A Star-Configured Optical Local Area Network Using Carrier Sense Multiple Access with a Novel Collision Detector

This work describes design and theoretical performance of Star-Configured optical local area network that uses a passive star coupler and employs carrier sense multiple access with collision detection, CSMA/CD. The local area network can support data rates up to 200 Mb/sec. Collision detection is a formidable design challenge in an optical medium because of the large signal level variations (dynamic range) normally expected. A number of methods of collision detection are proposed in the literature. However, all of them have limited reliability or limited dynamic range. We introduce a novel collision detection method that uses collision sequences constructed from cyclic error-correcting codes and a sequence weight violation rule. Each transmitter has its own unique sequence with a constant Hamming weight. The collision detection sequence is inserted in each packet header. Our method enables detection of collision subject to far wider dynamic range variation than competing methods proposed up to now and is extremely simple. The collision detector consists of a counter and it works both for non-return to zero and Manchester coding. The collision detector is analyzed for an avalanche photo diode receiver. The general performance analysis is done both with Gaussian approximations and with moment methods. A dynamic range of 17 dB seems possible for a transmitter with an extinction ratio of 100 and a simple receiver with a fixed threshold. An explicit table of 56 collision detection sequences based on Golay code is presented.