The Cambridge Backbone Ring

The Cambridge Backbone Ring (CBR) is a collaborative project between Olivetti Research Limited and the University of Cambridge Computer Laboratory. The project has designed and built an experimental ring communication network for integrated computer data, voice and real-time video applications. The network is currently aimed as a backbone facility for interconnecting groups of CFR (Cambridge Fast Ring) networks in the Cambridge area. Media access control is through the empty slot technique, with transmitting stations lling multiple slots each ring revolution. The network operates on monomode optical bre and is designed for an eventual line rate of 1000 Mbit/sec In the Backbone Ring architecture 1], the bandwidth of the bre optic channel can be partitioned into a number (currently four) of TDM channels. This enables stations of varying cost and bandwidth to be attached to one network , parameterised by the number of channels a station can use concurrently. Unlike several other TDM ring projects in the literature, the Backbone Ring architecture achieves full connectivity, even with the simplest type of station. 1 Background and Requirement One motive for the Cambridge Backbone Ring project is the provision of high band-width interconnection for clusters of Cambridge Fast Rings. The Cambridge Fast Ring (CFR) 2] is a slotted ring local-area network which operates at 75 Mbit/second over dual twisted pair and multimode optical bres. The Backbone Ring uses an empty slot access protocol which is similar to that used for the CFR. Consequently,