Resolving Performance Anomaly and Hidden Node Problem in IEEE 802.11n Wireless LANs

Abstract

The demand for wireless local area network (WLAN) has drastically increased due to the prevalence of the mobile devices such as smart phones and tablet PCs. In this dissertation, we study the two defective phenomenons in IEEE 802.11 WLANs, which are performance anomaly and hidden node problem. Each node may have a different amount of airtime because the basic channel access mechanism, distributed coordination function (DCF), in IEEE 802.11 was originally designed to provide fair chance to access the channel, regardless of packet size and data rate. This can lead to the degradation of overall network throughput and airtime fairness among nodes, which is known as performance anomaly. The hidden node problem occurs due to the failure of the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. When a transmitter cannot carrier sense the signal of another node even though its signal can interfere with the frame reception at the receiver, this node is called as a hidden node which leads to degradation of network performance. Recently, as service providers and personal users install access points (APs) without considering nearby basic service sets (BSSs), a large number of BSSs can overlap with each other, causing the hidden node problem to occur more frequently. To resolve the performance anomaly, we first study the problem of optimal frame aggregation in IEEE 802.11n, when a wireless channel is shared by heterogeneous nodes of different data rates and packet sizes. We employ a generalized two-level frame aggregation scheme that combines the conventional aggregated medium access control (MAC) service data unit (A-MSDU) and aggregated MAC protocol data unit (A-MPDU) schemes, and formulate an optimization problem to maximize the achievable throughput while assuring airtime fairness. On the basis of the solution of the optimization problem, we propose a frame size adaptation (FA) scheme that adjusts the number of packets in a frame according to the data rate and packet size. The FA scheme is fully compatible with the IEEE 802.11n standard and works in a distributed manner, which neither modifies the channel access mechanism nor resorts to a centralized scheduling algorithm. The simulation results confirm that the FA scheme tightly regulates the airtime usage of each node to be almost the same and significantly improves the overall network throughput compared to other existing schemes. The solutions to alleviate the hidden node problem, such as the request-to-send/clear-to-send (RTS/CTS) exchange, often contain protocol overhead that may lead to throughput degradation when there is no hidden node. Therefore, it is necessary to develop a hidden node detection mechanism that can be used to trigger a resolution mechanism for hidden nodes. By utilizing frame aggregation, block acknowledgement (ACK), and fast link adaptation (FLA) in IEEE 802.11n, we propose a novel hidden node detection (HD) mechanism that takes three main causes of frame losses, which are collisions, hidden nodes, and channel impairments, into consideration. The proposed HD mechanism detects hidden nodes based on measurable MAC layer statistics and the received block ACK frame, and determines whether or not to use the RTS/CTS exchange. We show in a simulation study that the HD mechanism can detect hidden nodes well under various circumstances and that the network throughput can be improved by using the RTS/CTS exchange adaptively in conjunction with the HD mechanism. Finally, we propose an enhanced mechanism for resolving the hidden node problem in conjunction with the FA scheme and the HD mechanism. We first classify all possible scenarios of the hidden node problem according to the relative location of nodes in two flow interaction. Through a simulation study, we investigate the effectiveness of the RTS/CTS mechanism for each scenario and show that the malfunction of the RTS/CTS mechanism is due to the limitation of the effective CTS range and the carrier sensing mechanism at the transmitter-side. The proposed mechanism enhance the capability of the RTS/CTS mechanism in two ways: by extending the effective CTS range, and by adopting the receiver-oriented contention (ROC) mechanism. We provide extensive simulation results to show that the proposed mechanism can effectively resolve the hidden node problem, and the network throughput and fairness can be significantly improved.