System for providing fair access for vlans to a shared transmission medium

Abstract

A method of controlling access to a common physical data transmission link from a multiplicity members of a packet-based local area network, wherein the members are partitioned into a plurality of virtual local area networks (VLANs) and wherein packets are identified according to the virtual local area network from which they come, comprises (a) monitoring packet traffic from the virtual local area networks on the link and thereby obtaining measures of the traffic flow from each of the virtual local area networks; (b) determining whether the traffic flow for each respective VLAN is above or below a threshold; (c) denoting a VLAN which is above its threshold as in a restricted state and denoting a VLAN which is below its threshold as in a priority state; (d) arbitrating between requests from the VLANs to transmit data packets on the link; and (e) according priority in the arbitration to a VLAN which is in a priority state over any VLAN which is in a restricted state. A request to transmit is gated with a signal denoting the state of the respective VLAN to provide a set of priority requests. In response to the presence of any priority request the arbitration is made only in respect of the set of priority requests and in the absence of a priority request arbitration is made in respect of the VLANs which indicate a request to transmit. A 'leaky bucket' counter may be used for monitoring the traffic for each VLAN.