance of other stations. This is a surprising result which however does not apply for bursty traffic, as shown by the results of our simulation studies.