A smart exponential‐threshold‐linear backoff mechanism for IEEE 802.11 WLANs

Based on the standardized IEEE 802.11 Distributed Coordination Function (DCF) protocol, this paper proposes a new backoff mechanism, called Smart Exponential‐Threshold‐Linear (SETL) Backoff Mechanism, to enhance the system performance of contention‐based wireless networks. In the IEEE 802.11 DCF scheme, the smaller contention window (CW) will increase the collision probability, but the larger CW will delay the transmission. Hence, in the proposed SETL scheme, a threshold is set to determine the behavior of CW after each transmission. When the CW is smaller than the threshold, the CW of a competing station is exponentially adjusted to lower collision probability. Conversely, if the CW is larger than the threshold, the CW size is tuned linearly to prevent large transmission delay. Through extensive simulations, the results show that the proposed SETL scheme provides a better system throughput and lower collision rate in both light and heavy network loads than the related backoff algorithm schemes, including Binary Exponential Backoff (BEB), Exponential Increase Exponential Decrease (EIED) and Linear Increase Linear Decrease (LILD). Copyright © 2011 John Wiley & Sons, Ltd.     

Contention resolution with EIED backoff for bandwidth request in IEEE 802.16 networks

In this letter, we suggest contention resolution with exponential increase and exponential decrease (EIED) backoff for bandwidth request in worldwide interoperability for microwave access (WiMAX) networks. In EIED, setting of backoff factor to overcome collision due to contention is very challenging and hence we suggest a method to compute backoff factor with average contention window. Further, to reduce access delay, we estimate the response time based on probability of failure and average contention window. Simulations validate the proposed EIED backoff in terms of contention efficiency, capacity and access delay. The contention efficiency and capacity is improved by 47.50% (for q value of 0.25) and 28.57% (for 25 numbers of transmission opportunity), respectively, when bandwidth request is made with the proposed EIED backoff mechanism.     

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.     

A backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF

The binary exponential backoff (BEB) mechanism is applied to the packet retransmission in lots of wireless network protocols including IEEE 802.11 and 802.15.4. In distributed dynamic network environments, the fixed contention window (CW) updating factor of BEB mechanism can’t adapt to the variety of network size properly, resulting in serious collisions. To solve this problem, this paper proposes a backoff algorithm based on self-adaptive contention window update factor for IEEE 802.11 DCF. In WLANs, this proposed backoff algorithm can greatly enhance the throughput by setting the optimal CW updating factor according to the theoretical analysis. When the number of active nodes varies, an intelligent scheme can adaptively adjust the CW updating factor to achieve the maximal throughput during run time. As a result, it effectively reduces the number of collisions, improves the channel utilization and retains the advantages of the binary exponential back-off algorithm, such as simplicity and zero cost. In IEEE 802.11 distributed coordination function (DCF) protocol, the numerical analysis of physical layer parameters show that the new backoff algorithm performance is much better than BEB, MIMD and MMS algorithm.     

A cross-layer congestion and contention window control scheme for TCP performance improvement in wireless LANs

Neither the current TCP protocol nor the standard backoff algorithm of IEEE 802.11 protocol is able to distinguish corruption loss from congestion or collision loss. Hence, high transmission errors and a varying latency inherent in wireless channel would have a seriously adverse effect on the performance of TCP. In this paper, we propose a novel and pragmatic cross-layer approach with joint congestion and contention window control scheme to improve the performance of TCP in IEEE 802.11 wireless environments. In addition to theoretical analysis, simulations are conducted to evaluate the proposed scheme. As it turns out, our design indeed provides a more efficient solution for frequent transmission loss and enables TCP to distinguish between congestion loses and transmission errors, thus to take proper remedial actions.     

一种基于实时优化思想的多址接入协议性能分析

该文提出了一种基于慢退避和实时优化思想的碰撞减少多址接入CRMA (Collision Reduced Multiple Access)协议。CRMA协议将实时优化的思想同慢退避的思想相结合,有效地解决了IEEE 802.11协议二进制指数退避算法成功发送数据帧后没有记录网络当前繁忙程度的缺点,能够更准确地记录数据帧成功发送后高负荷网络的退避阶数,降低数据帧接入信道的碰撞概率,提高无线信道的利用率。新的多址接入协议能够与现有的IEEE 802.11协议完全兼容,具有简单、无开销、完全分布性和自适应性的特点。分析和仿真结果表明,CRMA协议较之改进前的IEEE 802.11协议和其它最新的多址接入算法能够更有效地利用网络中已有的信息,更准确地估计网络当前的竞争状态,表现出更好的网络性能。     

An effective medium contention method to improve the performance of IEEE 802.11

In this paper, we propose an effective medium access mechanism to enhance performance of the IEEE 802.11 distributed coordination function (DCF). One of the primary issues of 802.11 is a contention-based medium access control (MAC) mechanism over a limited medium, which is shared by many mobile users. In the original 802.11 DCF, the binary exponential backoff algorithm with specific contention window size is employed to coordinate the competition for shared channel. Instead of binary exponential increase, we adopt linear increase for the contention window that is determined according to the competing number of nodes. We also assume that the access point can broadcast the number of mobile nodes to each station through management frames. An analytical model is developed for the throughput performance of the wireless medium. Using simulation results from the NS2 simulator, we show that our model can accurately predict the system saturation throughput, and can obtain better performance in terms of throughput, fairness, and packet drop.     

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.     

GDCF: game-theoretic distributed co-ordination function in WLANs

A game-theoretic DCF (GDCF) in IEEE 802.11 is presented, to improve the performance of WLANs by designing a simple auto-degressive backoff mechanism based on incompletely co-operative game theory. Simulation results show that GDCF can increase system throughput and decrease delay and packet-loss-rate while maintaining reasonable energy consumption     

A k-Round Elimination Contention Scheme for WLANs

The contention resolution scheme is a key component in carrier-sense-based wireless MAC protocols. It has a major impact on MAC'S performance metrics such as throughput, delay, and jitter. The IEEE 802.11 DCF adopts a simple contention resolution scheme, namely, the binary exponential backoff (BEB) scheme. The BEB scheme achieves a reasonable performance for transmitting best-effort packets in small-sized wireless networks. However, as the network size increases, it suffers from inefficiency because of the medium contention, which leads to reduced performance. The main reason is that the BEB mechanism incurs an ever- increasing collision rate as the number of contending nodes increases. We devise a novel contention resolution scheme, a k-round elimination contention (k-EC) scheme. The k-EC scheme exhibits high efficiency and robustness during the collision resolution. More importantly, it is insensitive to the number of contending nodes. This feature makes it feasible for use in networks of different sizes. Simulation results show that the k-EC scheme offers a powerful remedy to medium contention resolution. It significantly outperforms the IEEE 802.11 DCF scheme in all the MAC'S performance metrics and also exhibits better fairness.     

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.     

Lock step: an algorithm to reduce wi-fi jitter

The exponential backoff algorithm used in IEEE 802.11 does not guarantee short term fairness between flows. We propose an algorithm based on IdleSense where each of N flows tries to transmit every W = 8N slots; i.e. 8 slots per flow with 7 idle. Each flow adaptively adjusts its contention window W based on the observed proportion of empty slots. One key aspect of Lock Step is setting a deterministic backoff after a successful transmission so that, for persistent flows, eventually all users are in lock step and transmit every 8N slots. Another key aspect is forcing a second collision between colliding users so all users can better estimate the proportion of idle slots per transmission thus allowing users to estimate N and hence the optimal backoff. This second collision can also be engineered to assist the ZigZag algorithm to decode the colliding packets. Even with transient flows Lock Step reduces jitter and can substantially increase the number of flows carrying Voice over IP traffic through a single access point.     

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     

Enhanced binary exponential backoff algorithm for fair channel access in the ieee 802.11 medium access control protocol

The medium access control protocol determines system throughput in wireless mobile ad hoc networks following the ieee 802.11 standard. Under this standard, asynchronous data transmissions have a defined distributed coordination function that allows stations to contend for channel usage in a distributed manner via the carrier sensing multiple access with collision avoidance protocol. In distributed coordination function, a slotted binary exponential backoff (BEB) algorithm resolves collisions of packets transmitted simultaneously by different stations. The BEB algorithm prevents packet collisions during simultaneous access by randomizing moments at stations attempting to access the wireless channels. However, this randomization does not eliminate packet collisions entirely, leading to reduced system throughput and increased packet delay and drop. In addition, the BEB algorithm results in unfair channel access among stations. In this paper, we propose an enhanced binary exponential backoff algorithm to improve channel access fairness by adjusting the manner of increasing or decreasing the contention window based on the number of the successfully sent frames. We propose several configurations and use the NS2 simulator to analyze network performance. The enhanced binary exponential backoff algorithm improves channel access fairness, significantly increases network throughput capacity, and reduces packet delay and drop. Copyright © 2013 John Wiley & Sons, Ltd.     

IEEE802.11无线局域网中一种支持业务区分的回退算法

支持QoS的MAC机制是WLAN支持QoS的关键所在.我们已经给出了一种低分组碰撞概率的MAC层回退机制——RWBO+BEB.该文进一步讨论如何让RWBO+BEB支持业务区分的问题.首先提出了一个Markov链模型,分析如何根据无线终端的带宽比率设置最小竞争窗口,然后给出了一种新的支持业务区分的回退算法——DS-RWBO,仿真结果表明,DS-RWBO能根据局域网中每个终端的带宽比率分配无线信道的带宽资源.     

A study on the influence of transmission errors on WLAN IEEE 802.11 MAC performance

Since the advent of the first IEEE 802.11 standard for WLANs, several papers have been presented that evaluate the IEEE 802.11 DCF access method. In realistic WLAN environments frame errors usually occur due to non‐ideal channel conditions; in this way, papers including adverse transmission conditions in the evaluation have been published later in the literature. In this paper, we review existent analytical models that include the influence of transmission errors in IEEE 802.11 DCF performance. We modify current models and provide a more accurate analysis, thus allowing the evaluation in single rate and multi‐rate scenarios with stations subject to different link error conditions. Moreover, this paper exposes the unfairness problem that arises in IEEE 802.11 DCF networks with stations subject to different transmission conditions through analytical and simulation results. Stations are not able to distinguish collisions from failed transmissions due to link errors; both result in a missing ACK and, consequently, the transmitting stations apply the exponential backoff algorithm. This fact leads to a lower performance for stations in worse transmission conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimal Constant-Window Backoff Scheme for IEEE 802.11 DCF in Single-Hop Wireless Networks Under Finite Load Conditions

Existing backoff scheme’s optimization of IEEE 802.11 DCF MAC protocol consider only saturated networks or asymptotic conditions. In real situations, traffic is bursty or streamed at low rates so that stations do not operate usually in saturated regime. In this work, we propose and analyze a backoff enhancement for IEEE 802.11 DCF that requires information only about the network size and that is quasi-optimal under all traffic loads. We first analyze the performance of DCF multiple access scheme under general load conditions in single-hop configuration and we provide an accurate delay statistics model that consider the self-loop probability in every backoff state. We prove then the short-term unfairness of the binary exponential backoff used in IEEE 802.11 by defining channel capture probability as fairness metric. Motivated by the results on fairness, we introduce the constant-window backoff scheme and we compare its performance to IEEE 802.11 DCF with Binary exponential backoff. The quasi-optimality of the proposed scheme is proved analytically and numerical results show that it increases, both the throughput and fairness, of IEEE 802.11 DCF while remaining insensitive to traffic intensity. The analysis is then extended to consider the finite queuing capacity at nodes buffers using results from the delay analysis. NS2 simulations validate the obtained results. Institut Eurecom’s research is partially supported by its industrial members: BMW Group Research & Technology—BMW Group Company, Bouygues Telecom, Cisco Systems, France Telecom , Hitachi Europe, SFR, Sharp, STMicroelectronics, Swisscom, Thales.     

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     

Analysis of random access channel in UTRA‐TDD on AWGN channel

The random access channel (RACH) in UTRA‐TDD systems is an uplink contention‐based transport channel that is mainly used to carry control information from mobile stations to base stations. In this paper, we study the performance of RACH on an additive white Gaussian noise (AWGN) channel whereby successful transmission of a burst requires the spreading code chosen to be collision‐free and the burst is error‐free after convolutional decoding. Based on this model, the code‐collision probability, the data bit error probability and the RACH channel capacity are derived. The random retransmission delay mechanism is not specified in UTRA‐TDD. We therefore choose an access mechanism with binary exponential backoff delay procedure similar to that in IEEE 802.11. Based on that mechanism, the blocking probability and the first two moments of the delay are also derived. Compared with the mean, the standard deviation is found to be very high. Copyright © 2004 John Wiley & Sons, Ltd.     

Defining and maximizing PPT—a novel performance parameter for IEEE 802.11 DCF

Much research has been conducted on saturation throughput of IEEE802.11 DCF, and has led to some improvement. But increasing the successful transmission probability of packet is also important for saving stations' battery energy and decreasing the packet delay. In this paper, we define a new performance parameter, named Product of successful transmission Probability and saturation Throughput (PPT), for 802.11 DCF, which binds successful transmission probability and saturation throughput together. An analysis is given to maximize PPT. An expression of optimal minimum contention windows (CW_min) is obtained analytically for maximizing PPT. For simplicity, we give a name DCF‐PPT to the 802.11 DCF that sets its CW_min according to this expression. The performance of DCF‐PPT is simulated with different stations in terms of saturation throughput, successful transmission probability and PPT. The simulation results indicate that, compared to 802.11 DCF, DCF‐PPT can significantly increase the PPT and successful transmission probability (about 0.95) on condition that the saturation throughput is not decreased. Copyright © 2006 John Wiley & Sons, Ltd.