Selfish hosts in wireless networks that fail to adhere to the MAC protocol may obtain an unfair share of the channel bandwidth. We present modifications to the IEEE 802.11 backoff mechanism to simplify detection of such selfish hosts. We also present a correction scheme for penalizing greedy misbehavior which attempts to restrict the misbehaving nodes to a fair share of the channel bandwidth. Simulation results indicate that our detection and correction schemes are fairly successful in handling MAC layer misbehavior.

We study a fixed point formalisation of the well known analysis of Bianchi. We provide a significant simplication and generalisation of the analysis. In this more general framework, the fixed point solution and performance measures resulting from it are studied. Uniqueness of the fixed point is established. Simple and general throughput formulas are provided. It is shown that the throughput of any ow will be bounded by the one with the smallest transmission rate. The aggregate throughput is bounded by the reciprocal of the harmonic mean of the transmission rates. In an asymptotic regime with a large number of nodes, explicit formulas for the collision probability, the aggregate attempt rate and the aggregate throughput are provided. The results from the analysis are compared with ns2 simulations, and also with an exact Markov model of the back-off process. It is shown how the saturated network analysis can be used to obtain TCP transfer throughputs in some cases.

This paper presents' the first analytical model to derive the saturation throughput of collision avoidance protocols' in multi-hop ad hoc networks' with nodes randomly placed according to a two-dimensional Poisson distribution. We show that the sender-initiated collision-avoidance scheme performs much better than the ideal CSMA scheme with a separate channel for acknowledgments'. More importantly, we show that the collision-avoidance scheme can accom- modate much fewer competing nodes within a region in a network infested with hidden terminals' than in those cases without hidden terminals' or with just a few, if reasonable throughput is to be maintained. Simulations of the popular IEEE 802.11 MAC protocol show that it cannot ensure collision-free transmission of data packets' and thus throughput can degrade well below what is predicted by the analysis' of a correct collision avoidance protocol. Based on these results', a number of improvements' are proposed for the IEEE 802.11 MAC protocol.

Wide-spread deployment of infrastructure WLANs has made Wi-Fi an integral part of today’s Internet access technology. Despite its crucial role in affecting end-to-end performance, past research has focused on MACprotocol enhancement, analysis and simulation-based performance evaluation without sufficient consideration for modeling inaccuracies stemming from inter-layer dependencies, including physical layer diversity, that significantly impact performance. We take a fresh look at IEEE 802.11 WLANs, and using a combination of experiment, simulation, and analysis demonstrate its surprisingly agile performance traits. Our main findings are two-fold. First, contention-based MACthroughput degrades gracefully under congested conditions, enabled by physical layer channel diversity that reduces the effective level of MACcontention. In contrast, fairness and jitter significantly degrade at a critical offered load. This duality obviates the need for link layer flow control for throughput improvement but necessitates traffic control for fairness and QoS. Second, TCP-over-WLAN achieves high throughput commensurate with that of wireline TCP under saturated conditions, challenging the widely held perception that TCP throughput fares poorly over WLANs when subject to heavy contention. We show that TCP-over-WLAN prowess is facilitated by the self-regulating actions of DCF and TCP congestion control that jointly drive the shared physical channel at an effective load of 2–3 wireless stations, even when the number of active stations is very large. Our results highlight subtle inter-layer dependencies including the mitigating influence of TCP-over-WLAN on dynamic rate shifting.

Abstract—We study a fixed-point formalization of the well-known analysis of Bianchi. We provide a significant simplification and generalization of the analysis. In this more general framework, the fixed-point solution and performance measures resulting from it are studied. Uniqueness of the fixed point is established. Simple and general throughput formulas are provided. It is shown that the throughput of any flow will be bounded by the one with the smallest transmission rate. The aggregate throughput is bounded by the reciprocal of the harmonic mean of the transmission rates. In an asymptotic regime with a large number of nodes, explicit formulas for the collision probability, the aggregate attempt rate, and the aggregate throughput are provided. The results from the analysis are compared with ns2 simulations and also with an exact Markov model of the backoff process. It is shown how the saturated network analysis can be used to obtain TCP transfer throughputs in some cases. Index Terms—CSMA/CA, performance of MAC protocols, wireless networks. I.

This paper studies an important problem in the IEEE 802.11 distributed coordination function (DCF)-based wireless local area network (WLAN): how well can the network support quality of service (QoS). Specifically, this paper analyzes the network's performance in terms of maximum protocol capacity or throughput, delay, and packet loss rate. Although the performance of the 802.11 protocol, such as throughput or delay, has been extensively studied in the saturated case, it is demonstrated that maximum protocol capacity can only be achieved in the nonsaturated case and is almost independent of the number of active nodes. By analyzing packet delay, consisting of medium access control (MAC) service time and waiting time, accurate estimates were derived for delay and delay variation when the throughput increases from zero to the maximum value. Packet loss rate is also given for the nonsaturated case. Furthermore, it is shown that the channel busyness ratio provides precise and robust information about the current network status, which can be utilized to facilitate QoS provisioning. The authors have conducted a comprehensive simulation study to verify their analytical results and to tune the 802.11 to work at the optimal point with maximum throughput and low delay and packet loss rate. The simulation results show that by controlling the total traffic rate, the original 802.11 protocol can support strict QoS requirements, such as those required by voice over Internet protocol (VoIP) or streaming video, and at the same time achieve high channel utilization.

In this paper, we present an analytic model for evaluating the queueing delays at nodes in an IEEE 802.11 MAC based wireless network. The model can account for arbitrary arrival patterns, packet size distributions and number of nodes. Our model gives closed form expressions for obtaining the delay and queue length characteristics. We model each node as a discrete time G/G/1 queue and derive the service time distribution while accounting for a number of factors including the channel access delay due to the shared medium, impact of packet collisions, the resulting backoffs as well as the packet size distribution. The model is also extended for ongoing proposals under consideration for 802.11e wherein a number of packets may be transmitted in a burst once the channel is accessed. Our analytical results are verified through extensive simulations. The results of our model can also be used for providing probabilistic quality of service guarantees and determining the number of nodes that can be accommodated while satisfying a given delay constraint.

Collision avoidance is very important in contention-based medium access control protocols for multi-hop ad hoc networks due to the adverse effects of hidden terminals. Four-way sender-initiated schemes are the most popular collision-avoidance schemes to date. Although there has been considerable work on the performance evaluation of these schemes, most analytical work is confined to single-hop ad hoc networks or networks with very few hidden terminals. In this paper, we use a simple analytical model to derive the saturation throughput of collision avoidance protocols in multi-hop ad hoc networks with nodes randomly placed according to a two-dimensional Poisson distribution, which to our knowledge has not been investigated sufficiently before. We show that the sender-initiated collision-avoidance scheme achieves much higher throughput than the idealized carrier sense multiple access scheme with an ideal separate channel for acknowledgments. More importantly, we show that the collision avoidance scheme can accommodate much fewer competing nodes within a region in a network infested with hidden terminals' than in a fully-connected network, if reasonable throughput is to be maintained. This shows that the scalability problem of contention-based collision-avoidance protocols looms much earlier than people might expect. Simulation experiments of the popular IEEE 802.11 MAC protocol validate the predictions made in the analysis.

Quality of service (QoS) support for multimedia services in the IEEE 802.11 wireless LAN is an important issue for such WLANs to become a viable wireless access to the Internet. In this paper, we endeavor to propose a practical scheme to achieve this goal without changing the channel access mechanism. To this end, a novel call admission and rate control (CARC) scheme is proposed. The key idea of this scheme is to regulate the arriving traffic of the WLAN such that the network can work at an optimal point. We first show that the channel busyness ratio is a good indicator of the network status in the sense that it is easy to obtain and can accurately and timely represent channel utilization. Then we propose two algorithms based on the channel busyness ratio. The call admission control algorithm is used to regulate the admission of real-time or streaming traffic and the rate control algorithm to control the transmission rate of best effort traffic. As a result, the real-time or streaming traffic is supported with statistical QoS guarantees and the best effort traffic can fully utilize the residual channel capacity left by the real-time and streaming traffic. In addition, the rate control algorithm itself provides a solution that could be used above the media access mechanism to approach the maximal theoretical channel utilization. A comprehensive simulation study in ns-2 has verified the performance of our proposed CARC scheme, showing that the original 802.11 DCF protocol can statically support strict QoS requirements, such as those required by voice over IP or streaming video, and at the same time, achieve a high channel utilization.

This paper presents a new approach for performance evaluation of the IEEE 802.11 Medium Access Control (MAC) protocol. The approach is based on system approximations, where the statistical characteristics of the protocol operations are studied and approximated by an appropriate phase-type distribution. A queueing model that incorporates bursty arrival process as well as the statistical characteristics of the protocol operations is constructed. This model is used to study the performance of IEEE 802.11. The accuracy of the analytical results is verified by simulation.