3-tier structured access protocol to control unfairness in DQDB MANs

This paper proposes a 3-tier structured access protocol that divides the access mechanism into three domains (viz., low, normal and heavy), depending on the magnitude of the actual load in the network. Our approach is to identify the dominating cause of unfairness, if any, in each of these domains and provide the remedy accordingly. In a unidirectional bus, the traffic ordered by the network is very likely to follow a graded (or symmetric) pattern, decreasing along the direction of the bus. The proposed 3-tier protocol is designed to meet the needs of the symmetric demand pattern. We demonstrate that such a 3-tier can be successfully implemented and the symmetric demands can be served better. At low loads, because of the ample bandwidth available, unfairness is never an issue and hence the 802.6 performance is retained. At normal loads, the latency in transporting a request contributes to unfairness. Using an Anticipatory Demand prediction scheme (ADS), the unfairness in this domain is addressed. At heavy loads, the restrictions imposed by the request filing mechanism and also the actual load pattern (segment flooding and request flooding) result in different unfairness patterns. The heavy load domain unfairness is handled by an Access Protection Scheme (APS). The performance of the proposed protocol is studied under different traffic demand patterns. A tuning scheme, offering the potential for dynamic bandwidth allocation, is briefly discussed. Our studies demonstrate that the 3-tier domains can be successfully implemented. Under heavy symmetric demand pattern, the service pattern (in terms of bandwidth) matches the demand. Hence the symmetric performance is impressive, in comparison with the bandwidth balancing (BWB) mechanism. At other load patterns, the performance is found to be reasonable.