Exponential Backoff Mechanism Research Articles

Cross-Layer Scheme to Control Contention Window for Per-Flow in Asymmetric Multi-Hop Networks

The IEEE 802.11 MAC standard for wireless ad hoc networks adopts Binary Exponential Back-off (BEB) mechanism to resolve bandwidth contention between stations. BEB mechanism controls the bandwidth allocation for each station by choosing a back-off value from one to CW according to the uniform random distribution, where CW is the contention window size. However, in asymmetric multi-hop networks, some stations are disadvantaged in opportunity of access to the shared channel and may suffer severe throughput degradation when the traffic load is large. Then, the network performance is degraded in terms of throughput and fairness. In this paper, we propose a new cross-layer scheme aiming to solve the per-flow unfairness problem and achieve good throughput performance in IEEE 802.11 multi-hop ad hoc networks. Our cross-layer scheme collects useful information from the physical, MAC and link layers of own station. This information is used to determine the optimal Contention Window (CW) size for per-station fairness. We also use this information to adjust CW size for each flow in the station in order to achieve per-flow fairness. Performance of our cross-layer scheme is examined on various asymmetric multi-hop network topologies by using Network Simulator (NS-2).

Enhancing Performance of Bluetooth Piconets Using Priority Scheduling and Exponential Back-Off Mechanism

Bluetooth is a personal wireless communication technology and is being applied in many scenarios. It is an emerging standard for short range, low cost, low power wireless access technology. Current existing MAC (Medium Access Control) scheduling schemes only provide best-effort service for all masterslave connections. It is very challenging to provide QoS (Quality of Service) support for different connections due to the feature of Master Driven TDD (Time Division Duplex). However, there is no solution available to support both delay and bandwidth guarantees required by real time applications. This paper addresses the issue of how to enhance QoS support in a Bluetooth piconet. The Bluetooth specification proposes a Round Robin scheduler as possible solution for scheduling the transmissions in a Bluetooth Piconet. We propose an algorithm which will reduce the bandwidth waste and enhance the efficiency of network. We define token counters to estimate traffic of real-time slaves. To increase bandwidth utilization, a back-off mechanism is then presented for best-effort slaves to decrease the frequency of polling idle slaves. Simulation results demonstrate that our scheme achieves better performance over the Round Robin scheduling. Keywords—Piconet, Medium Access Control, Polling algorithm, Scheduling, QoS, Time Division Duplex (TDD).

Performance evaluation of wireless sensor network with hybrid channel access mechanism

In this paper, a hybrid channel access mechanism is proposed for the wireless sensor network that considers the channel access procedure of IEEE 802.15.4 and combines the binary exponential backoff mechanism of IEEE 802.11 due to packet collision of nodes after successful channel assessment. Taking the backlogged nodes due to collision, an extended linear feedback model is developed and a discrete-time Markov chain model is designed to analyze the successful and failure probabilities of the system model of the wireless sensor network. Besides, an energy consumption model for the one hop wireless sensor network is developed based on our models and hybrid channel access mechanism. Extensive performance analysis are done to study the effect of binary exponential contention window on energy consumption of the nodes and it is verified that our simulation results totally match with the theoretical results for different size of contention windows and node numbers.

Delay distribution and loss probability of bandwidth requests under truncated binary exponential backoff mechanism in IEEE 802.16e over Gilbert-Elliot error channel

This paper presents the mathematical analysis of the truncated binary exponential backoff (TBEB) mechanism as a contentionresolution for bandwidth requests in the broadcast polling and themulticast polling in the IEEE 802.16e. We derive the delaydistribution and the loss probability of request packets in the TBEBover Gilbert-Elliot error channel, by analytic methods on theassumption of Bernoulli arrival process and the unsaturatedcondition. The optimal contention period of transmissionopportunities for transmitting bandwidth requests can be obtainedwhile satisfying QoS on delay bound and loss bound. Furthermore, wefind the utilization of transmission opportunity to see efficiencyof the bandwidth. Numerical examples show that the analyticalresults are well-matched with simulations, and the performanceevaluations in the broadcast polling and multicast polling arecompared on the mean delay, the loss probability and the utilizationof transmission opportunity. Numerical results address that themulticast polling with more groups has better performance than thebroadcast polling in the sense of shorter delay, lower lossprobability and higher utilization of transmission opportunity.

Analysis of the IEEE 802.11 Back-Off Mechanism in Presence of Hidden Nodes

The binary exponential back-off mechanism is one of the basic elements that constitute the IEEE 802.11 protocol. The models of the back-off mechanism have been developed with the assumption that collisions occur only due to nodes within the carrier sensing range and the collision probability is constant in steady-state. However, the transmission collisions can occur due to hidden nodes and these tend to occur consecutively, contrary to the collisions due to nodes within the carrier sensing range. Consecutive collisions increase the back-off time exponentially, resulting in less frequent transmission attempts. Ignoring this collision characteristic in modeling the back-off mechanism can produce large errors in the performance analysis of networks. In this paper, we model the back-off process as a Markov renewal process by taking into account such consecutive collisions due to hidden nodes, and then compare this result with NS2 simulation results. According to the simulation results, the proposed model reduces the relative error in the attempt probability by more than 90% in the grid topology. We also propose a new collision model for a simple network considering consecutive collisions due to hidden nodes, and analyze the network under saturated traffic condition using the proposed models. The attempt and collision probabilities are estimated with high accuracy.

Performance Analysis Based on Packet Arrival Rate for the IEEE 802.11 DCF

A performance analytic model based on packet arrival rate is proposed for IEEE 802.11 DCF (distributed coordination function)in infrastructure-based WLAN(wireless local area network).The model does not only take into account the factors of number of terminals,traffic loads,the binary exponential back-off mechanism, but also analyzes the impact of the finite queue system at the MAC(medium access control)layer.Each terminal is modeled as an M/M/1/K queue,and the virtual time slot is introduced to discretize the state of the queue system. Based on this,a three dimensional discrete time Markov chain is used to model the system.By using the model, normalized throughput,packet delay and packet losing ratio are obtained.Simulation results show that the proposed model can validly predict the performance of the DCF under various packet arrival rates.

Removing exponential backoff from TCP

The well-accepted wisdom is that TCP's exponential backoff mechanism, introduced by Jacobson 20 years ago, is essential for preserving the stability of the Internet. In this paper, we show that removing exponential backoff from TCP altogether can be done without inducing any stability side-effects. We introduce the implicit packet conservation principle and show that as long as the endpoints uphold this principle, they can only improve their end-to-end performance relative to the exponential backoff case. By conducting large-scale simulations, modeling, and network experiments in Emulab and the Internet using a kernel-level FreeBSD TCP implementation, realistic traffic distributions, and complex network topologies, we demonstrate that TCP's binary exponential backoff mechanism can be safely removed. Moreover, we show that insuitability of TCP's exponential backoff is fundamental, i.e., independent from the currently-dominant Internet traffic properties or bottleneck capacities. Surprisingly, our results indicate that a path to incrementally deploying the change does exist.

Effectiveness of Explicit Stations Grouping in Crowded Wireless LANs

In this letter, we evaluate the effectiveness of a multi-stage contention scheme for wireless local area networks (WLANs) medium access control (MAC). Multi-stage contention schemes basically divide the stations into smaller groups to resolve the contention more efficiently. Previous researchers have proposed virtual grouping schemes for WLANs MAC. Here we quantitatively analyze what can be achieved with a simple grouping scheme, i.e. through multi-stage contention. Our analysis shows that the multi-stage scheme is efficient in resolving contention, making it a good alternative to the commonly used exponential backoff mechanism.

MAC Access Delay of IEEE 802.11 DCF

The MAC access delay in a saturated IEEE 802.11 DCF wireless LAN is analyzed. We develop a unified analytical model and obtain explicit expressions for the first two moments as well as the generating function. We show via comparison with simulation that our model accurately predicts the mean, standard deviation, and distribution of the access delay for a wide range of operating conditions. In addition, we show that the obtained generating function is much more accurate than others that have appeared in the literature. Using our model, we prove that the binary exponential backoff mechanism induces a heavy-tailed delay distribution for the case of unlimited retransmissions. We show using numerical examples that the distribution has a truncated power-law tail when a retransmission limit exists. This finding suggests that DCF is prone to long delays and not suited to carrying delay-sensitive applications

Performance Analysis of IEEE 802.11 DCF in Imperfect Channels

IEEE 802.11 is the most important standard for wireless local area networks (WLANs). In IEEE 802.11, the fundamental medium access control (MAC) scheme is the distributed coordination function (DCF). To understand the performance of WLANs, it is important to analyze IEEE 802.11 DCF. Recently, several analytical models have been proposed to evaluate the performance of DCF under different incoming traffic conditions. However, to the best of the authors' knowledge, there is no accurate model that takes into account both the incoming traffic loads and the effect of imperfect wireless channels, in which unsuccessful packet delivery may occur due to bit transmission errors. In this paper, the authors address this issue and provide an analytical model to evaluate the performance of DCF in imperfect wireless channels. The authors consider the impact of different factors together, including the binary exponential backoff mechanism in DCF, various incoming traffic loads, distribution of incoming packet size, queueing system at the MAC layer, and the imperfect wireless channels, which has never been done before. Extensive simulation and analysis results show that the proposed analytical model can accurately predict the delay and throughput performance of IEEE 802.11 DCF under different channel and traffic conditions

The IEEE 802.11 Distributed Coordination Function in Small-Scale Ad-Hoc Wireless LANs

The IEEE 802.11 standards for wireless local area networks define how the stations of an ad-hoc wireless network coordinate in order to share the medium efficiently. This work investigates the performance of such a network by considering the two different access mechanisms proposed in these standards. The IEEE 802.11 access mechanisms are based on the carrier sense multiple access with collision avoidance (CSMA/CA) protocol using a binary slotted exponential backoff mechanism. The basic CSMA/CA mechanism uses an acknowledgment message at the end of each transmitted packet, whereas the request to send/clear to send (RTS/CTS) CSMA/CA mechanism also uses a RTS/CTS message exchange before transmitting a packet. In this work, we analyze these two access mechanisms in terms of throughput and delay. Extensive numerical results are presented to highlight the characteristics of each access mechanism and to define the dependence of each mechanism on the backoff procedure parameters.

CSMA/CA performance under high traffic conditions: throughput and delay analysis

The IEEE802.11 standard for wireless local area networks is based on the CSMA/CA protocol for supporting asynchronous data transfers. CSMA/CA uses an acknowledgment mechanism for verifying successful transmissions and optionally, a handshaking mechanism for decreasing collisions overhead. In both cases, an exponential backoff mechanism is used. This work investigates the theoretical performance of both mechanisms in terms of throughput and delay under traffic conditions that correspond to the maximum load that the network can support in stable conditions. We present extensive numerical results in order to highlight the effect of the backoff mechanism parameters on network performance for both mechanisms.