On the use of free-space optical links for latency-tolerant traffic applications

The growing need for network capacity due to peer-to-peer (P2P) users causes tremendous costs for Internet service providers (ISPs). Traffic shaping schemes have been applied to limit P2P traffic while reducing customer satisfaction and affecting the performance of other applications. As an alternative, in this work we propose the use of inexpensive free-space optical (FSO) links from ISPs to fibre backbones to accommodate latency-tolerant traffic including P2P and other bulk data transfers for backup and storage. For such classes of data traffic, the instantaneous reliability of the link is not the primary concern, rather the average throughput of the link is important. The average throughput and outage characteristics of an FSO link for latency-tolerant traffic are studied using weather data from a major Canadian city. Rather than previous studies which focus on improving the reliability of FSO links, this work considers different modulation techniques with the goal of achieving the highest average throughput for data traffic. For a 1 km link with a 1 ns symbol period, an average annual rate of nearly 1.5 Gbps is possible using 4-PAM modulation as opposed to an average rate of 880 Mbps for conventional on-off keying.