Purushotham Kamath - 2/24/04
ISI

THE NEED FOR MEDIA ACCESS CONTROL IN OPTICAL CDMA NETWORKS

Optical CDMA Local Area Networks allow shared access to a broadcast medium. Every node on the network is assigned an Optical Orthogonal Codeword (OOC) to transmit or receive on. OOCs are designed to be pseudo-orthogonal, i.e., the correlation (and therefore the interference) between pairs of codewords is constrained. In this talk, we demonstrate that the use of optical CDMA does not preclude the need for a media access control (MAC) layer protocol to resolve contention for the shared media.

As more nodes on an optical CDMA network transmit simultaneously, the interference between codewords increases and the network throughput falls. In this talk we discuss a network architecture where there is
virtually no MAC layer, except for choice of the codeset, and show that its throughput degrades and collapses under moderate to heavy load. We discuss an alternate architecture called Interference Avoidance where nodes on the network use media access mechanisms to avoid causing interference on the line, thereby improving network throughput. Interference avoidance is analyzed and it is shown that it can provide up to 30% improvement in throughput with low delays and no throughput collapse. The analysis is validated through simulation with realistic network traffic traces.