An Improved Medium Access Protocol For WLANs

At present, IEEE 802.11 standard is the de facto standard for WLANs in the world. It defines the medium access control (MAC) and physical layer specifications. The basic access mechanism to wireless channel is the Distributed Coordination Function (DCF), which is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method. The performance of the network system is affected by the drawbacks of the DCF when the number of active stations linking in the WLANs is increasing. Because the IEEE 802.11 DCF basic access mechanism strongly depends on system parameters, such as the number of contending stations in the network and the minimum contention window size. It is necessary to improve and enhance the original 802.11 WLANs standard, since more and more wireless devices may be used in the WLANs. This paper focuses on improving the performance of the network system by lowering the contending probability when the members of the WLANs are increasing.Our paper improves some parts of the 802.11 standard based on the original DCF function. We discuss them from two perspectives as follows:First, we propose a multi-rounds contention medium access method. In this new method, we divide the original contention window of the IEEE 802.11 standard into multiple parts. Every part is called a contention round. The stations which are going to access the medium should contend for all contention rounds. Only the stations which win all rounds have rights to access the medium. The purpose that we increase the contention rounds is to decrease the number of stations which can enter the next contention rounds. This method can lower the collision probability when the winning stations access the medium.Second, we introduce a control frame. In the control frame, we develop a contention field which includes the information of all contention rounds. These control frames are sent by the stations when they win a contention round. Stations which receive the control frames can understand the contention information of the sending stations from the contention field in the control frame. If the stations receive the control frames, they should freeze the contending activity according to the content in the contention field of the frame. And they will be activated when the wireless medium is idle once again. We carry out the control function by using these control frames.With these simple modifications, we can decrease the collision probability when the stations access the wireless medium. The performance of the network system keeps in a stable condition even though the number of the stations is increasing gradually. We compared the throughput of the network system using improved method with the original 802.11 DCF function by simulations. The results show that the new method can significantly improve the performance of the network system, since the new method can lower the contention probability when the stations want to access the wireless medium. These simulation results point to that the improved medium access method achieves the goals which we made to reduce the contention probability and to improve the performance of the system.