Search

We study the MAC performance of IEEE 802.11 wireless local area networks in the presence of the cochannel interference. Simulation results show that the interfered stations experience increased contention due to the exposed terminal effect. Furthermore, the cochannel interference presents a hidden terminal problem which causes all stations suffer in receiving packets from the access point. Realizing the traditional RTS/CTS that used to cope with the hidden terminal effect is not throughput efficient in this cochannel interference setting, we propose an interference avoidance algorithm to mitigate the interference effect from the neighbor BSS operating at the same channel. In scenarios of web browsing interfered by neighbor traffics, our algorithm improves the MAC throughput by more than 15% and reduces the delay by nearly 100%.

IEEE 802.11 is becoming a standard of increasing importance for wireless LANs. The task group e is working to extend it in order to enhance the current medium access control (MAC) layer, in order to support multimedia transmission with QoS requirements. This paper suggests a new medium access method called the advanced point coordinator function (A-PCF) which uses a dynamic polling scheme supporting different services. We simulate the A-PCF using the NS-2 simulator. Then we evaluate and compare this method with point coordinator function (PCF), an access method proposed by the IEEE 802.11. The normalized throughput, the average delay and the loss rate are the three QoS metrics that illustrate the performance of the A-PCF strategy.

In this paper we study the effect of overlapping cells on the performance of wireless LAN hot spots. Our results show that the MAC protocol defined in the IEEE 802.11 standard cannot guarantee spatial coexistence of multiple hot spots. We identify the reasons for the observed problems and propose a prioritization algorithm that can provide fairness in terms of average throughput and alternating medium access to all involved clients. Finally, we summarize the problems of implementing this approach in hot spots based on the 802.11b standard.

The paper proposes a new scheme that can evaluate the cell throughput performance of wireless local area network (LAN) systems that use the carrier sense multiple access/collision avoidance (CSMA/CA) and multiple transmission bit-rate (multi-rate) techniques. We use the proposed scheme to evaluate the IEEE 802.11a system and systems whose signal-bandwidth is expanded from that of the IEEE 802.11a system. We find that a system with 35(75) MHz signal-bandwidth achieves about 1.3(1.25) times higher cell throughput than the IEEE 802.11a system. Furthermore, the system with 35(75) MHz signal-bandwidth is also shown to have the potential to achieve up to 1.5(1.8) times higher cell throughput performance than the IEEE 802.11a system if the transmission efficiency on the media access control (MAC) layer is improved. The proposed scheme confirms that the approach of expanding the signal bandwidth of the IEEE 802.11a system is effective in improving the cell throughput performance. This result is virtually impossible to derive with the conventional scheme.

In this paper, we analyze the throughput of the IEEE 802.11 DCF scheme using the RTS/CTS access mechanism in multi-hop ad hoc networks. The mathematical analysis is much more involved than that in a single-hop network due to the hidden node problem. A more realistic carrier sense range is considered in this paper. The impact of parameters, such as the number of stations, the packet size, and the carrier sense range, is also considered. Simulation results show good agreement with the analytical results.

In wireless networks, multiple stations contend for access to the shared channel. In the cases of collisions, the associated collision cost is much higher than wired networks since stations cannot detect the collision without explicit feedback from the receiver. For this reason, more efficient contention resolution algorithms are desired for wireless networks to reduce the collision probability among backlogged stations. With distributed multiple access control, each station usually goes through a contention resolution stage before initiating its transmission. As contention resolution stage consumes channel bandwidth without producing any goodput, ideally, we desire it to be as short as possible while reducing the possibility of collisions to as small as possible. However, in general, it is difficult to achieve an optimum tradeoff between these two desired features. In this paper, we propose to use pipelining techniques to resolve such conflicts and improve the performance of multiple access control in terms of channel utilization. We discuss several pipelining MAC schemes and present their advantages and disadvantages accordingly.

Traditionally IEEE 802.11 systems have used omni-directional antennas. Omni-directional antennas are most suitable for the CSMA/CA based 802.11 MAC protocol. Recently increased market penetration of 802.11 is demanding higher range and higher capacity access points (AP). The use of smart antennas to enhance the coverage and capacity of 802.11 systems has become a compelling application. In an AP that uses smart antennas, each transmission is in the form of a beam, with the main lobe directed to a particular station (STA). Thus the coverage area is no longer omni-directional, which causes some undesirable side effects to the 802.11 MAC. The directional nature of transmissions and receptions and the interaction between them introduce certain characteristic problems. The problems such as the hidden terminal problem, the hidden beam problem and the 'ACK suicide' problem along with some other problems involved are described in this paper. The impact of the problems is quantified using simulations. Some possible solutions to the above problems are discussed.

A number of service differentiation mechanisms have been proposed for, in general, CSMA-CA systems, and, in particular, the 802.11 enhanced distributed coordination function. An effective way to provide prioritized service support is to use different inter frame spaces (IFS) for stations belonging to different priority classes. This paper proposes a novel analytical approach to evaluate throughput and delay performance of IFS based priority mechanisms.