Improving Protocol Capacity with Model-Based Frame Scheduling in IEEE 802.11-operated WLANs

In this paper, we develop a model-based frame scheduling scheme, called MFS, to enhance the capacity of IEEE 802.11-operated wireless LANs (WLANs). In MFS each node estimates the current network status by keeping track of the number of collisions it encounters between its two consecutive successful frame transmissions, and, based on the the estimated information, computes the current network utilization. The result is then used to determine a scheduling delay that is introduced (with the objective of avoiding collision) before a node attempts for transmission of its pending frame. In order to accurately calculate the current utilization in WLANs, we develop an analytical model that characterizes data transmission activities in IEEE 802.11-operated WLANs with/without the RTS/CTS mechanism, and validate the model with ns-2 simulation. All the control overhead incurred in the physical and MAC layers, as well as system parameters specified in IEEE 802.11 are figured in. We have conducted a comprehensive simulation study to evaluate MFS in perspective of number of collisions, achievable throughput, and inter-transmission delay. The simulation results indicate that the performance improvement with respect to protocol capacity can be as high as 20% with RTS/CTS and 150% without RTS/CTS, in a WLAN of up to 300 nodes. In addition, the inter-transmission delay in MFS is smaller and exhibits less variation than that in IEEE 802.11.