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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 trafflc. 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 traosmit. 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 @PP) as an access mechanism for the uplink transmission. The transmission period in the modified PCF consists of a distributed polling protocol period @PPP) 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.0-7803-7788-5/03/$17.00 02003 IEEE.

Absfracf-A' modified version of the point coordination function (PCF) in the IEEE 802.11 standard called the Modified PCF has been introduced to improve the channel utilization over the standard PCF. The Modified PCF is a hybrid scheme incorporating polling and channel sensing. Therefore, a collision might occur in the channel if some stations cannot sense the change of channel status in case there is a station transmitting a packet, the so-called hidden station problem.In this paper, we present an investigation of the effect of the hidden station problem on the performance of the Modified PCF and propose a new collision resolution technique, which presumes a hidden station and then drops the station into a list of hidden stations. The simulation results show that the hidden station problem increase the overall delay of the system in the uplink However, using the proposed collision resolution technique, the delay can he effectively reduced.

Packet reservation multiple access (PRMA) was designed for supporting only voice and data traffics. To investigate the performance of PRMA for video transmission, a modified version of PRMA is needed to be considered. In this paper, a modification to the PRMA protocol proposed for supporting video traffic was investigated using a computer simulation. A real video Imce fde from H.261 codec was used as a video traffic for simplification and accuracy in the simulation study. The simulation results show that the moditied scheme for supporting E261 video traffic could provide acceptable quality-of-service at a given number of video terminal in the system. Furthermore, the modifed PRMA could support simultaneously 28 voice terminals operating at 32 kbps. 28 data terminals generating data packets at a rate of 1200 bps. and one H.261 video terminal at the mean bit rate of 64 kbps, at the channel rate of 720 kbps in the system.