Modeling and Optimizing IEEE 802.11 DCF for Long-Distance Links

Most rural areas in developing countries are isolated due to the lack of appropriate low-cost communication technologies. Previous experiences have shown that IEEE 802.11 can be used for the deployment of large static mesh networks with only minor changes to the MAC layer that enable WiFi transceivers to work properly even for very long distances (up to 100 km in point to point links, and almost 40 km in point to multipoint setups). However, the impact of distance on performance of such long links has not been deeply analyzed. In addition, previous analytical models of IEEE 802.11 DCF cannot be applied because they implicitly assume that the propagation time can be neglected. This paper formally studies the impact of the distance on the behavior of IEEE 802.11 DCF and presents an analytical model of IEEE 802.11 DCF that accounts for distances correctly. The model is validated with simulations and within a controlled experimental framework, based on wireless channel emulation. Finally, we propose adjustments for ACKTimeout, CTSTimeout, SlotTime, and CW_{min} parameters that improve significantly the performance of DCF over long distances.