Jin-Bong Chang

Performance enhancement of multirate IEEE 802.11 WLANs with geographically scattered stations

In todays IEEE 802.11 Wireless LANs (WLANs), e.g., the popular IEEE 802.11 b, stations support multiple transmission rates, and use them adaptively depending on the underlying channel condition via link adaptation. It has been known that when some stations use low transmission rates due to bad channel conditions, the performance of the stations using high rates is heavily degraded, and this phenomenon is often referred to as performance anomaly. In this paper, we model the WLAN incorporating stations with multiple transmission rates in order to demonstrate the performance anomaly analytically. Note that all the previously proposed models of the IEEE 802.11 assume a single transmission rate. We also develop possible remedies to improve the performance. Our solution is basically to control the access parameters such as the initial backoff window, the frame size, and the maximum backoff stage, depending on the employed transmission rate. Throughout simulations, we demonstrate that our analytical model is accurate, and the proposed mechanism can indeed provide the remedies to the performance anomaly by increasing the aggregate throughput up to six times.

A high-throughput MAC strategy for next-generation WLANs

Wireless LAN technology has been shown to he a revolutionary development during the last decade. Recently popularized IEEE 802.11a/g-based products can support up to 54 Mb/s physical layer rate and provide wireless access to the Internet. However, in order to deal robustly with the unreliable wireless nature, the 802.11 medium access control protocol has a relatively large overhead and hence, the throughput performance is much worse than the underlying physical layer rate. Moreover, along with many emerging applications and services over WLANs, such as voice over WLAN and audio/video streaming, the demand lor faster and higher- capacity WLANs has been growing recently. In this article, we propose a new medium access control protocol for the next-generation high-speed WLANs. The proposed medium access control, called multi-user polling controlled channel access, is composed of two components: multi-layer frame aggregation, which performs aggregation at both the medium access control and the physical layers; and multi-user polling, used to reduce the contention overhead and in turn, achieve higher network utilization. Multi-user polling controlled channel access is compared with the 802.11e-enhanced distributed channel access medium access control. Highly enhanced medium access control efficiency can be achieved by applying multi-user polling controlled channel access. We show the improved medium access control performance in terms of the aggregate throughput of non-QoS Hows with relevant QoS requirements.

Traffic Scheduling for MPDCF MAC to Support QoS-Sensitive Data in IEEE 802.11 Wireless LANs

In IEEE 802.11 wireless LAN (WLAN), it is very hard to meet QoS requirements (throughput, delay and loss rate) of multimedia traffic by using distributed coordination function (DCF). In order to support QoS in WLAN, enhanced IEEE 802.11 medium access control (MAC) protocol is discussed in IEEE 802.11 Task Group E. IEEE 802.11e uses the rather complex hybrid coordination function (HCF). In this paper, we propose a simple multipolling DCF (MPDCF) mechanism with the advantages of high channel efficiency and QoS support. In our proposed mechanism, an AP can decide the transmission order and duration of QoS-sensitive wireless stations and poll the stations whenever it needs based on the controlled backoff procedure. We propose a scheduling algorithm using the proposed MPDCF to serve real-time traffic with constant and variable bit rates. The simulation results show that QoS requirements of the traffic can be satisfied and the channel utilization is also increased with our scheduling mechanism