Fairness under Sustained Overload in a DQDB Subnetwork with Eraser Nodes

The Distributed Queue Dual Bus (DQDB) protocol has been specified by the IEEE 802.6 committee as part of its proposed standard for Metropolitan Area Network. DQDB is based on 'OR' writing. However, by allowing erasing capabilities in some stations, its transmission capacity can be increased considerably. In this paper, we propose fairness criteria for the apportionment of throughput among the different stations for a DQDB subnetwork involving eraser nodes for connectionless type traffic under sustained overload. These criteria are based on maximizing the minimum throughput available to any station. Idealistic values of capacity apportionment for each active station are analytically computed by a recursive procedure based on closed form solutions for linear programming subproblems. These idealistic values provide a quantitative standard against which the fairness of proposed eraser node protocols may be assessed. A numerical example is used to provide a detailed demonstration of our method for capacity apportionment. Specific network scenarios are used to show the trade-off between fairness and throughput.