Backoff Mechanism Research Articles

Exploiting maximum performance of multichannel multiple access using half-duplex transceiver

A slot-optimised multichannel medium access control (SO-MMAC) protocol is proposed to resolve the multichannel hidden terminal problem and control channel (CCH) bottleneck problem for multihop ad hoc networks. It adjusts the relationship between the reservation slot (RS) on the CCH and the data packet transmission slot on the data channel (DCH) to achieve the best load balance and default collision-free DCH selection without establishing a channel state table, and then greatly enhances multiple access efficiency. A reservation scheme with minimum handshakes and RS and contention micro-slot backoff mechanisms is also proposed to effectively decrease control overhead and packet collisions. Simulation results show that SO-MMAC has better performance compared to the traditional MAC protocol.

Period-Controlled MAC for High Performance in Wireless Networks

In this paper, we propose Period-Controlled Medium Access Control (PC-MAC), which can operate in pseudo-TDMA manner and achieves high throughput and fairness in simple networks. PC-MAC works like CSMA/CA initially and becomes a pseudo-TDMA scheme in a few seconds due to the periodic backoff mechanism along with the contention control that tries to maintain the number of idle slots to an optimal level. Simulation results show 10%-50% higher throughput than distributed coordination function (DCF), depending on the number of nodes, while maintaining nearly perfect fairness. Furthermore, we also show how PC-MAC can successfully be applied to complex networks.

An Adaptive Backoff Algorithm for Mobile Ad-Hoc Networks

Collision is a common problem in Mobile Ad Hoc Networks (MANETs). There are several backoff algorithms that have been proposed to avoid this problem. This paper proposes a new backoff algorithm called the Square Root Backoff (SRB). Moreover, it identifies that no algorithm can perform the best in all cases. Therefore, an adaptive strategy is proposed to choose the best backoff mechanism from a set of mechanisms based on network density and mobility parameters. The proposed adaptive algorithm is implemented in two phases, the offline phase and the online phase. Such design aims at reducing the time complexity of the algorithm by performing some of the computations prior to the actual deployment and of the network. Results from simulations demonstrate that the SRB algorithm achieved better performance than BEB and LB. Moreover, the adaptive backoff algorithm obtains the best throughput and end-to-end delay performance over the other backoff algorithms.

Tandem queueing networks with neighbor blocking and back-offs

We introduce a novel class of tandem queueing networks which arise in modeling the congestion behavior of wireless multi-hop networks with distributed medium access control. These models provide valuable insight in how the network performance in terms of throughput depends on the back-off mechanism that governs the competition among neighboring nodes for access to the medium. The models fall at the interface between classical queueing networks and interacting particle systems, and give rise to high-dimensional stochastic processes that challenge existing methodologies. We present various open problems and conjectures, which are supported by partial results for special cases and limit regimes as well as simulation experiments.

A Real-Time MAC Protocol with Extended Backoff Scheme for Wireless Sensor Networks

Wireless sensor networks (WSNs) are formed by a great quantity of sensor nodes, which are consisted of battery-powered and some tiny devices. In WSN, both efficient energy management and Quality of Service (QoS) are important issues for some applications. Real-time services are usually employed to satisfy QoS requirements in critical environment. This paper proposes a real-time MAC (Medium Access Control) protocol with extended backoff scheme for wireless sensor networks. The basic idea of the proposed protocol employs (m,k)-firm constraint scheduling which is to adjust the contention window (CW) around the optimal value for decreasing the dynamic failure and reducing collisions DBP (Distant Based Priority). In the proposed protocol, the scheduling algorithm dynamically assigns uniform transmitting opportunities to each node. Numerical results reveal the effect of the proposed backoff mechanism.

Fair Coexistence MAC Protocol for Contention-Based Heterogeneous Networks

This paper proposes a contention-based fair coexistence mechanism among heterogeneous networks that have different transmission power and/or coverage. First, we show that the existing carrier sensing multiple access (CSMA) mechanism, that is a prevailing contention-based protocol, results in significant unfairness in channel access when heterogeneous networks coexist; a system with lower transmission power hardly occupies the shared channel due to interference from a system with higher transmission power. We analyze the causes of unfairness in terms of (i) the asymmetry of carrier sensing and (ii) the blindness of binary exponential backoff mechanism and the link adaptation mechanism, and we derive an analytical model of per-system throughput to investigate the effects of these causes. To resolve this problem, we propose a fair coexistence CSMA protocol consisting of access etiquette and interference-aware backoff. The former adaptively controls the contention window size so that the high-power system allows transmission opportunities to the low-power system in a fair and efficient manner. The latter differentiates between the response to transmission failure caused by collision and the response to failure caused by interference. The simulation results confirm that the proposed scheme effectively mitigates the unfairness of channel sharing while attaining high spectral efficiency.

Throughput analysis of the IEEE 802.11 distributed coordination function considering erroneous channel and capture effects

This paper presents a performance study of the distributed coordination function (DCF) of 802.11 networks considering erroneous channel and capture effects under non-saturated traffic conditions employing a basic access method. The aggregate throughput of a practical wireless local area network (WLAN) strongly depends on the channel conditions. In a real radio environment, the received signal power at the access point from a station is subjected to deterministic path loss, shadowing, and fast multipath fading. The binary exponential backoff (BEB) mechanism of IEEE 802.11 DCF severely suffers from more channel idle time under high bit error rate (BER). To alleviate the low performance of IEEE 802.11 DCF, a new mechanism is introduced, which greatly outperforms the existing methods under a high BER. A multidimensional Markov chain model is used to characterize the behavior of DCF in order to account both non-ideal channel conditions and capture effects.

An Efficient Medium Access Control Protocol for under water Acoustic Sensor Network

a terrestrial wireless communication which uses radio waves, underwater communication relies on acoustic waves. The long latency and limited bandwidth pose great challenges in Underwater Media Access Control (MAC) design. So this paper proposes a new MAC protocol for wireless sensor acoustic network which are adaptable to underwater environment. This new protocol is based on the classic MACA to avoid hidden and exposed node problem. It also address the fragmentation issue, instead of sending a large packet it proposes to send fragmented packets to improve the throughput of the network. It introduces a new binary exponential back-off mechanism to reduce the possibilities of collision and hence to improve the throughput. The simulation result of the proposed protocol shows that its performance is better than the other MAC protocol. The simplicity and throughput stability of proposed MAC make it an appropriate reference MAC protocol.

Design and performance analysis on adaptive reservation-assisted collision resolution protocol for WLANs

In conventional IEEE 802.11 medium access control protocol, the distributed coordination function is designed for the wireless stations (WSs) to perform channel contention within the wireless local area networks (WLANs). Packet collision is considered one of the major issues within this type of contention-based scheme, which can severely degrade network performance for the WLANs. Research work has been conducted to modify the random backoff mechanism in order to alleviate the packet collision problem while the WSs are contending for channel access. However, most of the existing work can only provide limited throughput enhancement under specific number of WSs within the network. In this paper, an adaptive reservation-assisted collision resolution (ARCR) protocol is proposed to improve packet collision resulting from the random access schemes. With its adaptable reservation period, the contention-based channel access can be adaptively transformed into a reservation-based system if there are pending packets required to be transmitted between the WSs and the access point. Analytical model is derived for the proposed ARCR scheme in order to evaluate and validate its throughput performance. It can be observed from both analytical and simulation results that the proposed protocol outperforms existing schemes with enhanced channel utilization and network throughput.

On the Design of Opportunistic MAC Protocols for Multihop Wireless ; Networks with Beamforming Antennas

Beamforming antennas promise a significant increase in the spatial reuse of the wireless medium when deployed in multihop wireless networks. However, existing directional Medium Access Control (MAC) protocols with the default binary exponential backoff mechanism are not capable of fully exploiting the offered potential. In this paper, we discuss various issues involved in the design of MAC protocols specific for beamforming antennas. Based on our discussion, we argue that the traditional binary exponential backoff mechanism limits the possible spatial reuse and aggravates some beamforming-related problems such as deafness and head-of-line blocking. To grasp the transmission opportunities offered by beamforming antennas, we design an Opportunistic Directional MAC (OPDMAC) protocol for multihop wireless networks. The OPDMAC protocol employs a novel backoff mechanism in which the node is not forced to undergo idle backoff after a transmission failure but can rather take the opportunity of transmitting other outstanding packets in other directions. This mechanism minimizes the idle waiting time and increases the channel utilization significantly and thereby enables OPDMAC to enhance the spatial reusability of the wireless medium and reduce the impact of the deafness problem without additional overhead. Through extensive simulations, we demonstrate that OPDMAC enhances the performance in terms of throughput, delay, packet delivery ratio, and fairness. To further improve its performance, we discuss and evaluate the benefits of carefully choosing some protocol parameters instead of using the default values commonly used by other directional MAC protocols.

Efficient Backoff Algorithm in Wireless Multihop Ad Hoc Networks

The IEEE 802.11 standard, currently used in wireless multihop ad hoc networks, wastes bandwidth capacity and energy resources because of many collisions. Previous studies have shown that controlling the contention window size at a given node can decrease its probability of collisions. In this paper, we propose an efficient backoff scheme and evaluate its performance in ad hoc networks. Our backoff mechanism that we devised grants node access to the channel, based on its probability of collision for a transmitted frame in comparison with those nodes in the two-hop contention area. In addition, the new stage minimum contention window size is the previous stage maximum contention window size. We use simulation experiments to evaluate the effective performance of our scheme in an ad hoc network. Our extensive ns-2-based simulation results have shown that the proposed scheme provides excellent performance in terms of bandwidth, energy awareness, end-to-end goodput, as well as packet delivery ratio.

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

AbstractBased 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.

Design and analysis of new generation protocols for triple-play networks

Abstract Transmission Control Protocol (TCP)'s proven stability and scalability has made it the most widely used transport layer protocol for more than twenty years. However, as multimedia applications become ubiquitous over the internet, TCP has been found incapable of meeting their requirements, which place more emphasis on timeliness than on reliability. Because of that, many multimedia applications turn to UDP as their underlying transport protocol. However, the majority of video on demand and live broadcast applications predominantly use TCP over UDP (User Datagram Protocol), due to UDP's unresponsiveness to network conditions and problems with firewalls and NATs (Network Address Translations). TCP's poor performance in delivering real-time media is due to the following reasons: 1) TCP's emphasis on reliable in-order delivery causes frame jitter that interrupt media play out. 2) TCP's coarse-grained retransmission timeout (RTO) and its back-off mechanism is detrimental to any real-time based application. In this study, we propose a new variant of TCP with an early retransmission scheme as an enhancement to make it more suitable for streaming media. We call this new protocol TCP-ER. We performed extensive NS-2 simulations to show that: 1) the early retransmission scheme can reduce the number of retransmission timeouts in a variety of network environments, which results in a considerable decrease in number of retransmission timeouts and packet delay jitter. 2) Under same network conditions, constrained streaming over TCP-ER has a considerably lower number of late packets than its normal TCP counterpart. 3) TCP-ER has a higher throughput in severely congested network conditions, whereas it stays relatively fair with typical TCP implementations (specifically TCP-SACK) as congestion gets alleviated.

Adaptive backoff scheme for ad hoc networks based on IEEE 802.11

AbstractThe performance of backoff scheme plays an important role in designing efficient Medium Access Protocols for ad hoc networks. In this paper, we propose an adaptive backoff scheme and evaluate the performance of the proposed scheme for ad hoc networks. The backoff mechanism devised by us grants a node access to the channel based on its probability of collision for a transmitted frame in comparison to the nodes in the two‐hop contention area. We use both an analytical model and simulation experiments to evaluate the performance of our adaptive backoff mechanism in an ad hoc network. The results show that our protocol exhibits a significant improvement in power saving, end‐to‐end goodput, packet delivery ratio, and hop‐put, compared with the existing IEEE 802.11 DCF. Copyright © 2010 John Wiley & Sons, Ltd.

The effect of contention in CSMA networks: Model and fairness protocol

Multi-hop wireless networks using Carrier Sense Multiple Access (CSMA) suffer from various forms of unfairness. A majority of the existing work focuses on fairness issues due to hidden terminals and the impact on backoff mechanisms. This paper focuses on a more fundamental form of unfairness that arises due to unequal contention opportunities at a node; some nodes rarely observe an idle channel since two or more interferers can transmit together. Contention unfairness is unrelated to hidden terminals. The paper analyzes the impact of contention and provides insight into developing solutions for contention unfairness. We first develop a model–from first principles–for contention in IEEE 802.11 networks. The accuracy of the model is validated through simulations and the results show that such unfairness is a common phenomenon. Based on the insights gained from the model, we propose and evaluate distributed schemes that reduce the effect of unfairness due to contention. We show that contention unfairness occurs frequently in general scenarios. Simulation results indicate that the proposed protocols improve the overall system performance by providing greater fairness with a small reduction in throughput.

Listen before you talk, but on the frequency domain

WiFi networks adopt a random backoff mechanism to mediate channel contention. Although effective, backing off has been shown to reduce channel utilization. This is because backing off occurs in the time domain, forcing the channel to remain idle for multiple time slots. This poster proposes to break away from convention, and recreate the time domain operations in the frequency domain. Our basic idea is to pretend that OFDM subcarriers are integer numbers, and hence, counting down from a random backoff value may be equivalent to transmitting on a randomly chosen subcarrier. By looking at all active subcarriers and comparing with one's own chosen subcarrier, it is possible to select the winner of the contention. In fact, a logical ordering among nodes is feasible, enabling a train of scheduled transmissions from only one round of contention. We develop these ideas into a T2F protocol and prototype it on a small testbed of 5 USRPs. Early experimental results confirm its feasibility, along with promising throughput gains of up to 50%.

Modified Distributed Laxity Based Priority Scheduling Scheme

This paper proposes a medium access control (MAC) protocol for mobile Adhoc networks (MANET). The MAC layer is responsible for selecting the next packet to be transmitted and the timing of its transmission. Various new factors are introduced to determine the priority of the flow. The back-off mechanism devised by us grants node access to the channel based on the rank of its highest priority packets. It is clear that the proposed algorithm works better than the existing distributed laxity based priority-scheduling scheme and the same is proven using the simulation results using Glomosim.

Performance analysis of bandwidth requests under unicast, multicast and broadcast pollings in IEEE 802.16d/e

In the polling mode in IEEE 802.16d/e, one of three modes: unicast, multicast and broadcast pollings, is used to reserve bandwidth for data transmission. In the unicast polling, the BS polls each individual MS to allow to transmit a bandwidth request packet, while in the multicast and broadcast pollings, the truncated binary exponential backoff (TBEB) mechanism is adopted as a contention resolution among mobile stations (MSs) in a multicast or broadcast group. This paper investigates the delay of bandwidth requests in the unicast, multicast and broadcast pollings, by deriving the delay distribution of the unicast polling and the TBEB by means of analytical methods. We consider an error-free channel as well as an error-prone channel with i.i.d. constant packet error rate per frame. Furthermore, we find the utilization of transmission opportunity to see efficiency of the bandwidth in the TBEB. Performance evaluations are provided to show that analytical results are well-matched with simulations. By the numerical results, we can find the optimal parameters such as the initial backoff window size of the TBEB and the number of transmission opportunities (or slots) satisfying quality of service (QoS) requirement on delay and loss, and thus we can determine which scheme is better than others depending on the probability of a request arrival during one frame. Numerical examples address that the TBEB performs better than the unicast polling for light traffic loads and vice versa for heavy traffic loads. Also, it is shown that the multicast polling has better performance than the broadcast polling in the sense of shorter delay, lower loss probability and higher utilization of transmission opportunity.

Collision-Aware Backoff Mechanisms for IEEE 802.11 WLANs

The distributed coordination function (DCF) is the most widely deployed medium access control (MAC) protocol for IEEE 802.11 WLANs. It is a carrier sense multiple access with collision avoidance (CSMA/CA) scheme which employs a binary exponential backoff (BEB) algorithm to reduce the collision probability. In this paper we propose a novel backoff algorithm, collision-aware backoff mechanism (CABM), which dynamically selects a suitable contention window size according to the contention level of current network. A contention level is separately estimated by each station depending on unsuccessful frame transmission ratios. However, frame losses can result from not only collisions but also link errors. If the bit error rate (BER) of a channel is available, CABM can be enhanced by a more accurate estimation of collision probability. The enhanced variant, CABM+, is also proposed for improving the system performance in an error-prone channel. Simulation results show that our schemes outperform the standard protocol in terms of the collision probability and system throughput, especially when the number of contending stations is large.

A reservation based backoff method for video streaming in 802.11 home networks

QoS provisioning for video streaming over 802.11 home networks is challenging due to the tight bandwidth and delay constraints. Currently, both 802.11e HCCA and EDCA have their disadvantages when delivering video over 802.11 networks. EDCA exhibits degraded QoS performance for increased number of stations, while HCCA has high complexity, and also suffers from the impact of inter-cell interference. In this paper, we propose a new backoff mechanism which is fundamentally different from traditional random backoff methods in 802.11. The mechanism achieves resource reservation by reusing one or multiple time slots for transmission in consecutive backoff cycles. The basic idea is borrowed from R-ALOHA, but several modifications are made to allow it to work in the context of CSMA network. An additive increase multiplicative decrease based algorithm is proposed to control resource allocation for video streaming in the 802.11 network. Theoretical analysis and numerical simulations are conducted to validate the effectiveness of the method and the algorithm.