Two well-known problems that can cause performance degradations in IEEE 802.11 wireless networks are the exposed-node (EN) and hidden-node (HN) problems. Although there have been isolated and incidental studies of EN and HN, a comprehensive treatment has not been attempted. The contributions of this paper are threefold: First, we provide rigorous mathematical definitions for EN and HN in wireless networks (including wireless local area networks (WLANs) with multiple access points (APs) and ad hoc networks). Second, we relate EN to the nonscalability of network throughput and HN to unfair throughput distributions. Third, we provide schemes to eliminate EN and HN, respectively. We show that the standard 802.11 technology is not scalable because, due to EN, more APs do not yield higher total throughput. By removing EN, our schemes make it possible to achieve scalable throughput commensurate with the seminal theoretical results in [1] and [2]. In addition, by removing HN, our schemes solve the performance problems triggered by HN, including throughput unfairness/starvation and rerouting instability.

Unlike wired networks, wireless networks are characterized by channel errors. In wireless LANs (WLANs), link-layer ARQ can be used for error recovery. However, this technique assumes packet losses are due to packet collisions. With FIFO queuing at the access point (AP), ARQ may give rise to a "head-of-line (HOL) blocking" phenomenon that severely degrades the throughput performance. We study a simple adaptive round robin (ARR) scheduler at the LLC (logical link control) layer as a solution to the HOL blocking problem. Salient features of ARR include: 1) an explicit estimate of the channel state is not required; 2) compatibility with the existing IEEE 802.11 MAC protocol; 3) ability to achieve near-optimal throughput; 4) flexibility for meeting various throughput-fairness objectives. Besides extensive simulations, we also give the analytical upper and lower bounds for WLAN throughput with ARR. Our analysis closely matches the simulation results.

The paper considers scenarios in which fairness and efficiency are two conflicting objectives in wireless networks, and investigates the use of a proportional fairness objective to strike a balance between the two objectives. We explain the physical meaning of proportional fairness in a wireless network and give an analysis showing that proportional fairness is equivalent or close to max-min fairness in terms of air-time usage (as opposed to bandwidth usage). For infrastructure WLANs, two approaches to achieving proportional fairness are discussed. For ad hoc networks, achieving proportional fairness is more complex and requires global information on contention among different traffic flows. We propose and evaluate the use of a distributed max-min air-time allocation algorithm to approximate the proportional fairness objective.

In large-scale wireless networks, interference among nodes limits channel spatial re-use and is a main hurdle for scalable performance. There are two types of interference: (1) physical interference due to the receiver's inability to decode a signal when the power received from other signals is large; (2) protocol interference imposed by the specific multi-access protocol being used. The paper models both interference types in terms of a set of inequality constraints for the IEEE 802.11 CSMA/CA protocol. Based on the inequalities, we investigate the impact of some parameters (basic-rate, data-rate, and physical-preamble-rate) on channel spatial re-use. Regardless of the parameter settings, the total capacity in an 802.11 network reaches a ceiling as the number of nodes, n, increases. We identify the fundamental causes for the non-scalable performance, and show that 802.11 can be made to be scalable with a simple modification, achieving O(n) throughput without adaptive power control. We believe that this is the first paper to demonstrate this.