Star Network With Collision-Avoidance Circuits

Carrier-Sense Multiple-Access local-area networks with collision detection (CSMA-CD)1 have been implemented using lightwave technology,2 but the use of lightguides is questioned because the networks do not benefit from the full bandwidth of the lightguides. In these networks, the lightguides are used to reduce ground-loop voltage, electromagnetic interference (EMI), size, and weight of the cable. CSMA-CD networks have the disadvantage of requiring a large packet size when they operate at high bit rates. For example, a 1-kmlong (2-km round trip) passive star network,2 operating at 150 Mb/s, has an efficiency of 5 percent for 256 bits/packet, 44 percent for 4096 bits/packet, and 93 percent for 65,536 bits/packet. In addition to the low efficiency there is an associated instability problem that appears when the network reaches the channel capacity.3 Without traffic restrictions or under heavy traffic conditions the network becomes unstable and crashes. This paper describes a new method to eliminate the collisions caused by simultaneous transmission of packets in a star network. The method consists of placing collision-avoidance circuits in the node at the center of the star. This collision-avoidance circuit does the work of a "traffic cop," in that it lets pass the packets that arrive while the node is idle and blocks those that arrive while the node is busy. The packets that 631