Performance analysis of the IEEE 802.11 distributed coordinationfunction

The IEEE has standardized the 802.11 protocol for wireless local area networks. The primary medium access control (MAC) technique of 802.11 is called the distributed coordination function (DCF). The DCF is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme with binary slotted exponential backoff. This paper provides a simple, but nevertheless extremely accurate, analytical model to compute the 802.11 DCF throughput, in the assumption of finite number of terminals and ideal channel conditions. The proposed analysis applies to both the packet transmission schemes employed by DCF, namely, the basic access and the RTS/CTS access mechanisms. In addition, it also applies to a combination of the two schemes, in which packets longer than a given threshold are transmitted according to the RTS/CTS mechanism. By means of the proposed model, we provide an extensive throughput performance evaluation of both access mechanisms of the 802.11 protocol     

IEEE 802.11协议中分布式协调机制的性能模型

IEEE 802.11采用异步传输方式作为媒体层的主要技术,而基于载波检测碰撞避免的分布式接入机制则是其最大的特点.关于分布式接入机制的研究,目前已经有了许多的模型,但是,大部分的模型都是研究终端所产生的数据包是固定长度,很少有模型来研究终端数据包是可变长度的情况.这种情况下的难点就是不易求得碰撞发生时信道所消耗的时间长度.本文则研究在终端数据包长度的分布函数为f(x)下协议的吞吐量和延迟性能模型.首先本文将原标准协议的退避算法看成是有固定大小的竞争窗口,用以求得站点的发送概率;然后,分析信道的工作状态,给出了性能模型,重点在求解碰撞消耗的信道时间,在文章的最后,我们通过仿真试验来验证了模型的正确性.     

Collision reduction mechanism for masked node problem in ad hoc networks

IEEE 802.11 MAC uses RTS/CTS mechanism to avoid DATA packet collisions. RTS/CTS mechanism has been introduced to solve the problems of carrier sense multiple access (CSMA) in ad hoc networks such as hidden/exposed node problem. However, it creates a new problem called masked node problem. In this paper, a collision reduction mechanism named RTS/CTS/TTM with resume is introduced. This mechanism aims to minimize the probability of DATA packet collisions due to the masked nodes in an ad hoc network. We develop a new control packet called time-to-mask (TTM), which contains the time that the node will be masked. The proposed mechanism has been evaluated with a mathematical analysis and a simulation on a small IEEE 802.11 ad hoc network. The numerical results indicate that the RTS/CTS/TTM with resume reduces the probability of DATA packet collision.     

Achieving performance enhancement in IEEE 802.11 WLANs by using the DIDD backoff mechanism

Wireless local area networks (WLANs) based on the IEEE 802.11 standards have been widely implemented mainly because of their easy deployment and low cost. The IEEE 802.11 collision avoidance procedures utilize the binary exponential backoff (BEB) scheme that reduces the collision probability by doubling the contention window after a packet collision. In this paper, we propose an easy‐to‐implement and effective contention window‐resetting scheme, called double increment double decrement (DIDD), in order to enhance the performance of IEEE 802.11 WLANs. DIDD is simple, fully compatible with IEEE 802.11 and does not require any estimation of the number of contending wireless stations. We develop an alternative mathematical analysis for the proposed DIDD scheme that is based on elementary conditional probability arguments rather than bi‐dimensional Markov chains that have been extensively utilized in the literature. We carry out a detailed performance study and we identify the improvement of DIDD comparing to the legacy BEB for both basic access and request‐to‐send/clear‐to‐send (RTS/CTS) medium access mechanisms. Copyright © 2006 John Wiley & Sons, Ltd.     

IEEE 802.11-saturation throughput analysis

To satisfy the emerging need of wireless data communications, the IEEE is currently standardizing the 802.11 protocol for wireless local area networks. This standard adopts a CSMA/CA medium access control protocol with exponential backoff. We present a simple analytical model to compute the saturation throughput performance in the presence of a finite number of terminals and in the assumption of ideal channel conditions. The model applies to both basic and request-to-send/clear-to-send (RTS/CTS) access mechanisms. Comparison with simulation results shows that the model is extremely accurate in predicting the system throughput     

A novel hidden station detection mechanism in IEEE 802.11 WLAN

The popular IEEE 802.11 wireless local area network (WLAN) is based on a carrier sense multiple access with collision avoidance (CSMA/CA), where a station listens to the medium before transmission in order to avoid collision. If there exist stations which can not hear each other, i.e., hidden stations, the potential collision probability increases, thus dramatically degrading the network throughput. The RTS/CTS (request-to-send/clear-to-send) frame exchange is a solution for the hidden station problem, but the RTS/CTS exchange itself consumes the network resources by transmitting the control frames. In order to maximize the network throughput, we need to use the RTS/CTS exchange adaptively only when hidden stations exist in the network. In this letter, a simple but very effective hidden station detection mechanism is proposed. Once a station detects the hidden stations via the proposed detection mechanism, it can trigger the usage of the RTS/CTS exchange. The simulation results demonstrate that the proposed mechanism can provide the maximum system throughput performance.     

Improving protocol capacity for UDP/TCP traffic with model-based frame scheduling in IEEE 802.11-operated WLANs

In this paper, we develop a model-based frame scheduling scheme, called MFS, to enhance the capacity of IEEE 802.11-operated wireless local area networks (WLANs) for both transmission control protocol (TCP) and user datagram protocol (UDP) traffic. In MFS each node estimates the current network status by keeping track of the number of collisions it encounters between its two consecutive successful frame transmissions, and computes accordingly the current network utilization. The result is then used to determine a scheduling delay to be introduced before a node attempts to transmit its pending frame. MFS does not require any change in IEEE 802.11, but instead lays a thin layer between the LL and medium access control (MAC) layers. In order to accurately calculate the current utilization in WLANs, we develop an analytical model that characterizes data transmission activities in IEEE 802.11-operated WLANs with/without the request to send/clear to send (RTS/CTS) mechanism, and validate the model with ns-2 simulation. All the control overhead incurred in the physical and MAC layers, as well as system parameters specified in IEEE 802.11, are figured in. We conduct a comprehensive simulation study to evaluate MFS in perspective of the number of collisions, achievable throughput, intertransmission delay, and fairness in the cases of TCP and UDP traffic. The simulation results indicate that the performance improvement with respect to the protocol capacity in a WLAN of up to 300 nodes is 1) as high as 20% with the RTS/CTS and 70% without the RTS/CTS in the case of UDP traffic and 2) as high as 10% with the RTS/CTS and 40% without the RTS/CTS in the case of TCP traffic. Moreover, the intertransmission delay in MFS is smaller and exhibits less variation than that in IEEE 802.11; the fairness among wireless nodes in MFS is better than, or equal to, that in IEEE 802.11.     

Estimation Algorithm of Contending Stations Based on Improved DCF Model in IEEE 802.11

The fundamental access method of IEEE 802.11 is a DCF known as carrier sense multiple access with collision avoidance (CSMA/CA) scheme with exponential back-off. RTS_threshold is used to determine whether to deploy RTS/CTS access method. This threshold should vary with the number of contending stations which contend wireless media to get better throughput. The paper proposes an algorithm which estimates the number of contending stations in BSS. The algorithm is shown to be accurate which is verified by elaborate simulations.     

Evaluation of the masked node problem in ad hoc wireless LANs

IEEE 802.11 wireless networks employ the so-called RTS/CTS mechanism in order to avoid data packet collisions. The main design assumption is that all the nodes in the vicinity of a sender and a receiver will hear the RTS or CTS packets, and defer their transmission appropriately. This assumption happens to not hold, in general, even under perfect operating conditions. Often, neighboring nodes are "masked" by other ongoing transmissions nearby and, hence, are unable to receive the RTS or CTS packets correctly. We refer to such nodes as masked nodes. In this paper, we describe the masked node problem and show scenarios leading to data packet collisions. We evaluate the impact of masked nodes through mathematical analysis and real experiments on a small IEEE 802.11 ad hoc network. The analytical and experimental data closely match and reveal that the presence of a masked node in a network can result in an order of magnitude increase in data packet loss compared to a network without masked nodes. These results are further validated by extensive simulations on a large-scale network, which show that masked nodes also significantly affect delay and throughput performance. Therefore, masked nodes severely limit the effectiveness of the RTS/CTS mechanism in preventing performance degradation in wireless LANs.     

Analytical Analysis of Applying Packet Fragmentation Mechanism on Both Basic and RTS/CTS Access Methods of the IEEE 802.11b DCF Network Under Imperfect Channel and Finite Load Conditions

The mathematical modeling and performance evaluation of the IEEE 802.11 network in all its various extensions (802.11b, 802.11a, 802.11g, 802.11e, 802.11n, etc.) have already been widely explored over the past years. However, the Packet Fragmentation Mechanism (PFM), which is proposed by the IEEE work group to enhance the MAC sub-layer of the IEEE 802.11 standard in an error-prone channel, has been missed in the available literature. Yet, the PFM is the only existing solution to reduce the influence of bit error rate and the length of data packets on the packet error rate, and consequently on the performances of IEEE 802.11 networks. In this paper, we propose a new three-dimensional Markov chain in order to model, for the first time in the literature, the PFM in both Basic and RTS/CTS access methods of the IEEE 802.11b DCF network under imperfect channel and finite load conditions. Then, we develop mathematical models to derive a variety of performance metrics, such as: the overall throughput, the average packet delay successfully transmitted, the average packet drop time, the delay jitter and the packet delay distribution. Performance analysis of applying PFM on both Basic and RTS/CTS access methods of the IEEE 802.11b DCF network under imperfect channel and finite load conditions shows original results and leads to new conclusions that could not be intuitively expected.     

Saturation throughput analysis of multi‐rate IEEE 802.11 wireless networks

IEEE 802.11 protocol supports adaptive rate mechanism, which selects the transmission rate according to the condition of the wireless channel, to enhance the system performance. Thus, research of multi‐rate IEEE 802.11 medium access control (MAC) performance has become one of the hot research topics. In this paper, we study the performance of multi‐rate IEEE 802.11 MAC over a Gaussian channel. An accurate analytical model is presented to compute the system saturation throughput. We validate our model in both single‐rate and multi‐rate networks through various simulations. The results show that our model is accurate and channel error has a significant impact on system performance. In addition, our numerical results show that the performance of single‐rate IEEE 802.11 DCF with basic access method is better than that with RTS/CTS mechanism in a high‐rate and high‐load network and vice versa. In a multi‐rate network, the performance of IEEE 802.11 DCF with RTS/CTS mechanism is better than that with basic access method in a congested and error‐prone wireless environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Collision-free operation in ad hoc carrier sense multiple access wireless networks

The CSMA/CA algorithm proposed in the IEEE 802.11 standard does not exclude collisions between transmitted packets. Regardless of whether RTS/CTS packets are used or not, these collisions do always have a more or less negative effect on system performance, especially at high loads. In this letter a method of avoiding collisions by using busy energy bursts is proposed. The algorithm used is based on the 802.11 standard and depends only on the assumption that each node can hear the transmissions of all other nodes. Collisions are avoided by transmitting short sequences of energy bursts without the need of any further communication between nodes contending for the use of the channel. The proposed method provides better average packet delays also as higher maximum system loads than conventional CSMA/CA. In addition to this, it renders the use of acknowledgment packets unnecessary.     

A Game-Theoretic Study of CSMA/CA Under a Backoff Attack

CSMA/CA, the contention mechanism of the IEEE 802.11 DCF medium access protocol, has recently been found vulnerable to selfish backoff attacks consisting in nonstandard configuration of the constituent backoff scheme. Such attacks can greatly increase a selfish station's bandwidth share at the expense of honest stations applying a standard configuration. The paper investigates the distribution of bandwidth among anonymous network stations, some of which are selfish. A station's obtained bandwidth share is regarded as a payoff in a noncooperative CSMA/CA game. Regardless of the IEEE 802.11 parameter setting, the payoff function is found similar to a multiplayer Prisoners' Dilemma; moreover, the number (though not the identities) of selfish stations can be inferred by observation of successful transmission attempts. Further, a repeated CSMA/CA game is defined, where a station can toggle between standard and nonstandard backoff configurations with a view of maximizing a long-term utility. It is argued that a desirable station strategy should yield a fair, Pareto efficient, and subgame perfect Nash equilibrium. One such strategy, called CRISP, is described and evaluated     

Analysis of the Request to Send/Clear to Send Exchange in WLAN Over Fiber Networks

A study of the effect of the optical path delay on the effectiveness of the request to send/clear to send (RTS/CTS) exchange in high-speed IEEE 802.11 wireless LAN (WLAN) over fiber networks has been carried out. It is shown that although the fiber delay might violate some of the timing boundaries of the medium access control (MAC) protocol, with a careful choice of the RTS threshold parameter, which determines when the RTS/CTS is used, these networks can benefit significantly from the four-way handshake even without the need for modifying the existing protocol.      

A combined approach for detecting hidden nodes in 802.11 wireless LANs

Hidden node is a fundamental problem that severely degrades the performance of wireless networks. The problem occurs when nodes that do not hear each other transmit at the same time, which leads to data packet collision. IEEE 802.11 Wireless Local Area Networks (WLANs) tries to solve this problem through the Request to Send/Clear to Send (RTS/CTS) mechanism. However, the mechanism is not wholly successful. The RTS/CTS idea is based on the assumption that all nodes in the vicinity of Access Points will hear CTS packets and consequently defer their transmissions. The shortcoming of RTS/CTS stems from the fact that such packets introduce high overhead if extensively used. In this article, we propose a hybrid approach for detecting hidden nodes in 802.11 WLANs. The approach is mainly based on adaptive learning about collisions in the network. We think that the approach will be useful for controlling the tuning of RTS/CTS threshold and therefore reduce the overhead those packets introduce. Detailed simulation experiments have shown the strength of the proposed approach compared with other approaches.     

EBA: an enhancement of the IEEE 802.11 DCF via distributed reservation

The IEEE 802.11 standard for wireless local area networks (WLANs) employs a medium access control (MAC), called distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). The collision avoidance mechanism utilizes the random backoff prior to each frame transmission attempt. The random nature of the backoff reduces the collision probability, but cannot completely eliminate collisions. It is known that the throughput performance of the 802.11 WLAN is significantly compromised as the number of stations increases. In this paper, we propose a novel distributed reservation-based MAC protocol, called early backoff announcement (EBA), which is backward compatible with the legacy DCF. Under EBA, a station announces its future backoff information in terms of the number of backoff slots via the MAC header of its frame being transmitted. All the stations receiving the information avoid collisions by excluding the same backoff duration when selecting their future backoff value. Through extensive simulations, EBA is found to achieve a significant increase in the throughput performance as well as a higher degree of fairness compared to the 802.11 DCF.     

无线局域网性能参数仿真分析

无线信道由于自身的特点,在通信环境较差的情况下其通信质量尤其难以保证.在无线局域网中,通过引入数据包拆分和RTS/CTS会话机制可以对恶劣环境下的通信质量起到改善作用.利用OPNET网络仿真平台,构建了一个集中式网络,设置不同的参数运行仿真,分析包拆分和RTS/CTS会话对网络性能(吞吐量、端到端时延等)产生的影响,为无线局域网的应用开发提供了参考.     

Performance analysis of the IEEE 802.11 MAC protocol for wireless LANs

Wireless local area networks (WLANs) are extremely popular being almost everywhere including business, office and home deployments. The IEEE 802.11 protocol is the dominating standard for WLANs. The essential medium access control (MAC) mechanism of 802.11 is called distributed co‐ordination function (DCF). This paper provides a simple and accurate analysis using Markov chain modelling to compute IEEE 802.11 DCF performance, in the absence of hidden stations and transmission errors. This mathematical analysis calculates in addition to the throughput efficiency, the average packet delay, the packet drop probability and the average time to drop a packet for both basic access and RTS/CTS medium access schemes. The derived analysis, which takes into account packet retry limits, is validated by comparison with OPNET simulation results. We demonstrate that a Markov chain model presented in the literature, which also calculates throughput and packet delay by introducing an additional transition state to the Markov chain model, does not appear to model IEEE 802.11 correctly, leading to ambiguous conclusions for its performance. We also carry out an extensive and detailed study on the influence on performance of the initial contention window size (CW), maximum CW size and data rate. Performance results are presented to identify the dependence on the backoff procedure parameters and to give insights on the issues affecting IEEE 802.11 DCF performance. Copyright © 2005 John Wiley & Sons, Ltd.     

An Improvement of Slotted CSMA/CA Algorithm in IEEE 802.15.4 Medium Access Layer

The standard of IEEE 802.15.4 is able to achieve low-power transmissions in low-rate and short-distance wireless personal area networks. It employs the slotted carrier sense multiple access with collision avoidance (CSMA/CA) for the contention mechanism. The blind backoff process in the slotted CSMA/CA will cause lower channel utilization. Sensor node performs backoff process immediately when the clear channel assessment (CCA) detecting busy channel. It may neglect the implicit information of CCA failed detection and further cause the redundant senses. This paper proposes an additional carrier sensing (ACS) algorithm based on IEEE 802.15.4 to enhance the carrier sensing mechanisms in original slotted CSMA/CA. An analytic model is developed to evaluate the performance of ACS algorithm. The analytical and simulation results demonstrate that the proposed scheme significantly improves the throughput, average medium access control delay and power consumption of CCA detecting.     

Model-based admission control for IEEE 802.11e enhanced distributed channel access

IEEE 802.11e enhanced distributed channel access (EDCA) is a distributed medium access scheme based on carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this paper, a model-based admission control (MBAC) scheme that performs real-timely at medium access control (MAC) layer is proposed for the decision of accepting or rejecting requests for adding traffic streams to an IEEE 802.11e EDCA wireless local area network (WLAN). The admission control strategy is implemented in access point (AP), which employs collision probability and access delay measures from active flows to estimate throughput and packet delay of each traffic class by the proposed unsaturation analytical model. Simulation results prove accuracy of the proposed analytical model and effectiveness of MBAC scheme.