A Performance Study of the Local Fairness Algorithm for MetaRing MAC Protocol

The MetaRing is a Medium Access Control (MAC) protocol for Gigabit LANs and MANs with cells removed by the destination stations (slot reuse). Slot reuse increases the aggregate throughput beyond the capacity of single links but can cause starvation. In order to prevent this the MetaRing MAC protocol includes a fairness algorithm. Two types of fairness algorithms have been proposed: "global" and "local". The paper analyses the local fairness algorithm specified in [CHEN93] by focusing on a worst-case network scenario which is particularly critical with respect to slot reuse. This scenario consists of a group of N equally spaced stations located in one half of the ring and which all send data traffic to a gateway only. The analysis can be divided into two parts. In the first part stations are assumed to operate in asymptotic conditions (never empty queues) and closed formulae for the aggregate and station throughputs are derived. In the second part of the paper the performances of the local fairness algorithm are studied in normal conditions, that is when the offered load is lower (underload conditions) or slightly higher (overload conditions) than the aggregate throughput achievable in asymptotic conditions. The analysis is performed (by simulation) on the same worst-case network scenario. The results obtained show that in asymptotic conditions the local fairness algorithm always allows a complete utilization of the medium capacity. Although it is not perfectly fair, the unfairness can be reduced by choosing of the algorithm parameters appropriately. This choice guarantees fairness in overload conditions as well, but causes unfairness in underload conditions.