them as an abstract cellular model where users access information through access points. ract model consists of a base station, a number of clients, and a number of channels. A client can acquire an uplink channel to send a request to the base station and receive the result from a downlink channel. While we assume that many users can listen to the same downlink channel to achieve broadcasting from the base station, an uplink channel is dedicated to one client for transmission at any time. There are two basic approaches to disseminating data to mobile clients. In on-demand access, a mobile client submits a request to the server, which then returns the results to the mobile client directly via point-to-point connection. In periodic broadcast, data is broadcast periodically on a wireless channel. A mobile client listens to the broadcast channel and downloads the