CRMA-II: A MAC protocol for ring-based Gb/s LANs and MANs

Abstract

CRMA-II is a MAC protocol for Gb/s LANs and MANs that are based on a single or dual-ring, with bus structures being covered as well. Data are transmitted in slots and buffer-insertion techniques are applied whenever data frames must be transmitted in contiguous slots or when immediate access to the media is required. Spatial slot reuse is achieved by freeing slots at their destination. Access to the media is controlled by a simple busy/free mechanism; it is enhanced by a reservation-based fairness mechanism. This results in fast access when the network is lightly loaded, while fair access and a tight delay bound are achieved during periods of sustained heavy load or overload. Fairness scheduling automatically adapts to the traffic requested by the nodes and causes minimal control intervention. Following current trends in communications, the standard ATM cell format has been adopted for both the medium and the attachment interfaces. CRMA-II control is only visible in an access sublayer between the standard ATM layer and the ATM transmission technology. © 1994.