Evaluating transport protocol performance over a wireless mesh backbone

IEEE 802.11n wireless physical layer technology increases the deployment of high throughput wireless indoor mesh backbones for ubiquitous Internet connectivity at the urban and metropolitan areas. Most of the network traffic flows in today’s Internet use ‘Transmission Control Protocol’ (TCP) as the transport layer protocol. There has been extensive works that deal with TCP issues over wireless mesh networks as well as noisy wireless channels. Further, IEEE 802.11n is well known for its susceptibility to increased channel losses during high data rate communication. This paper investigates the dynamics of an end-to-end transport layer protocol like TCP in the presence of burst and correlated losses during IEEE 802.11n high data rate communication, while maintaining fairness among all the end-to-end flows. For this purpose, we evaluate four TCP variants-Loss Tolerant TCP (LT-TCP), Network Coded TCP (TCP/NC), TCP-Horizon and Wireless Control Protocol (WCP), where the first two protocols are known to perform very well in extreme lossy networks, and the last two are specifically designed for mesh networks. Our evaluation shows that WCP performs better in a IEEE 802.11n supported mesh networks compared to other three variants. However, WCP also results in negative impact at high data rates, where end-to-end goodput drops with the increase in physical data rate. The analysis of the results reveals that explicit loss notifications and flow balancing are not sufficient to improve transport protocol performance in an IEEE 802.11n supported mesh backbone, rather a specific mechanism is required to synchronize the transport queue management with lower layer scheduling that depends on IEEE 802.11n features, like channel bonding and frame aggregation. The findings of this paper give the direction to design a new transport protocol that can utilize the full capacity of IEEE 802.11n mesh backbone.     

Throughput analysis of IEEE 802.11 multirate WLANs with collision aware rate adaptation algorithm

This paper presents a mathematical model that analyzes the throughput of the IEEE 802.11b distributed coordination function (DCF) with the collision aware rate adaptation (CARA) algorithm. IEEE 802.11 WLANs provide multiple transmission rates to improve system throughput by adapting the transmission rate to the current channel conditions. The system throughput is determined by some stations using low transmission rates due to bad channel conditions. CARA algorithm does not disturb the existing IEEE 802.11b formats and it can be easily incorporated into the commercial wireless local area networks (WLAN) devices. Finally, we verify our findings with simulation.     

一种基于方向天线和时间同步机制的多信道无线网状网MAC协议

在城市小区和广阔的农村建立低成本的无线网状网,为无线终端用户提供高速的接入服务,是一项极具现实意义的工作.设计了一种适合于此类网络的MAC层协议称为Mesh-MAC,该协议建立在已有的IEEE 802.11硬件产品基础上,每两个节点之间的通信是由一对方向天线完成,通过一种全新的时间同步方案,可以实现M ESH网内高效的数据转发.仿真结果表明,与IEEE 802.11及2P协议相比,Mesh-MAC协议在吞吐量和端到端传输延时等方面,都有明显的改善.     

Design and Performance Evaluation of a Distributed Contention Control (DCC) Mechanism for IEEE 802.11 Wireless Local Area Networks

This paper focuses on the design and performance evaluation of a new mechanism, named Distributed Contention Control (DCC), for the adaptive contention reduction in LAN networks that utilize random access MAC protocols. The proposed mechanism could be executed on the top of a preexistent access protocol, with no additional overhead introduced. Specifically, we consider the IEEE 802.11 wireless LAN (WLAN). The DCC mechanism requires a simple and rough estimate of the contention level, and this can be achieved by estimating any parameter, directly connected with the amount of contention on the shared channel. The main characteristics of the proposed mechanism are represented by its simplicity, integration with the Standard, complete distribution, absence of overheads, and prompt reaction to changes in the network congestion. The protocol automatically adapts to the network congestion by monitoring the channel contention level through the estimation of the contention parameter. In this paper we show that the information needed for the contention estimation is already available to a 802.11 station, with no additional costs. Simulation experiments to evaluate the performance of an IEEE 802.11 WLAN, with and without the DCC mechanism, have been carried out. Results confirmed the effectiveness of the DCC mechanism in improving the performance, stability, and congestion reaction of the IEEE 802.11 access scheme. The DCC mechanism also provides a simple way to implement a distributed priority mechanism.     

A simple recursive scheme for adjusting the contention window size in IEEE 802.11e wireless ad hoc networks

The IEEE 802.11e standard has been introduced recently for providing Quality of Service (QoS) capabilities in the emerging wireless local area networks. This standard introduces a contention window based Enhanced Distributed Channel Access (EDCA) technique that provides a prioritized traffic to guarantee the minimum bandwidth needed for time critical applications. However, the EDCA technique resets the Contention Window (CW) of the mobile station statically after each successful transmission. This static behavior does not adapt to the network state since it reduces the network usage and results in bad performance and poor link utilization whenever the demand for link utilization increases. This paper proposes a new adaptive differentiation technique for IEEE 802.11e Wireless Local Area Networks that takes into account the network state before resetting the contention window. In the new technique, the congestion level of the network is sensed by using previous CW values. Three other enhancement techniques that focus on network adaptation are also discussed. Their main limitations are the high complexity of the implemented algorithms and their slow adaptation to the network state when the channel experiences bursty traffic. The proposed technique is compared to the original differentiation techniques of IEEE 802.11a and IEEE 802.11e standards, as well as to the enhancement schemes. Results show that the proposed adaptive technique outperforms IEEE 802.11e and is comparable to the other enhancement schemes while maintaining relatively low complexity requirements.     

基于变化率的802.11竞争窗口退避算法

在分析典型的退避算法基础上,提出一种改进的IEEE 802.11协议退避算法,引入结点碰撞频率的相对变化率和结点成功发送频率的相对变化率,以此衡量网络当前拥塞状况,根据上述2种变化率动态调整竞争窗口,降低信道接入的竞争。NS2仿真结果表明,该算法可以适应网络负载的变化,提高系统的吞吐量,降低丢包率和端到端时延。     

无线网络中MAC层违规行为的检测和惩罚

802.11无线媒体访问控制(MAC)协议利用分布式争夺解决机制处理无线信道的共享。在这种环境下,那些没有执行MAC协议的节点可以获得不公平的信道带宽。IEEE 802.11要求节点等待一个随机时间间隔之后再竞争访问信道,如果某些节点等待一个较小的时间间隔,那么对其他普通节点来说,这是不公平的。针对这种情况对IEEE 802.11 MAC协议做简单的改进来检测这样的违规行为并对其进行惩罚。     

Rate avalanche: Effects on the performance of multi-rate 802.11 wireless networks

Originally designed to deal with the hidden node problem, the Request-to-Send/Clear-to-Send (RTS/CTS) exchange is often turned off in most infrastructure-based 802.11 networks with the belief that the benefit it brings might not even be able to pay off its transmission overhead. While this is often true for networks using fixed transmission rate, our investigation leads to the opposite conclusion when multiple transmission rates are exploited in WLANs. Through extensive simulations using realistic channel propagation and reception models, we found out that in a heavily loaded multi-rate WLAN, a situation that we call rate avalanche often happens if RTS/CTS is turned off: high collision rates not only lead to retransmissions but also drive the nodes to switch to lower date rates; the retransmissions and the longer channel occupation caused by lower rates will further deteriorate the channel contention, which yields more collisions. This vicious circle could significantly degrade the network performance even no hidden node presents. Our investigation also reveals that, in the absence of effective and practical loss differentiation mechanisms, simply turning on the RTS/CTS could effectively suppress the rate avalanche effect. Various scenarios/conditions are extensively examined to study the impact of RTS/CTS on the network performance. Our study provides some important insights about using the RTS/CTS exchange in multi-rate 802.11 WLANs.     

OAR速率自适应机制的改进与实现

以最大化AdHoc网络吞吐量为目标,研究了基于IEEE802.11协议的物理层和MAC层信道接入机制和速率选择机制,分析了经典的速率自适应机制,但这些速率自适应机制绝大多数是针对数据帧提出的,而针对控制帧的速率自适应机制还非常少见。在研究基础上提出了一种控制帧速率自适应机制,详细阐述了实现过程,并对改进机制进行了仿真。仿真结果表明:通过控制帧速率自适应能有效地降低系统总开销,提高网络吞吐量。     

An interference avoidance MAC protocol design in mobile ad hoc networks

The multi-rate IEEE 802.11 DCF MAC protocol can transmit control signals at a basic transmission rate and data signals at various transmission rates. When the transmission rates of the control signals and the data signals differ, the transmission range of the lower transmission rate is larger than the transmission range of the higher transmission rate. Since a lower transmission rate increases the transmission range, it also increases the nodes in the Network Allocation Vector (NAV) status and decreases the network throughput. However, if a neighbor receiving the control signal of the ongoing communication pair communicates with another node, it may occur signal interference. This study proposes a Space Overlapping MAC (SO-MAC) protocol to increase the communication pairs and avoid interferences in single radio for multi-rate wireless network. The SO-MAC protocol uses a channel division mechanism to avoid interference between the data and control signals. This study also proposes a bandwidth allocation strategy for the sub-channels to maximize the utilization of the bandwidth of the divided sub-channels. To solve the interference between the data signals, SO-MAC allows a neighbor of the sender and the receiver to use the received signal strength to determine whether it can send or receive the data signal to increase the communication pairs. Simulation results show that, compared to the IEEE 802.11 DCF protocol and DCA protocol, the proposed SO-MAC protocol with the bandwidth allocation strategy can increase the communication pairs, achieve better throughput, reduce the number of handshake failures, and decrease the delay of transmitting a packet.     

Modeling multiple hop wireless networks with varying transmission power and data rate

Wireless multi-hop networks can vary both the transmission power and modulation of links. Those two parameters provide several design choices, which influence the performance of wireless multi-hop networks, e.g. minimize energy consumption, increase throughput, reduce contention, and maximize link quality. However, only network-wide metrics are considered in previous works. Further, per-flow performance metrics, such as the end-to-end energy consumption and latency, have not been studied. Those parameters directly impact the experience of users, which should be considered in capacity and performance studies. Our model incorporates per-flow metrics while also considering fading, contention, hidden terminals and packet error probabilities. We instantiate the model into an IEEE 802.11 multi-hop scenario, and evaluate common routing decisions such as maximizing link quality, maximizing data rate or minimizing the transmission power.     

Performance analysis of IEEE 802.11 WLANs with rate adaptation in time-varying fading channels

The IEEE 802.11 supports multiple transmission bit rates by using different modulation and coding schemes. Due to different bit error characteristics and transmission efficiencies of the rates, stations may benefit from an adaptive use of them for a varying channel condition, called rate adaptation. The accuracy of rate adaptation is expected to be highly affected by a time varying nature of typical radio channels due to multipath fading. This paper presents an analytic model of the IEEE distributed coordination function (DCF) with the automatic rate fallback (ARF) rate adaptation algorithm, which is the most widely used one in the 802.11 market, under time-correlated Rayleigh fading. The key idea behind the approach is to exploit the first-order Markovian approximation of Rayleigh fading channels, based on which transmission failure probabilities are obtained depending on the current and previous transmission status. By using those probabilities, the ARF process of a station is modeled as a Markov chain, then, the rate distribution obtained by solving the Markov chain is fed to a DCF model. The proposed DCF model is described in a per-station manner, thus enables the analysis of heterogeneous channel conditions and medium access control (MAC) configurations among stations.     

Runtime optimization of IEEE 802.11 wireless LANs performance

IEEE 802.11 is the standard for wireless local area networks (WLANs) promoted by the Institute of Electrical and Electronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is the most mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to its optimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a real case, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the message transmitted on the channel), but it, at most, can estimate it. We present and evaluate a distributed mechanism for contention control in IEEE 802.11 wireless LANs. Our mechanism, named asymptotically optimal backoff (AOB), dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimal channel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization and the average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additional hardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated through simulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.     

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.     

A new adaptive MAC protocol with QoS support based on IEEE 802.11 in ad hoc networks

With the expanding of applications, the demand of quality of service (QoS) has become strongly increased in ad hoc networks. Since the efficient and reasonable MAC protocol is a key factor for providing QoS in ad hoc networks, in this paper we propose an adaptive QoS MAC protocol (AMP) based on IEEE 802.11. In AMP, we introduce the concept of transmission license, where only the node which holds transmission license can participate in the channel contention for changing the number of licenses according to the load of the network adaptively, controlling the number of the nodes that participate in the channel contention, and ensuring the nodes with licenses share the channels through contention. In addition, AMP assigns different priority classes for different traffic according to the special characteristics and performance types of the different networks, and it sets the different contention parameters for the different priorities services for guaranteeing these services performances to have advantages in the channel contention. Simulation shows that compared to IEEE 802.11 protocol, AMP not only can meet the QoS requirement with high priority in the networks but also can well solve the hidden terminal problems and the fairness issues between different network nodes; that is, it can satisfy the high efficiency, pertinence, spatial-reuse, etc. to the largest extent at the same time in limited channels.     

Spectrum sharing in IEEE 802.11s wireless mesh networks

With current amendments, transmission rates of 100 Mb/s and more become possible with IEEE 802.11 WLANs. On the one hand, this allows the end user to change from wired to wireless infrastructure in even more application scenarios; on the other hand interference sensitive modes reduce the maximum range between the mobile station and the access point (AP). To extend the transmission range transparently, relay APs form a mesh network and provide wireless connection over large areas.Besides path selection, a crucial capability of a wireless mesh network is the ability to share the available spectrum among the participants. In this work, we classify two inherently different MAC protocols according to this ability. The well-known IEEE 802.11 DCF takes the position of a typical CSMA/CA protocol, whereas the Mesh Network Alliance (MNA) represents a distributed, reservation-based approach.To assess their performance, we follow a dual approach: first we develop a method to compute the capacity bounds of the protocols in the considered scenarios. It helps to estimate the absolute gain of spectrum sharing in wireless mesh networks. Second, the WARP2 simulation engine is used to compare the distributed behaviour of both protocols. This results in a relative evaluation. A final conclusion is drawn by combining the simulation and the theoretical results. It underlines the significant possibilities of the MNA approach and shows future directions for capacity gains.     

一种基于IEEE802.11的无线自组网MAC协议

基于IEEE802.11DCF提出了一种应用于无线自组网的媒质接入控制协议，该协议包括网络适应性退避算法和拥塞反馈两个关键机制。协议的主要思想是根据节点周围实际竞争状况和网络拥塞情况进行包调度。仿真结果表明，提出的机制有效地降低了平均端到端时延和数据丢包率,提高了信道接入公平性。     

An energy-efficient,transport-controlled MAC protocol for wireless sensor networks

In wireless sensor networks (WSNs), one major cause of wasted energy is that the wireless network interface is always on to accept possible traffic. Many medium access control (MAC) protocols therefore adopted a periodic listen-and-sleep scheme to save energy, at sacrifice of end-to-end latency and throughput. Another cause is packet dropping due to network congestion, necessitating a lightweight transport protocol for WSNs. In this paper, we suggest a transport-controlled MAC protocol (TC-MAC) that combines the transport protocol into the MAC protocol with the aims of achieving high performance as well as energy efficiency in multi-hop forwarding. Although TC-MAC also works through a periodic listen-and-sleep scheme, it lowers end-to-end latency by reserving data forwarding schedules across multi-hop nodes during the listen period and by forwarding data during the sleep period, all while increasing throughput by piggybacking the subsequent data forwarding schedule on current data transmissions and forwarding data consecutively. In addition, TC-MAC gives a fairness-aware lightweight transport control mechanism based on benefits of using the MAC-layer information. The results show that TC-MAC performs as well as an 802.11-like MAC in end-to-end latency and throughput, and is more efficient than S-MAC in energy consumption, with the additional advantage of supporting fairness-aware congestion control.     

Analyzing the effective throughput in multi-hop IEEE 802.11n networks

In this paper we characterize the effective throughput for multi-hop paths in IEEE 802.11n based wireless mesh networks. We derive an analytical model capturing the effects of frame aggregation and block acknowledgements, features found in the new IEEE 802.11n standard. We describe the throughput at MAC layer as a function of physical data rate, error rate, aggregation level and path length. While being mathematically tractable, the proposed model is flexible enough to account for complex and realistic error characteristics of the wireless channel, such as long-term fluctuations and burstiness. We further show how to integrate the well-known Gilbert-Elliot channel model into our model and evaluate both models in our indoor wireless testbed.     

On the rate adaptation for IEEE 802.11 wireless networks

Rate adaptation (RA) is a mechanism to choose transmission rate based on the dynamic channel quality in wireless networks. This paper studied the adaptation algorithm run solely at the sender-side in IEEE 802.11 networks. The key insight is the inference discrepancy in inferring the relative order of transmission rates with respect to the expected performance, which indicates that one cannot always reach the correct order based solely on the channel state information collected by the sender itself. The consequence is wrong rate decision and significant performance loss. Therefore, we present a new RA structure to mitigate such effect by using a novel component, rate testing. Further, by employing the active measurement, a lightweight and effective testing mechanism, SFB, short frame burst, is proposed to detect and filter out the unsuitable transmission rate. Finally, an active measurement-based rate adaptation mechanism (AMRA) is designed and implemented. The experimental results show that AMRA outperforms many other well-known RA solutions in most scenarios.