Contention Window Size Research Articles

Enhancing the selection of backoff interval using fuzzy logic over wireless Ad Hoc networks.

IEEE 802.11 is the de facto standard for medium access over wireless ad hoc network. The collision avoidance mechanism (i.e., random binary exponential backoff—BEB) of IEEE 802.11 DCF (distributed coordination function) is inefficient and unfair especially under heavy load. In the literature, many algorithms have been proposed to tune the contention window (CW) size. However, these algorithms make every node select its backoff interval between [0, CW] in a random and uniform manner. This randomness is incorporated to avoid collisions among the nodes. But this random backoff interval can change the optimal order and frequency of channel access among competing nodes which results in unfairness and increased delay. In this paper, we propose an algorithm that schedules the medium access in a fair and effective manner. This algorithm enhances IEEE 802.11 DCF with additional level of contention resolution that prioritizes the contending nodes according to its queue length and waiting time. Each node computes its unique backoff interval using fuzzy logic based on the input parameters collected from contending nodes through overhearing. We evaluate our algorithm against IEEE 802.11, GDCF (gentle distributed coordination function) protocols using ns-2.35 simulator and show that our algorithm achieves good performance.

Hybrid Control of Contention Window and Frame Aggregation for Performance Enhancement in Multirate WLANs

The IEEE 802.11 standard has been evolved to support multiple transmission rates in wireless local area networks (WLANs) to cope with diverse channel conditions and to increase throughput. However, when stations with different transmission rates coexist, the basic channel access mechanism of WLAN, distributed coordination function (DCF), not only fails to assure airtime fairness among competing stations but also decreases overall network throughput, because DCF was designed to provide fair opportunity of channel access, regardless of transmission rate. As an effective solution to this problem, we propose a hybrid control mechanism that integrates contention window control and frame aggregation. The former adjusts the size of contention window and differentiates the channel access opportunity depending on the transmission rates of stations. The latter controls the number of packets in the aggregated frame to tightly assure per-station airtime fairness with the reduced channel access overheads. Moreover, we derive an analytical model to evaluate the performance of the proposed mechanism in terms of throughput and fairness. Along with the analysis results, the extensive simulation results confirm that the proposed mechanism significantly increases the overall throughput by about three times compared to the conventional DCF, while assuring airtime fairness strictly.

Modeling of initial contention window size for successful initial ranging process in IEEE 802.22 WRAN cell

Avoiding collision among contending customer premise equipments (CPEs) attempting to associate with a base station (BS) in a wireless regional area network (WRAN) is a challenging issue. The collision probability is highly dependent upon the size of the initial contention window and the number of contending CPEs. To reduce the collision probability among CPEs in order to start the ranging process in an IEEE 802.22 network, the BS needs to adjust the initial contention window size. This paper provides an analytical framework to estimate the ranging request collision probability depending upon the size of the initial contention window and the number of CPEs attempting to join the IEEE WRAN cell. The accuracy of the estimated curve is analyzed for various numbers of contention CPEs on the basis of the relative errors. The numerical results confirm that the approximation works reasonably well for finding the ranging request collision probability for any number of contention CPEs at a particular value of initial contention window size. Moreover, this approximation provides the threshold size for a contention window to start the initial ranging process for any number of CPEs in an IEEE 802.22 network.

Mobility and QoS oriented 802.11p MAC scheme for vehicle-to-infrastructure communications

Vehicles in Intelligent Transportation System (ITS) can communicate with other vehicles or roadside unit (RSU) within a specific area. In ITS, different services have different requirements of Quality of Service (QoS). For delay-tolerant services, such as FTP and Email, a longtime fairness should be guaranteed, which means vehicles with different speeds should have the same chance to communicate with RSU. While for real-time services, such as voice and video, a low average delay should be achieved. Based on original DCF scheme, much modification work has been done to achieve better QoS for mixed services, but fail to consider the unfairness problem brought by mobility. In this paper, we study the performance of 802.11p MAC scheme under high- speed mobility conditions and build a Mixed-Service-Mobility model to analyze the interaction among different services. By adjusting the minimum contention window size of 802.11p MAC scheme according to different speeds, we succeed in providing a balance between the longtime fairness of delay-tolerant services and the average delay of real-time services. The proposed analytical model is validated against simulation results to demonstrate its accuracy and effectiveness.

Pareto Optimized EDCA Parameter Control for Wireless Local Area Networks

The performance of IEEE 802.11e enhanced distributed channel access (EDCA) is influenced by several interactive parameters that make quality of service (QoS) control complex and difficult. In EDCA, the most critical performance influencing parameters are the arbitration interframe space (AIFS) and contention window size (CW) of each access category (AC). The objective of this paper is to provide a scheme for parameter control such that the throughput per station as well as the overall system throughput of the network is maximized and controllable. For this purpose, a simple and accurate analytical model describing the throughput behavior of EDCA networks is presented in this paper. Based on this model, the paper further provides a scheme in which a Pareto optimal system configuration is obtained via an appropriate CW control for a given AIFS value, which is a different approach compared to relevant papers in the literature that deal with CW control only. The simulation results confirm the effectiveness of the proposed method which shows significant performance improvements compared to other existing algorithms.

TCP-CC: cross-layer TCP pacing protocol by contention control on wireless networks

Transmission Control Protocol (TCP) performs poorly over wireless networks. Some research indicates that the TCP congestion control mechanism may cause burstiness in the traffic flow. Numerous TCP segments are delivered simultaneously, while an acknowledgement of a retransmission is successfully received. Burstiness leads to a highly contentious network, which markedly increases the probability of packet loss on wireless networks. TCP pacing is a possible solutions for TCP burstiness on multi-hop networks. In this algorithm, TCP segment transmissions are distributed over the whole Round Trip Time. Most pacing protocols attempt to insert a delay interval into the TCP transmissions. However, there is a similar pacing algorithm in IEEE802.11, known as the contention window mechanism. In this paper, we first measure and analyze the way that the contention window size affects TCP throughput in different scenarios. We propose a cross-layer TCP pacing protocol by contention control in the MAC layer, called TCP Contention Control (TCP-CC). It adjusts the lower bound of the contention window in order to optimize the overall TCP throughput in both one-hop and multi-hop topology. Finally, comparative simulations are conducted in order to verify the improvements of our protocol on both TCP Reno and TCP Vegas.

English

IEEE 802.11 DCF is the MAC protocol currently used in wireless LANs. However, due to idle and collision times, 802.11 DCF performs poorly when it comes to channel utilization, system throughput, and channel access time. To overcome these sources of inefficiency in 802.11 DCF, we propose a distributed and dynamically adaptive MAC protocol for wireless networks, called Token-DCF with Append process and change contention window size. Main focus of our approach is on reducing idle and collision times by introducing an algorithm of token passing using Append process and change contention window size. In Append process strong data searched by DCF process and its queue length if greater than the threshold level, will focus on the minimum queue length in header list and add it with the new neighboring node by increasing system throughput and channel access time. We simulate in ns-2 to measure and compare performance of MAC protocols.

Real-time message scheduling for ISA100.11a networks

This paper proposes two new message scheduling methods on shared timeslots of the ISA100.11a standard to enhance real-time performance, namely, traffic-aware message scheduling (TAMS) and contention window size adjustment (CWSA). In TAMS, instead of competing to transmit sporadic messages in consecutive cycles, end-nodes are divided into groups, which then access the channel in specific cycles when the probability of timeslots getting involved in collisions exceeds a specified threshold. Conversely, in CWSA, the contention window is adjusted when the probability of timeslots getting involved in collisions exceeds the threshold. The results of simulations conducted indicate that these two proposed methods provide performance improvements in terms of success probability and end-to-end delay.

Performance modeling and analysis of IEEE 802.11 DCF based fair channel access for vehicle-to-roadside communication in a non-saturated state

This paper considers the fair access problem in vehicular ad hoc networks and develops analytical models for analyzing the performance of an IEEE 802.11 distributed coordination function based fair channel access protocol in a non-saturated state. We first derive the relationship between the transmission probability and the minimum contention window size of a vehicle, and the relationship between the velocity and the minimum contention window size of a vehicle in a non-saturated state. Based on the analytical model, the minimum contention window size of a vehicle for a given velocity can be determined in order to achieve fair access among different vehicles. Moreover, an analytical model is also developed for analyzing the throughput performance of the fair channel access protocol in a non-saturated state. The effectiveness of the analytical models is justified through simulation results.

CSMA/CA‐based medium access control for indoor millimeter wave networks

AbstractMillimeter wave (mmWave) communication is a promising technology to support high‐rate (e.g., multi‐Gbps) multimedia applications because of its large available bandwidth. Multipacket reception is one of the important capabilities of mmWave networks to capture a few packets simultaneously. This capability has the potential to improve medium access control layer performance. Because of the severe propagation loss in mmWave band, traditional backoff mechanisms in carrier sensing multiple access/collision avoidance (CSMA/CA) designed for narrowband systems can result not only in unfairness but also in significant throughput reduction. This paper proposes a novel backoff mechanism in CSMA/CA by giving a higher transmission probability to the node with a transmission failure than that with a transmission success, aiming to improve the system throughput. The transmission probability is adjusted by changing the contention window size according to the congestion status of each node and the whole network. The analysis demonstrates the effectiveness of the proposed backoff mechanism on reducing transmission collisions and increasing network throughput. Extensive simulations show that the proposed backoff mechanism can efficiently utilize network resources and significantly improve the network performance on system throughput and fairness. Copyright © 2014 John Wiley & Sons, Ltd.

Several Contention Window Adjustment Techniques for Improving Unsaturated throughput of Wireless LANs

This paper proposes the several contention window adjustment schemes in backoff process as well-known backoff algorithm (BA) for improving the performance of wireless local area network (WLAN). In addition, this research introduces a new unsaturated discrete Markov chain model in fixed backoff stages and fixed contention window sizes technique (FBFC). The proposed contention window adjustment schemes are designed by applying the moment generating function concept in random variable and process theorem. Unsaturated throughput parameters are used to compare the performance of all contention window size adjustment techniques based on IEEE802.11b WLAN standards. The comparison results show that Bernoulli and Double adjustment schemes are good contention window size adjustments at light traffic load, and the Even contention window size adjustment operates well at high traffic load condition.

The Effect of MAC Parameters on Energy Efficiency and Delay in Wireless Sensor Networks

As the most important indices to evaluate the performance of wireless sensor networks, energy efficiency and delay have been achieved more and more attention by researchers. In order to build the mathematical relationship between network parameters and performances, this paper starts from the three basic elements of communication and takes account backoff mechanism in sending period, bit error rate in transmission period and collision rate in receiving period. Using theoretical analysis and simulation confirmation, we can make a conclusion that packet payload length, bit error rate, node number, contention window size and the maximum retransmission times allowed have influences on packet drop rate, and then affect network performances

Homeplug-AV CSMA/CA Cross-Layer Extension for QoS Improvement of Multimedia Services

This letter proposes a cross-layer extension of the CSMA/CA protocol used in the Power-line Communications standard HomePlug-AV. Its main goal is to allocate more bandwidth to nodes with QoS needs by giving them more channel access opportunities. An easy way to do this is by appropriately modifying the contention window size of the participants. The presented protocol considers both the QoS requirements of the upper-layer services and the physical layer restrictions. It is mathematically modeled and evaluated by means of a realistic simulator in an in-home scenario, showing that it significantly improves the system performance compared to the standard version of the protocol and to any other previously presented extension.

Improving broadcast reliability for neighbor discovery, link estimation and collection tree construction in wireless sensor networks

Neighbor Discovery and Link Estimation (NDLE) phase and Collection Tree Construction (CTC) phase are essential for correct and efficient operation of network protocols. However, the accuracy of these phases is highly affected by packet collisions, because CSMA is used for access arbitration and it does not support collision avoidance with broadcast transmissions. To improve NDLE accuracy: (i) We propose contention window adjustment mechanisms that rely on collision detection through the capture effect. In contrast to the existing approaches that utilize a long inter-packet duration for collision avoidance, the proposed mechanisms do not depend on network configuration and can provide adaptive collision avoidance with respect to the local collision intensity. (ii) We propose a mathematical model through which the MAC protocol can be configured to achieve a desired broadcasting success probability. (iii) We investigate and show the potential benefits of exploiting partially recovered packets during the NDLE phase. To improve CTC accuracy, we propose the Geowindow algorithm, which reduces packet collisions through contention window size management and transmission prioritization. Our results show that the Geowindow algorithm can improve the efficiency of the TinyOS’s Collection Tree Protocol up to 74% in terms of tree cost, without increasing duration or energy consumption. Also, it can improve the packet delivery performance up to 70% in data gathering scenarios. The proposed MAC mechanisms of this paper are not only suitable for the initialization phases, but they can also be used for NDLE and CTC updates during the regular network operation, as well as other broadcast-based traffic patterns.

Virtual Continuous CWmin Control Scheme of WLAN

In this paper, a priority control problem between uplink and downlink flows in IEEE 802.11 wireless LANs is considered. The minimum contention window size (CWmin) has a nonnegative integer value. CWmin control scheme is one of the solutions for priority control to achieve the fairness between links. However, it has the problem that CWmin control scheme cannot achieve precise priority control when the CWmin values become small. As the solution of this problem, this paper proposes a new CWmin control method called a virtual continuous CWmin control (VCCC) scheme. The key concept of this method is that it involves the use of small and large CWmin values probabilistically. The proposed scheme realizes the expected value of CWmin as a nonnegative real number and solves the precise priority control problem. Moreover, we proposed a theoretical analysis model for the proposed VCCC scheme. Computer simulation results show that the proposed scheme improves the throughput performance and achieves fairness between the uplink and the downlink flows in an infrastructure mode of the IEEE 802.11 based wireless LAN. Throughput of the proposed scheme is 31% higher than that of a conventional scheme when the number of wireless stations is 18. The difference between the theoretical analysis results and computer simulation results of the throughput is within 1% when the number of STAs is less than 10.

Unsaturated Throughput Analysis of Physical-Layer Network Coding Based on IEEE 802.11 Distributed Coordination Function

In this paper, we investigate the throughput performance of \rev{physical-layer network coding} (PNC) under the IEEE 802.11 distributed coordination function (DCF). We consider the wireless network that two client groups communicate with each other across one relay node, and focus on the unsaturated network case. The difficulty in modeling the relay systems under the IEEE 802.11 DCF is that the minimum contention window sizes of the client nodes and the relay node may be different, which makes the traditional throughput analysis methods for the non-relay wireless networks inapplicable. Fortunately, we find that the relay system can be decomposed into four parts and respectively modeled. Analytical results show that the throughput gain of PNC scheme is heavily affected by the probability that a transmitted network-coding (NC) packet contains the information of two packets. The implication is that the throughput benefit of PNC is more significant for bidirectional isochronous traffic with rate requirements. \rev{We further derive an approximate closed-form solution of the optimal transmission probability of client nodes that maximizes the PNC network throughput.} We validate our analytical model through extensive simulations and discuss the relationship between the PNC network throughput and other system parameters, such as the minimum contention window sizes of both the client nodes and the relay node.

An Efficient Backoff Algorithm for IEEE 802.15.4 Wireless Sensor Networks

IEEE 802.15.4 is one of the most prominent MAC protocol standard designed to achieve low-power, low-cost, and low-rate wireless personal area networks. The contention access period of IEEE 802.15.4 employs carrier sense multiple access with collision avoidance (CSMA/CA) algorithm. A long random backoff time causes longer average delay, while a small one gives a high collision rate. In this paper, we propose an efficient backoff algorithm, called EBA-15.4MAC that enhances the performance of slotted CSMA/CA algorithm. EBA-15.4MAC is designed based on two new techniques; firstly, it updates the contention window size based on the probability of collision parameter. Secondly, EBA-15.4MAC resolves the problem of access collision via the deployment of a novel Temporary Backoff (TB) and Next Temporary Backoff (NTB). In this case, the nodes not choose backoff exponent randomly as mentioned in the standard but they select TB and NTB values which can be 10–50 % of the actual backoff delay selected by the node randomly. By using these two new methods, EBA-15.4MAC minimizes the level of collision since the probability of two nodes selecting the same backoff period will be low. To evaluate the performance of EBA-15.4MAC mechanism, the network simulator has been conducted. Simulation results demonstrate that the proposed scheme significantly improves the throughput, delivery ratio, power consumption and average delay.

Synchronized CSMA Contention: Model, Implementation, and Evaluation

A class of carrier sense multiple access (CSMA) protocols used in a broad range of wireless applications uses synchronized contention where nodes periodically contend at intervals of fixed duration. While several models exist for asynchronous CSMA contention used in protocols like IEEE 802.11 MAC, no model exists for synchronized CSMA contention that also incorporates realistic factors like clock drifts. In this paper, we introduce a model that quantifies the interplay of clock drifts with contention window size, control packet size, and carrier sense regulated by usage of guard time. Using a field programmable gate array (FPGA)-based MAC protocol implementation and controlled experiments on a wireless testbed, we evaluate the model predictions on the isolated and combined impact of these key performance factors to per-flow throughput and fairness properties in both single-hop and multihop networks. Our model and experimental evaluation reveal conditions on protocol parameters under which the throughput of certain flows can exponentially decrease; while at the same time, it enables solutions that can offset such problems in a predictable manner.

Throughput Analysis Model for IEEE 802.11e EDCA with Multiple Access Categories

IEEE 802.11e standard has been specified to support differentiated quality of service (QoS), one of the critical issues on the conventional IEEE 802.11 wireless local area networks (WLANs). Enhanced Distributed Channel Access (EDCA) is the fundamental and mandatory contention-based channel access method of IEEE 802.11e, and delivers traffic based on differentiated Access Categories (ACs). A general three dimensional Markov chain model of IEEE 802.11e EDCA for performance analysis is proposed in this paper. The analytical model considers multiple stations with an arbitrary number of different ACs. It also differentiates the contention window (CW) sizes and the arbitration interframe spaces (AIFSs), and considers virtual collision mechanism. Based on the model, the saturation throughput of EDCA is derived, and the accuracy of the proposed model is validated via simulations.

베이지안 게임모델을 적용한 P-persistent MAC 기반 주기적 안정 메시지 전송 방법

For the safety messages in IEEE 802.11p/WAVE vehicles network environment, strict periodic beacon broadcasting requires status advertisement to assist the driver for safety. In crowded networks where beacon message are broadcasted at a high number of frequencies by many vehicles, which used for beacon sending, will be congested by the wireless medium due to the contention-window based IEEE 802.11p MAC. To resolve the congestion, we consider a MAC scheme based on slotted p-persistent CSMA as a simple non-cooperative Bayesian game which involves payoffs reflecting the attempt probability. Then, we derive Bayesian Nash Equilibrium (BNE) in a closed form. Using the BNE, we propose new congestion control algorithm to improve the performance of the beacon rate under saturation condition in IEEE 802.11p/WAVE vehicular networks. This algorithm explicitly computes packet delivery probability as a function of contention window (CW) size and number of vehicles. The proposed algorithm is validated against numerical simulation results to demonstrate its stability.