Distributed Coordination Function Research Articles

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.

Performance Analysis of MAC with DCF on CCH and Reservation for Multi-Channel for IEEE 802.11p/1609.4 WAVE Networks

We propose and analyze a MAC Protocol for multiple service channels (SCHs) based on reservation by distributed coordination function (DCF) on control channel (CCH) for IEEE 802.11p and IEEE 1609.4 wireless access in vehicular environments (WAVE) networks. Specifically each user sends a request for service (RFS) packet to the Road-Side Unit (RSU) on the CCH by DCF method in order to reserve one SCH over which a data file is transmitted at a once. We construct a Markov Chain to find collision probability of RFS packet. We obtain probability of successful delivery and head-of-line (HOL) delay of RFS packet, and service time of data file.

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.

Modelling collision alleviating DCF protocol with finite retry limits

IEEE 802.11-based wireless ad hoc networks are well suited for critical scenarios such as rescue operations during natural calamities, military applications on battle fields and so on where the existing infrastructure fails. In these networks, since nodes have limited battery power, the reduction in packet collision is very important, otherwise the nodes have to spend more energy for packet transmission. In these wireless networks, the packet transmission procedure plays a pivotal role in performance. Several packet transmission procedures represented with Markov chain models have been suggested by the authors to improve the network performance. However, these procedures have not been suitable for dense networks due to the high collision rate. A new packet transmission procedure for the IEEE 802.11 distributed coordination function (DCF) protocol is proposed to reduce the number of collisions and enhance the throughput of wireless networks. A mathematical framework is carried out for transmission probability and performance analysis is presented showing that the new procedure greatly enhances the network performance and as a result can be applied in the design and performance optimisation of wireless network protocols.

Investigation on the Dependence of TCP Upstream Throughput on Snr for Single and Multiple Links in a Wlan System

An empirical investigation on the dependence of TCP upstream throughput (TCPupT) against signal to noise ratio (SNR) in an IEEE802.11b WLAN system was carried out in various environments and varieties of QoS traffic using an Infrastructure based IEEE802.11b WLAN system set up consecutively in various environments (open corridor, small offices with block walls or plaster boards and free space). Empirical models describing the TCPupT against SNR for different signal ranges (all signals, strong signals, grey signals and weak signals) were developed and validated for both single and multiple users on the network. Our results show a strong dependence of TCPupT on the received SNR which varied as the SNR values changed from high (strong signals) through low (grey signals) to very low (weak signals). Our models showed lower RMS errors when they and other similar models were compared with validation data. We observed RMS errors of 0.5605471Mbps, 0.4479389Mbps, 1.04536603Mbps and 0.5813471Mbps for the General, Strong signals, Grey signals and Weak signals models respectively for the single user scenario. We also observed RMS errors of 1.3319396Mbps, 0.9457431Mbps, 0.8092979Mbps and 0.4220292Mbps for the multiple users scenario. An appreciable difference was observed between the throughput behavior for single and multiple users on the network showing the inefficiency of the distributed coordination function and Point coordination function used in WLANs as access mechanisms. Our models will provide researchers and WLAN systems users with a tool to estimate the TCP upstream throughput in a real network in various environments by monitoring the SNR.

Modeling and Analysis of IEEE 802.11 DCF MAC

Abstract In this paper, an analytic model is present for evaluating the MAC layer Collision Probability and mean delay at wireless nodes using the Distributed Coordination Function of IEEE 802.11 MAC Model. This model is valid for finite loads and can account for arbitrary arrival patterns, packet size distributions and number of nodes. Each node is modeled as a discrete time M/G/1 queue and impact of packet collisions, the resulting back-offs as well as the packet size distribution. We analyze this Model for different number of nodes in any network. The impact of arrival pattern on collision probability and delay is also analyzed.

Modelling of IEEE 802.11 DCF in the presence of hidden nodes

Today, the most commonly used technology for wireless networks is IEEE 802.11. The performance of the Medium Access Control MAC layer, which consists of Distributed Coordination Function DCF and Point Coordination Function PCF, has been examined over a long time period. The DCF is the random access scheme that uses CSMA/CA in IEEE 802.11 MAC layer protocol. There are various issues related to the performance evaluation of IEEE 802.11 DCF and one of the important issues is the problem of hidden terminals which exists in practice in any wireless network. In this paper, a two-dimensional analytical model is proposed for the analysis of IEEE 802.11 DCF in the presence of hidden terminals for both basic access and RTS/CTS modes by making use of the Markov chain.

Optimal Stochastic Policies in a Network Coding Capable Ad Hoc Networks

Network coding is a promising technology that increases system throughput by reducing the number of packet transmissions from the source node to the destination node in a saturated traffic scenario. Nevertheless, some packets can suffer from end-to-end delay, because of a queuing delay in an intermediate node waiting for other packets to be encoded with exclusive or (XOR). In this paper, we analyze the delay according to packet arrival rate and propose two network coding schemes, iXOR (Intelligent XOR) and oXOR (Optimal XOR) with Markov Decision Process (MDP). They reduce the average delay, even under an unsaturated traffic load, through the Holding-χ strategy. In particular, we are interested in the unsaturated network scenario. The unsaturated network is more practical because, in a real wireless network, nodes do not always have packets waiting to be sent. Through analysis and extensive simulations, we show that iXOR and oXOR are better than the Distributed Coordination Function (DCF) without XOR (the general forwarding scheme) and XOR with DCF with respect to average delay as well as delivery ratio.

A comprehensive DCF performance analysis in noisy industrial wireless networks

SummaryThe use of wireless technology in industrial networks is becoming more popular because of its flexibility, reduction of cable cost, and deployment time. Providing an accurate model to study the most important parameters of these networks, the timeliness and reliability, is essential in assessing the network metrics and choosing proper protocol settings. The Institute of Electrical and Electronics Engineers (IEEE) 802.11 is a common and established wireless technology, and several analytical models have been proposed to assess its performance; however, most of them are accurate only for a limited network situation, especially data networks that have large packet payloads and are used at high signal to noise ratios, and cannot be applied to study the performance of industrial networks that have short packet lengths and are used in harsh and noisy environments. In this paper, a novel three‐dimensional discrete‐time Markov chain model has been proposed for the IEEE 802.11‐based industrial wireless networks using the distributed coordination function as the medium access control mechanism in the worst‐case saturated traffic. It considers both causes of the backoff freezing: busy channel and the successive interframe space waiting time. In this way, it provides a much more accurate estimation of the channel access and error probabilities, resulting in a more accurate network parameter calculation. Also, based on the proposed model, a comprehensive packet delay analysis, including average, jitter, and cumulative distribution function, has been provided for the near 100% reliable industrial scenario and error‐prone channel condition, which in comparison with similar pieces of work provides much more accurate results. Copyright © 2014 John Wiley & Sons, Ltd.

Analysis of Optimal Carrier Sensing Range in Wireless Ad Hoc Networks with Order Dependent Capture Capability

IEEE 802.11 wireless ad hoc network performance in distributed coordination function mode is limited by hidden and exposed terminals problem. Request-to-send/clear-to-send based hand shake reduces ...

Development of Collision Alleviating DCF Protocol with Efficient Backoff Algorithm for Wireless Ad hoc Networks

Ad hoc networks have been proposed for emergency communication wherein the required infrastructure is unavailable. However, a major concern in Ad hoc networks is collisions. Even in infrastructure based wireless networks, when the number of contending nodes is high, more number of frame collisions occur which leads to drastic reduction in network performance. In all IEEE 802.11 based wireless and Ad hoc networks, the backoff algorithm dynamically controls the contention window of the nodes experiencing collisions. Even though several algorithms such as Binary Exponential Backoff, Double Increment Double Decrement backoff, Exponential Increase Exponential Decrease backoff, Hybrid Backoff, Binary Negative Exponential Backoff etc. have been proposed in the literature to enhance the performance of IEEE 802.11 Distributed Coordination Function (DCF) protocol, most of them have not been developed for real- traffic scenarios. Also the packet collision rate is high using these algorithms. So, in this paper, a new Contention Window based Multiplicative Increase Decrease Backoff (CWMIDB) algorithm is proposed for the DCF protocol to alleviate the number of collisions. Furthermore, the packet transmission procedure of the DCF protocol is modified to avoid channel capture effect and this is represented with a Markov chain model. A simple mathematical model is developed for transmission probability considering the non-saturated traffic and channel errors. Results show that the proposed CWMIDB algorithm provides superior quality-of-service parameters over existing backoff algorithms.

Performance Analysis of Group-Synchronized DCF for Dense IEEE 802.11 Networks

In dense IEEE 802.11 networks, improving the efficiency of contention-based media access control is an important and challenging issue. Recently, the IEEE802.11ah Task Group has discussed a group-synchronized distributed coordination function (GS-DCF) for densely deployed wireless networks with a large number of stations. By using the restricted access window (RAW) and RAW slots, the GS-DCF is anticipated to improve the throughput substantially, primarily due to relieving the channel contention. However, optimizing the MAC configurations for the RAW, i.e., the number and duration of RAW slots, is still an open issue. In this paper, we first build an analytical model to track the performance of the GS-DCF in saturated 802.11 networks. Then, we study and compare the GS-DCF throughput using both centralized and decentralized grouping schemes. The accuracy of our model has been validated with simulation results. It is observed that the GS-DCF obtains a throughput gain of seven times or more over DCF in a network of 512 or more stations. Moreover, it is demonstrated that the decentralized grouping scheme can be implemented with a small throughput loss when compared with the centralized grouping scheme.

Distributed Coordination Function Protocol with Unequal Frame Error Rates

This letter provides an analysis of the saturation throughputs of the individual stations (STAs) in a distributed coordination function (DCF) network. Compared with the previous DCF models, our new proposed model is suitable for more general environments where the frame error rates (FERs) are different depending on the transmitting STAs and the numbers of retransmission attempts. Numerical analyses show that FERs significantly affect the STAs' throughputs.

A Novel Multi-channel MAC Protocol for Directional Antennas in Ad Hoc Networks

In recent years, the employment of directional antennas in ad hoc networks has increased significantly. In this paper, we propose a multi-channel MAC protocol for directional antennas (MCMDA) for IEEE 802.11 based ad hoc networks. To improve the network performance, the so-called hidden terminals and deafness should be decreased as much as possible. In MCMDA, not only multiple channels are effectively utilized but also vertically opposite clear-to-send is employed to minimize the effects of hidden terminals and deafness. Each node is equipped with multiple interfaces for multiple directional antennas such that concurrent transmissions and receptions take place in a node at the same time, resulting in further improvement of the network capacity. The performance study based on extensive computer simulation shows that the proposed MCMDA achieves significant performance improvement in comparison to the conventional directional MAC as well as IEEE 802.11 distributed coordination function in terms of network throughput and end-to-end delay.

A MAC Throughput in the IEEE 802.11ac over Error-Prone Channel

This paper analyzes a MAC (Medium Access Control) layer throughput over error-prone channel in the IEEE 802.11ac-based wireless LAN with DCF (Distributed Coordination Function) protocol and A-MPDU (MAC Protocol Data Unit Aggregation) scheme, using theoretical analysis method. The MAC saturation throughput is evaluated by using a PER (Packet Error Rate) on the condition that the number of station, transmission probability, the number of parallel beams and the number of frames in each A-MPDU are variables. When the PER is 10-2 and the number of aggregated MPDUs in each A-MPDU is 20, it is identified that the MAC layer throughput of IEEE 802.11ac can be maximally attained up to a 92.8% of physical transmission rate.

Formal modeling and verification of an enhanced variant of the IEEE 802.11 CSMA/CA protocol

In this paper, we present a formal method for modeling and checking an enhanced version of the carrier sense multiple access with collision avoidance protocol related to the IEEE 802.11 MAC layer, which has been proposed as the standard protocol for wireless local area networks. We deal mainly with the distributed coordination function (DCF) procedure of this protocol throughout a sequence of transformation steps. First, we use the unified modeling language state machines to thoroughly capture the behavior of wireless stations implementing a DCF, and then translate them into the input language of the UPPAAL model checking tool, which is a network of communicating timed automata. Finally, we proceed by checking of some of the safety and liveness properties, such as deadlock-freedom, using this tool.

Saturated distributed coordination function Markov model for noisy soft‐real‐time industrial wireless networks

The use of wireless technology in industrial networks is becoming more popular due to its numerous advantages. Providing an accurate model to study its performance parameters, is an essential step in designing a reliable and real-time industrial wireless network protocol. Most of the existing models are accurate only for data networks which have large packet payloads and high signal to noise ratio (SNR) characteristics, and cannot be applied to study the performance of industrial networks which have short packet lengths and are used in harsh and noisy environments. In this paper a novel three-dimensional discrete-time Markov chain (DTMC) model has been proposed for the IEEE 802.11-based industrial wireless networks using the distributed coordination function (DCF) as the medium access control (MAC) mechanism. It considers both causes of the backoff freezing: busy channel and the successive inter-frame space (IFS) waiting phase. In this way, it provides a much more accurate estimation of the channel access and collision probabilities, resulting in more accurate network metric calculations. Also, based on the proposed model, a simple and accurate packet delay analysis is provided in the worst-case saturated traffic and error-prone channel condition which in comparison with similar works provides more accurate results.

IEEE 802.11e EDCA Networks: Modeling, Differentiation and Optimization

Enhanced distributed channel access (EDCA) is an extension of the distributed coordination function to support quality-of-service for IEEE 802.11 wireless local area networks. By assigning distinct backoff parameters to each access category (AC), differentiated throughput performance can be achieved when the network is saturated. Although it has been long observed that the network throughput with the current EDCA standard setting may significantly degrade as the network size grows, how to properly tune the backoff parameters to optimize the network throughput under a certain differentiation requirement remains largely unknown. In this paper, a new analytical model is proposed to address this open issue. Specifically, we focus on an M-AC IEEE 802.11e EDCA network where nodes in the same AC have identical backoff parameters, including the initial backoff window sizeW <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(g)</sup> , the cutoff phaseK <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(g)</sup> , and the arbitration interframe spaces (AIFS) numberA <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(g)</sup> , g = 1, . . . , M. The network steady-state operating point in saturated conditions, i.e., pA, is characterized by using the steady-state probability of successful transmission of head-of-line (HOL) packets given that the channel is idle, based on which explicit expressions of node throughput and network throughput are further obtained. For given target ratios of node throughput of ACs, the optimal initial backoff window sizes and AIFS numbers to maximize the network throughput are derived and verified by simulation results. The analysis reveals that the maximum network throughput is solely determined by the holding time of HOL packets in successful transmission and collision states. To achieve the maximum network throughput, the initial backoff window size of each AC should be linearly increased with the network size. In the meantime, the increasing rate of the initial backoff window size, or the AIFS number, of each AC should be also carefully set according to the target ratios of node throughput. Although the maximum network throughput with pre-specified target ratios of node throughput of ACs can be achieved in both ways, the backoff window size differentiation could be a more preferable option as it requires fewer tuning parameters and provides better precision than the AIFS differentiation.

Energy-Efficient Fragmentation Scheme for IEEE 802.11 DCF: Delay and Fairness Considerations

The interplay between the internet of things, big data, cloud computing and mobility has brought up a number of challenges. One of them is the reliance of handheld wireless communication devices, e.g. smartphones, on limited battery power. In this paper we contribute to efforts that address this challenge. We investigate the energy efficiency of MAC layer fragmentation scheme. Despite the usefulness of fragmentation in improving transmission reliability in poor quality wireless channels, it comes with overhead costs that lead to its energy inefficiency. When a fragment transmission fails (due to collisions or channel induced errors), extra contentions are executed and hence extra overheads are incurred. We propose a novel fragmentation scheme called backoff-free fragment retransmission (BFFR). BFFR addresses classical fragmentation overheads. It aims at improving classical fragmentation energy efficiency without adversely affecting its throughput. We implement BFFR in NS3 and performance evaluation showed that BFFR outperforms classical fragmentation scheme in both throughput and energy efficiency. Further performance evaluation results indicate that our scheme maintained MAC delay performance of distributed coordination function at a small cost of medium access fairness in small sized wireless local area networks (WLANs). BFFR can be employed in legacy IEEE 802.11 networks to enhance their energy efficiency and throughput in poor quality wireless channels.

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.