We 5. Summary and Future Work

are working on techniques to make this protocol more scalable. This includes a quantitative analysis and measurement of the resource utilization in the wired network and base stations, and techniques to reduce this consumption. As part of the Daedalus project, we are also investigating ways of having multiple wireless communication devices on a mobile host, each with different network characteristics and coverage, and performing handoffs to another interface when the quality of the network deteriorates on a given device. We expect to develop algorithms based on monitoring the status and quality of the communication channel to perform such vertical handoffs. every twentieth packet sequence number has been plotted. Most of the transfer is smooth and the handoffs are hardly noticeable on the graph (they take less than 15 ms). However, there is one place, marked by the circle, where the handoff latency is about 900 ms. In our experiments, this happens about 10-15% of the time when the transfer is at the highest possible rate. An investigation of this situation reveals MAC-level effects of the WaveLAN to be largely responsible for this phenomenon. The WaveLAN is a shared medium and adjacent base stations contend for the link based on the CSMA/ CA algorithm. The problem is that the implementation of this algorithm on the WaveLAN has some artifacts that exacerbate the inherent unfairness of the scheme. Under heavy load through one of the base stations, the other base station is denied access for much longer than it should even if it has packets to send. Its MAC-layer protocol continually performs exponential backoffs; when it finally becomes the forwarding base station, it has backed off to the point where the link remains idle for several hundred milliseconds. We were able to reproduce this artifact several times and traced the times at which various packets were being sent through the WaveLAN interface. Recall that the beacons from the base stations are transmitted at a periodic rate of one packet per second, per base station. These beacons should therefore be transmitted and arrive at the mobile host through the WaveLAN interface once per second. Instead, several (between seven and ten) beacons from the new base station arrive all at once at the mobile host, after being queued at the old base station for many seconds (and experiencing the associated Ethernet backoffs). Unfortunately, the wireless link remains idle for between 200 and 950 ms …