The full dissertation is available here
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 msec. 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 algorithms 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.

The full dissertation is available here
Host mobility and wireless access are two emerging design considerations that pose challenging problems at all layers of the networking protocol stack. This dissertation investigates their impact on the design of link, network, and transport layer protocols. At the network layer, we have designed and implemented a new routing architecture that allows the current set of Internet standards to support routing to mobile hosts. At the link and transport layers, we have designed mechanisms to improve throughput over error-prone wireless channels.

At the network layer, the most crucial problem is that of routing. The existing Internet routing mechanisms cannot route packets to hosts whose points of attachment to the network change over time. Exploiting IP's Loose Source Route option, we have designed and implemented a routing scheme which provides location independent network access to TCP/IP compliant mobile hosts. It also allows mobile hosts equipped with multiple network interfaces to dynamically migrate active network sessions from one network interface to another. The proposed scheme only requires the addition of two new entity types, Mobile Routers and Mobile Access Stations. These entities perform all required mobility-aware functions, such as address translation, user tracking and location management. No modifications to existing host or router software are required.

Although MobileIP provides continuous network connectivity to mobile hosts, the effects of host movement and wireless medium characteristics are often visible at the transport layer. We consider the effect of wireless medium characteristics on the performance of Transmission Control Protocol (TCP) sessions. Unlike wired networks, packets transmitted on wireless channels are often subject to burst errors which cause back to back packet losses. We show that TCP's error-recovery mechanisms perform poorly when packets from a TCP session are subject to burst errors. Unlike other approaches which require modification to TCP, our solution requires enhancements only at the wireless link layer, thus making it applicable to other transport protocols as well. We use a Channel State Dependent Packet (CSDP) scheduler which takes wireless channel characteristics into consideration in making packet dispatching decisions. Our results show that the CSDP technique provides improved throughput, better channel utilization, and fairness among multiple TCP streams.

The full dissertation is available here
Integration of mobile computers within existing data networks introduces new issues in the design of distributed algorithms and services. Location of a mobile host changes with time, and so the message count of a distributed algorithm should account for the "search" necessary to locate mobile participants. Further, mobile hosts are faced with resource constraints not commonly encountered by their tethered counterparts, viz. a low-bandwidth connection to the rest of the network, and tight restrictions on power consumption.

This dissertation introduces a system model for networks with mobile hosts. To bridge the resource disparity between mobile and static hosts, we propose a two-tier principle for structuring distributed algorithms in this model. We also propose that location-management of mobile participants be integrated with algorithm design.

We first consider a simple, yet fundamental distributed algorithm: a logical ring with a token circulating amongst participants, and restructure it for servicing token requests from mobile hosts. Second, we tackle the problem of delivering a multicast message to mobile recipients from exactly-one location. Third, we present a checkpointing algorithm to record a consistent global state of a distributed application executed on mobile hosts.