Description
In this dissertation, we examine the problem of performing handoff quickly in cellular data networks. We define handoff as the process of reconfiguring the mobile host, wireless network and backbone wired network to support communication after a user enters a different cell of the wireless network. In order to support applications and protocols used on wired networks, the handoff processing must not significantly affect the typical end-to-end loss or delay of any communications. This dissertation concentrates on two specific areas of handoff processing: routing updates and state distribution. The techniques we use to solve these problems are:
1. Multicast to set up routing in advance of handoff.
2. Hints, based on information from the cellular wireless system, to predict handoff.
3. Intelligent buffering, enabled by the multicast of data, to prevent data loss without the use of complicated forwarding.
4. State replication, enabled by the multicast, to avoid explicit state transfers during the handoff processing.
This dissertation describes the design, implementation and evaluation of these techniques in a variety of networking and computing environments. We have shown that any necessary routing updates and state transfers can be performed in a few milliseconds. For example, our implementation in an IP-based testbed completes typical handoffs in 5-15 msecs. In addition, the handoff processing introduces no additional packet delays or data loss. The primary cost of our algorithms to improve handoff latency is the use of excess bandwidth on the wired backbone networks. However, we have introduced base station layout diagrams that reduce this cost. In current systems, the performance improvement provided by these techniques easily outweigh the resources consumed. Since wired backbone networks will continue to have much greater available bandwidth than their wireless counterparts, this trade-off between handoff performance and network resources will continue to be advantageous in the future.