Modern distributed systems involving large number of nonstationary clients (mobile hosts, MH) connected via unreliable low-bandwidth communication channels are very prone to frequent disconnections. This disconnection may occur because of different reasons: The clients may voluntarily switch off (to save battery power), or a client may be involuntarily disconnected due to its own movement in a mobile network (hand-off, wireless link failures, etc.). A mobile computing environment is characterized by slow wireless links and relatively underprivileged hosts with limited battery powers. Still, when data at the server changes, the client hosts must be made aware of this fact in order for them to invalidate their cache, otherwise the host would continue to answer queries with the cached values returning incorrect data. The nature of the physical medium coupled with the fact that disconnections from the network are very frequent in mobile computing environments demand a cache invalidation strategy with minimum possible overheads. In this paper, we present a new cache maintenance scheme, called AS. The objective of the proposed scheme is to minimize the overhead for the MHs to validate their cache upon reconnection, to allow stateless servers, and to minimize the bandwidth requirement. The general approach is (1) to use asynchronous invalidation messages and (2) to buffer invalidation messages from servers at the MH's Home Location Cache (HLC) while the MH is disconnected from the network and redeliver these invalidation messages to the MH when it gets reconnected to the network. Use of asynchronous invalidation messages minimizes access latency, buffering of invalidation messages minimizes the overhead of validating MH's cache after each disconnection and use of HLC off-loads the overhead of maintaining state of MH's cache from the servers. The MH can be disconnected from the server either voluntarily or involuntarily. We capture the effects of both by using a single parameter: The percentage of time a mobile host is disconnected from the network. We demonstrate the efficacy of our scheme through simulation and performance modeling. In particular, we show that the average data access latency and the number of uplink requests by a MH decrease by using the proposed strategy at the cost of using buffer space at the HLC. We provide analytical comparison between our proposed scheme and the existing scheme for cache management in a mobile environment. Extensive experimental results are provided to compare the schemes in terms of performance metrics like latency, number of uplink requests, etc., under both a high and a low rate of change of data at servers for various values of the parameters. A mathematical model for the scheme is developed which matches closely with the simulation results.

We present a new cache maintenance scheme, called AS, suitable for the wireless mobile environment. Our scheme integrates the mobility management scheme of Mobile IP with cache maintenance scheme used in the CODA file system. As opposed to broadcasting invalidation report schemes, AS supports arbitrary disconnection patterns and uses less wireless bandwidth. We present analytical and simulation results to show the superiority of our caching scheme.

This paper investigates the performance of IEEE 802.11 wireless local area network (WLAN) protocol's distributed coordination function (DCF) in the presence of mobile and hidden terminals. In order to study the joint effect of hidden terminals and user mobility on the performance of IEEE 802.11 DCF, we extend Tobagi and Kleinrock's (1975) hearing graph framework to model hidden terminals in a static environment. We derive a combined mobility and hidden terminal model using a Markov chain from the hearing graph of a given physical layout. The simple model uses two parameters: /spl alpha/, which controls the number of hidden terminals in the steady state, and /spl lambda/, which controls the rate of mobility of each terminal. By varying the values of /spl alpha/ and /spl lambda/ we can systematically generate scenario with different number of hidden terminals and different mobility rates for a particular physical layout with static obstructions. We have developed a discrete event simulator which uses the parameterized model to obtain the throughput and blocking probability behavior of an IEEE 802.11 based ad hoc network in the presence of certain static obstructions. Our simulations suggest that the IEEE 802.11 DCF protocol is robust enough to handle moderate conditions of hidden terminals and mobility, but the performance may degrade under extreme conditions. Carefully selecting protocol parameters (RTS and fragmentation threshold) can help improve the performance even under extreme conditions.

The hidden terminal problem is unique to wireless networks and as of now there is very limited understanding about its effects on network performance. Results are presented from a simulation study of the IEEE 802.11 MAC protocol when operating in the presence of hidden terminals. We also propose a framework for modeling hidden terminals which can handle complex scenarios of both mobility and static obstructions. Our simulations indicate that hidden terminals can have a very detrimental effect on the performance of the IEEE 802. 11 MAC protocol. Although the throughput is acceptable when about 10 percent of station pairs are hidden, the packet delay can increase by an order of magnitude. The performance of the protocol drops sharply when the number of hidden pairs exceeds 10 percent.