Mixed-mode wireless networks: framework and power control issues

Wireless networks have been widely deployed in recent years to provide high-speed Internet access to mobile users. In traditional IEEE 802:11 wireless LAN using infrastructure mode, all users share the same channel and all packets are forwarded by an access point (AP). As a result, when the number of users increases, the throughput of each user degrades correspondingly. If users within a cell communicate with each other (as in some conferencing or file exchange scenarios), the throughput of the users could be improved by their using ad hoc connections instead of the AP. In such a network, a node can dynamically switch between the two modes with the help of the AP, transparent to the users. There are two important issues to be addressed in the system: (i) the framework on how the system is designed, and (ii) as regards to the ad hoc networks, unfairness due to channel capture.With respect to the first issue, we propose and present a framework for mixed-mode wireless LAN (M2-WLAN) to seamlessly integrate infrastructure and ad hoc modes. We also investigate the use of ad hoc connection to divert traffic from one AP to another, hence alleviating the hot-spot problem in public wireless networks. Using simulation, we show that M2-WLAN can substantially improve system throughput or capacity without user's manual configuration. With respect to the second issue, we note that due to the existence of hidden terminal and absence of central administration, IEEE 802.11 ad hoc networks may lead to capture problem. Capture problem refers to that flows of a few hops monopolize, or so-called “capture,” the channel while others suffer from starvation. We propose and investigate a distributed power control scheme termed “Power Adaptation for Starvation Avoidance” (PASA) to mitigate the problem. PASA dynamically adjusts the transmission power so as to break starvation, hence improving short-term fairness among contending flows. We also investigate the fairness and throughput issues on delivering multimedia over ad hoc networks. Via extensive simulations, PASA is shown to be able to achieve substantially better fairness without compromising TCP and UDP throughput.