A modified point coordination function in IEEE 802.11 wireless LAN

Abstract

With recent advances in wireless technologies, wireless LANs are becoming increasingly widespread as an alternative to fixed access technologies. The IEEE 802.11 standard has gained the most popularity among the different standards and is currently being deployed both within enterprises as well as being used for public access. One of the weaknesses of the basic medium access in the 802.11 standard is its relatively poor support for real-time traffic. In order to provide this support, the MAC layer implements a Point Coordination Function (PCF). However, since the PCF is based on a centralized polling protocol, some bandwidth is wasted due to the polling overheads and null packets in case the polled stations do not have any data to transmit. In order to reduce the waste and increase the channel utilization this paper presents a modified version of the standard PCF. The modified PCF uses a distributed polling protocol (DPP) as an access mechanism for the uplink transmission. The transmission period in the modified PCF consists of a distributed polling protocol period (DPPP) which is controlled by the DPP and the real-time traffic downlink period (RTDP). The paper further introduces a technique for dealing with the hidden station problem for use together with the proposed modification. This problem occurs when one or more stations misinterpret the status of the medium leading to unforeseen collisions. Using simulation we compare the performance of the modified PCF with the standard PCF when they are used to support voice transmissions. The results show that the modified PCF significantly improves the channel utilization since it can support a higher number of stations than the standard PCF.