In traditional all
. IntroductionWireless local area networks have come into great use in recent years. Although a variety of wireless network technologies have been in the market, wireless LANs based on the IEEE 802.11 standard are the most popular. There are two operation modes, i.e., the infrastructure mode and the ad hoc mode, specified in IEEE 802.11 standard. In the ad * This work was partially supported by the National Science Council and the Ministry of Education of ROC under the contract No. NSC92-2622-E-002-002 and 89E-FA06-2-4-8.hoc mode, all mobile nodes are free to directly communicate with each other in a peer-to-peer manner. Another option of the operation mode in WLAN is the infrastructure mode, where each mobile node associates itself with an access point (AP) within its direct transmission range. The AP is connected to a wired network and provides a conduit to the external network for the mobile nodes. Communications can only take place between the wireless node and the AP but not directly between the wireless nodes.In this paper, we consider the problem of distributing popular data at the server to users in a wireless LAN environment. The data dissemination problems correspond to a set of important applications. These applications include distribution of course slides in a class, dissemination of useful information in the info-station, online documents distribution in a conference, and many more. In a traditional infrastructure-based WLAN, all communications will go through the APs, but an AP can only provide a certain amount of bandwidth (e.g., 802.11b is 11 Mbps) for all users within its transmission coverage. In such wireless network environment, there are two major inefficiencies: (a) load imbalance problem among the APs: when the distribution of the location of the mobile nodes is non-uniform, this might lead to a situation where a lot of nodes access a single AP but other APs are idle, and (b) poor resource utilization: most of the nodes contend for the wireless channel of the AP while there are still some unused but available wireless channels, which can be used to set up ad hoc groups. When the number of users increases within an AP's cell, the contention for the wireless channel will lead to more collisions, degradation in system throughput, and longer data delivery time for the mobile nodes. To increase the system throughput and reduce the user's response time, a simple solution is to allocate more bandwidth for users by installing more APs, which use different wireless channels, but this approach is expensive and not flexible to deal with the "temporary" fluctuation of the network load.In a wireless network environment, an important observation for the data dissemination applications is that most local users are interested in nearly the same set of objects.