A New CSMA-CD Protocol for Local Area Networks with Dynamic Priorities and Low Collision Probability

This paper reports on the implementation of a local area network (LAN) operating under a new CSMA-CD protocol with dynamic priorities (CSMA-CD-DP). User terminals, host computers, and other servers are connected to a common broad-band channel through N network access stations in a clustered manner. This concept reduces the number of network access stations and enhances the utilization of hardware and software resources greatly. A new protocol has been developed which organizes the decentralized operation of the distributed network access stations and which allows for a number of specific features. In the idle state the channel is operated in the contention mode. After the beginning of a transmission, the channel is operated in a reservation mode. Channel arbitration after a completed transmission is resolved by staggered delays; at any time, each station owns a distinct transmission delay which is changed after every successful transmission by broadcasted acknowledgments. This protocol strictly limits the possibility of collisions and approaches the effectiveness of token and polling protocols with increasing load. Through specific allocations of transmission delays, static priorities or dynamic overload control can be realized easily. The performance of the CSMA-CD-DP protocol has been modeled and analyzed analytically as well as by simulation. Results for normal load and overload reveal high throughput and low transfer times which are basic for a wide range of applications in LAN's.