Towards providing optimal energy‐efficiency and throughput for IEEE 802.11 WLANs

IEEE 802.11 wireless network standard has become one of the most used wireless networking technologies for smart devices as it offers mobility support and low cost deployment. However, these devices deeply rely on the energy provided by their batteries, which results in limited running time. IEEE 802.11 network standard provides stations with carrier sense multiple access with collision avoidance for the medium access. Yet it results in stations to consume an important amount of power. Therefore, minimizing WiFi‐based energy consumption in smart devices has been received substantial attention in both academia and industry. Accordingly, this paper * proposes a novel beacon‐based energy‐efficient collision‐free medium access control protocol for any type of IEEE 802.11 stations, regardless of being stationary or mobile, or having different amount of traffic flow, transmission rates, or traffic types. The proposed scheme is valid for all types of low or wide bandwidth, single or multiuser multiple‐input multiple‐output WLAN channels, such as IEEE 802.11a\b\g\n\ac. In the proposed scheme, energy saving is achieved, enabling stations to transmit on the right time and maintaining stations in the doze state during a predetermined sleep_time interval after each successful frame transmission, by making use of modified control and management frames of the standard IEEE 802.11 protocol. The proposed scheme reduces the probability of collisions and may allow stations to enter the collision‐free state, regardless of the number of stations on the channel and their traffic types. Widespread simulations have been executed to validate the efficiency of the proposed method. The results demonstrate that the proposed method significantly increases overall throughput and reduces power consumption of stations over IEEE 802.11 WLANs.     

DeepSleep: IEEE 802.11 enhancement for energy-harvesting machine-to-machine communications

As future Machine-to-Machine (M2M) communications aim at supporting wireless networks with large coverage range and a huge number of devices without human intervention, energy-efficient protocol design for M2M communications networks becomes notably significant. The emerging energy harvesting technology, allowing devices to harvest energy from external sources automatically without human intervention, is promisingly applied to M2M communications networks, which can therefore operate permanently. However, currently available IEEE 802.11 protocols do not consider supporting energy-harvesting devices efficiently. Our research focuses effort in enhancing IEEE 802.11 power saving mode (PSM) with widely-deployed numerous devices powered by energy-harvesting modules so as to realize an energy-efficient M2M communications network. We propose DeepSleep with the aim of improving energy-efficiency and reducing the overall outage probability, application layer loss rate and collision probability. The effectiveness of DeepSleep is demonstrated by NS-2 platform. An analytical model is provided to select DeepSleep parameters. Applying DeepSleep, an energy-harvesting device can have less energy wastage on idle listening and overhearing, and have a higher channel access priority when waking up from a relatively longer period of sleeping. In addition, the channel access fairness is considered in DeepSleep design. In addition, all devices benefit when DeepSleep and 802.11 PSM co-exist in the network, which implies DeepSleep has potential to be deployed in existing WLANs.     

A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs

The finite battery power of mobile computers represents one of the greatest limitations to the utility of portable computers. Furthermore, portable computers often need to perform power consuming activities, such as transmitting and receiving data by means of a random-access, wireless channel. The amount of power consumed to transfer the data on the wireless channel is negatively affected by the channel congestion level, and significantly depends on the MAC protocol adopted. This paper illustrates the design and the performance evaluation of a new mechanism that, by controlling the accesses to the shared transmission channel of a wireless LAN, leads each station to an optimal Power Consumption level. Specifically, we considered the Standard IEEE 802.11 Distributed Coordination Function (DCF) access scheme for WLANs. For this protocol we analytically derived the optimal average Power Consumption levels required for a frame transmission. By exploiting these analytical results, we define a Power Save, Distributed Contention Control (PS-DCC) mechanism that can be adopted to enhance the performance of the Standard IEEE 802.11 DCF protocol from a power saving standpoint. The performance of an IEEE 802.11 network enhanced with the PS-DCC mechanism has been investigated by simulation. Results show that the enhanced protocol closely approximates the optimal power consumption level, and provides a channel utilization close to the theoretical upper bound for the IEEE 802.11 protocol capacity. In addition, even in low load situations, the enhanced protocol does not introduce additional overheads with respect to the standard protocol.     

Relay-volunteered multi-rate cooperative MAC protocol for IEEE 802.11 WLANs

In IEEE 802.11, the rate of a station (STA) is dynamically determined by link adaptation. Low-rate STAs tend to hog more channel time than high-rate STAs due to fair characteristics of carrier sense multiple access/collision avoidance, leading to overall throughput degradation. It can be improved by limiting the transmission opportunities of low-rate STAs by backoff parameters. This, however, may cause unfair transmission opportunities to low-rate STAs. In an attempt to increase overall throughput by volunteer high-rate relay STAs while maintaining fairness, we propose a new cooperative medium access control (MAC) protocol, relay-volunteered multi-rate cooperative MAC (RM-CMAC) based on ready to send/clear to send in multi-rate IEEE 802.11. In the RM-CMAC protocol, we show that the effect of hogging channel time by low-rate STAs can be remedied by controlling the initial backoff window size of low-rate STAs and the reduced transmission opportunity of low-rate STAs can be compensated by the help of volunteer high-rate relay STAs. We analyze the performance of RM-CMAC, i.e., throughput and MAC delay, by a multi-rate embedded Markov chain model. We demonstrate that our analysis is accurate and the RM-CMAC protocol enhances the network throughput and MAC delay while maintaining the fairness of low-rate STAs.     

Cooperative Relay Service in a Wireless LAN

As a family of wireless local area network (WLAN) protocols between physical layer and higher layer protocols, IEEE 802.11 has to accommodate the features and requirements of both ends. However, current practice has addressed the problems of these two layers separately and is far from satisfactory. On one end, due to varying channel conditions, WLANs have to provide multiple physical channel rates to support various signal qualities. A low channel rate station not only suffers low throughput, but also significantly degrades the throughput of other stations. On the other end, the power saving mechanism of 802.11 is ineffective in TCP-based communications, in which the wireless network interface (WNI) has to stay awake to quickly acknowledge senders, and hence, the energy is wasted on channel listening during idle awake time. In this paper, considering the needs of both ends, we utilize the idle communication power of the WNI to provide a Cooperative Relay Service (CRS) for WLANs with multiple channel rates. We characterize energy efficiency as energy per bit, instead of energy per second. In CRS, a high channel rate station relays data frames as a proxy between its neighboring stations with low channel rates and the Access Point, improving their throughput and energy efficiency. Different from traditional relaying approaches, CRS compensates a proxy for the energy consumed in data forwarding. The proxy obtains additional channel access time from its clients, leading to the increase of its own throughput without compromising its energy efficiency. Extensive experiments are conducted through a prototype implementation and ns-2 simulations to evaluate our proposed CRS. The experimental results show that CRS achieves significant performance improvements for both low and high channel rate stations     

WLAN中基于IEEE802.11u协议的终端节能功耗管理方案

支持IEEE 802.11u标准的移动终端在无线局域网提供的网络服务覆盖范围内可自动接入无线局域网。针对IEEE 802.11u无线局域网的特点,提出了一种基于时间序列感知的PSM（S-PSM）方案。采用IEEE 802.11u标准中定义的公共广播服务（GAS）来广播传输序列信息,所有终端根据该序列信息相应地控制自身的工作状态。为了降低竞争信道的碰撞概率,引入了响应竞争窗口。当接入点（AP）中没有数据分组时,AP 广播GAS帧并激活空闲计时器。所有终端进入睡眠状态直到空闲计时器到期。仿真结果表明,与传统的PSM相比,该方案能显著降低功耗,提升终端能效。     

Admission control in IEEE 802.11e wireless LANs

Although IEEE 802.11 based wireless local area networks have become more and more popular due to low cost and easy deployment, they can only provide best effort services and do not have quality of service supports for multimedia applications. Recently, a new standard, IEEE 802.11e, has been proposed, which introduces a so-called hybrid coordination function containing two medium access mechanisms: contention-based channel access and controlled channel access. In this article we first give a brief tutorial on the various MAC-layer QoS mechanisms provided by 802.11e. We show that the 802.11e standard provides a very powerful platform for QoS supports in WLANs. Then we provide an extensive survey of recent advances in admission control algorithms/protocols in IEEE 802.11e WLANs. Our survey covers the research work in admission control for both EDCA and HCCA. We show that the new MAC-layer QoS schemes and parameters provided in EDCA and HCCA can be well utilized to fulfill the requirements of admission control so that QoS for multimedia applications can be provided in WLANs. Last, we give a summary of the design of admission control in EDCA and HCCA, and point out the remaining challenges.     

Saving Energy during Channel Contention in 802.11 WLANs

We focus on energy saving in 802.11-based WLANs. Previous work has shown that, on the one hand, 802.11 wireless interfaces consume a significant amount of energy, on the other hand the use of current power management schemes can severely degrade the QoS performance of several Internet-based applications. Furthermore, the energy spent by wireless devices may even increase when the standard 802.11 power-saving mode (PSM) is implemented. These facts suggest that other solutions to energy saving are highly needed. In this paper, we consider the 802.11 distributed access scheme and we propose a novel approach that enables a station to enter a low-power operational state during channel contention. More specifically, our technique exploits the virtual carrier sense mechanism and the backoff function specified in the IEEE 802.11 DCF, so that a station can dramatically reduce its energy consumption without significant degradation of the QoS performance. To efficiently implement our mechanism, a low-power state with negligible transition time into the active state must be identified. This can be any of the non-standard, low-power states defined by proprietary solutions in the current or next-generation products [7,15,22]. By using the network simulator ns2, we evaluate the performance improvement that is obtained when the proposed mechanism is implemented, against the results attained through the standard DCF. The results show that we can achieve a reduction in energy consumption as large as 80% and 28% under, respectively, UDP and TCP traffic. This work was supported by the Italian Ministry of University and Research through the PATTERN and the PRIMO projects. Valeria Baiamonte graduated from Politecnico di Torino with a degree in Telecommunications Engineering in 2002, with 110/110. Between March 2002 and November 2002, she was supported by TiLab (Italian Public Telephone Research Company) while developing her thesis work on packet scheduling algorithms for WCDMA systems networks. From January 2003 to December 2003, she was a CNIT researcher at the Electronics Department of Politecnico working on the VICOM project. Currently she is a PhD student at the Electronics Department of Politecnico di Torino. Carla-Fabiana Chiasserini graduated with a summa cum laude degree in Electrical Engineering from the University of Florence in 1996. She did her graduate work at the Politecnico di Torino, Italy, receiving the Ph.D. degree in 1999. Since then she has been with the department of Electrical Engineering at Politecnico di Torino, where she is currently an assistant professor. Since 1999, she has worked as a visiting researcher at the University of California, San Diego, California. Her research interests include architectures, protocols and performance analysis of wireless networks for integrated multi-media services. She is a member of the editorial board of the Ad Hoc Networks Journal (Elsevier), and has served as an associate editor of the IEEE Communications Letters since 2004.     

Power saving access points for IEEE 802-11 wireless network infrastructure

In the past decade, there has been a huge proliferation of wireless local area networks (WLANs) based on the IEEE 802.11 WLAN standard. As 802.11 connectivity becomes more ubiquitous, multihop communications will be increasingly used for access point range extension and coverage enhancement. In this paper, we present a design for an IEEE 802. 11 -based power saving access point (PSAP), intended for use in multihop battery and solar/battery powered applications. These types of APs have many practical applications and can be deployed very quickly and inexpensively to provide coverage enhancement in situations such as campuses, building complexes, and fast deployment scenarios. Unlike conventional wired access points, in this type of system, power saving on the AP itself is an important objective. A key design constraint is that the proposed PSAP be backward compatible to a wide range of IEEE 802.11 functionality and existing wired access points. In this paper, we introduce the protocols required to achieve this compatibility, show the constraints imposed by this restriction, and present performance results for the proposed system.     

Study on bandwidth control for multiple transmission rates in IEEE 802.11e EDCAF

IEEE 802.11e supports the guaranteed quality of service (QoS) by providing different transmission priorities. IEEE 802.11e improves the media access control layer of IEEE 802.11 to satisfy the different QoS requirements by introducing two channel access functions: the enhanced distributed channel access (EDCA) and the hybrid coordination function (HCF) controlled channel access (HCCA). Signal quality may affect the available bandwidth and transmission rate, because the characteristic of communication channel in wireless environment is in random time‐variation manner. Generally a station using a lower transmission rate will occupy communication channel for a longer time and degrade system performance, which causes unfairness and cannot provide the guaranteed QoS for the stations with higher transmission rates. We propose a bandwidth control scheme (BCS) by combining the IEEE 802.11e enhanced distributed channel access function (EDCAF) protocol to overcome the guaranteed bandwidth issue in multirate environments. A multirate discrete Markov chain model is analyzed for the multirate transmission system in this paper. According to the obtained results, BCS improves performance especially in throughput and makes the different QoS requirements be processed efficiently and flexibly. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance and fairness enhancement in IEEE 802.11 WLAN networks

The multi-rate transmission mechanism in IEEE 802.11 can improve its reliability and robustness. However, it causes a performance anomaly. After analyzing the reasons for the performance anomaly in multi-rate mechanism, we propose a new scheme to solve the performance anomaly. By adjusting packet size according to the transmission rate, this scheme guarantees that these nodes with different transmit rates can access wireless channel fairly. Theoretical analysis and performance evaluation show that the proposed scheme can well solve the performance anomaly problem.     

一种基于IEEE802.11EDCF的自适应能量调整算法

简述了IEEE802.11及IEEE802.11e的两种访问控制机制DCF和EDCF,提出了一种基于IEEE802.11 EDCF的自适应能量调整算法,并在NS2上进行了仿真实现.仿真实验结果表明,该算法能够根据分组的优先权水平和节点当前能量值动态调度信道接入,延长了全网的生存期.     

Two-step multipolling MAC protocol for wireless LANs

The IEEE 802.11 standard defines two coordination functions: distributed coordination function (DCF) and point coordination function (PCF). These coordination functions coordinate the shared wireless medium. The PCF uses a centralized polling-based channel access method to support time-bounded services. To design an efficient polling scheme, the point coordinator (PC) needs to obtain information about the current transmission status and channel condition for each station. To reduce overhead caused by polling frames, it is better to poll all stations using one polling frame containing the transmission schedule. In this paper, we propose an efficient polling scheme, referred to as two-step multipolling (TS-MP), for the PCF in wireless local area networks (WLANs). In this new scheme, we propose to use two multipolling frames with different purposes. The first frame is broadcast to collect information such as the numbers of pending frames and the physical-layer transmission rates for the communication links among all stations. The second frame contains a polling sequence for data transmissions designed based on the collected information. This frame is broadcast to all stations. Extensive simulation studies show that TS-MP not only overcomes the aforementioned deficiencies, but also help to implement rate adaptation over time-varying wireless channel.     

Applying Opportunistic Medium Access and Multiuser MIMO Techniques in Multi-channel Multi-radio WLANs

Opportunistic medium access (i.e., multiuser diversity) and MIMO techniques (i.e., multiple-antenna techniques) are two effective ways to achieve a substantial throughput gain in a multiuser wireless system. In this paper, we propose a medium access control (MAC) protocol with opportunistic medium access and multiuser MIMO techniques (MAC-OMA/MM) in Multi-channel Multi-radio Wireless Local Area Networks (WLANs) to explore the utility of the joint design of these two techniques for the challenging MAC design. Specifically, in addition to utilizing multiple channels simultaneously and assigning multiple radio transceivers dynamically, multiuser MIMO technique is applied on each frequency channel to enhance the data rates. The key ideas of MAC-OMA/MM can be summarized as follows. By utilizing ATIM (Ad-hoc Traffic Indication Message) windows as in IEEE 802.11 power saving mechanism (PSM), user selection and channel negotiation are conducted between the access point (AP) and multiple users via ATIM messages on a common channel. Multiuser diversity are employed to opportunistically schedule among multiple candidate users to optimize data transmission in each negotiation. During data exchange, on each frequency channel, the AP is capable to communicate with two distinct users simultaneously both for the downlink and uplink data transmissions with the help of multiuser MIMO technique, which creates an extra dimension in spatial domain to further leverage the effect of multiuser diversity and multi-channel gains. Another contribution of this paper is to provide an analytical model to characterize the impact of our protocol on the system throughput and energy efficiency performance. Extensive simulations have been conducted and the results demonstrate that our protocol outperforms existing multi-channel MAC protocols with only minimal additional overhead and minor enhancements to the IEEE 802.11 PSM.

基于IEEE802.11协议的WLAN节省能耗的策略

在WLAN中移动终端设备的电池寿命是一个关键问题.文章概述了基于IEEE802.11协议的WLAN节省能耗的策略,并通过对IEEE802.11MAC协议层节能机制的分析提出一种改进的轮询方案.该方案能克服IEEE802.11在PSM工作模式下,当无线网络流量负载较重时不能显著降低能耗的缺点.     

Analysis of the integration of IEEE 802.11e capabilities in battery limited mobile devices

IEEE 802.11 wireless LANs (WLANs) is widely recognized as a complementary technology to cellular access networks in hot-spot areas due to its lower cost and higher data rates. Different interworking approaches are being studied, but one common element is considered by all of them: mobile devices can include capabilities for connecting through both access technologies, allowing the best option to be chosen depending on the availability at a specific moment. However, several challenges need to be addressed in order to achieve seamless integration of WLAN and cellular systems in such a mobile device. The IEEE 802.11e standard, which defines mechanisms to provide QoS in a WLAN, represents a basic step toward this integration, since it provides the means to support key applications such as voice over IP. The IEEE 802.11 power-save mode (PSM) is another necessary element for devices with severe battery limitations (e.g., cellular phones) in order to ensure reasonable battery duration. The resulting performance when both QoS and power-saving mechanisms are used together, however, is uncertain and requires further study. We analyze the implications of the interaction of the 802.11 PSM with 802.11e QoS mechanisms by determining if the desired QoS is still provided, detecting functionality conflicts, and quantifying the impact of the PSM upon the 802.11e QoS efficiency and system performance. The evaluation is performed via simulation.     

A Novel Scheduled Power Saving Mechanism for 802.11 Wireless LANs

Power conservation is a general concern for mobile computing and communication. In this paper, we investigate the performance of the current 802.11 power saving mechanism (unscheduled PSM) and identify that background network traffic can have a significant impact on the power consumption of mobile stations. To improve power efficiency, a novel scheduled PSM protocol based on time slicing is proposed in this paper. The protocol adopts the mechanism of time division, schedules the access point to deliver pending data at designated time slices, and adaptively adjusts the power state of the mobile stations. The proposed scheme is near theoretical optimal for power saving. It greatly reduces the effect of background traffic, minimizes the station idle time, and maximizes its energy utilization. Comprehensive analysis and simulations are conducted to evaluate the new protocol. The results show that the new protocol provides significant energy saving over the unscheduled PSM, particularly in circumstances where multiple traffic streams coexist in a network. Moreover, it achieves the saving at the cost of only a slight degradation of the one-way-delay performance.     

A novel contention-based MAC protocol with channel reservation for wireless LANs

In this paper, we present a novel contention-based medium access control (MAC) protocol, namely, the Channel Reservation MAC (CR-MAC) protocol. The CR-MAC protocol takes advantage of the overhearing feature of the shared wireless channel to exchange channel reservation information with little extra overhead. Each node can reserve the channel for the next packet waiting in the transmission queue during the current transmission. We theoretically prove that the CR-MAC protocol achieves much higher throughput than the IEEE 802.11 RTS/CTS mode under saturated traffic. The protocol also reduces packet collision, thereby saving the energy for retransmission. We evaluate the protocol by simulations under both saturated traffic and unsaturated traffic. Our simulation results not only validate the theoretical analysis on saturated throughput, but also reveal other good features of the protocol. For example, under saturated traffic, both the saturated throughput and fairness measures of the CR-MAC are very close to the theoretical upper bounds. Moreover, under unsaturated traffic, the protocol also achieves higher throughput and better fairness than IEEE 802.11 RTS/CTS.     

A study on the influence of transmission errors on WLAN IEEE 802.11 MAC performance

Since the advent of the first IEEE 802.11 standard for WLANs, several papers have been presented that evaluate the IEEE 802.11 DCF access method. In realistic WLAN environments frame errors usually occur due to non‐ideal channel conditions; in this way, papers including adverse transmission conditions in the evaluation have been published later in the literature. In this paper, we review existent analytical models that include the influence of transmission errors in IEEE 802.11 DCF performance. We modify current models and provide a more accurate analysis, thus allowing the evaluation in single rate and multi‐rate scenarios with stations subject to different link error conditions. Moreover, this paper exposes the unfairness problem that arises in IEEE 802.11 DCF networks with stations subject to different transmission conditions through analytical and simulation results. Stations are not able to distinguish collisions from failed transmissions due to link errors; both result in a missing ACK and, consequently, the transmitting stations apply the exponential backoff algorithm. This fact leads to a lower performance for stations in worse transmission conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Energy Efficient Schedulers in Wireless Networks: Design and Optimization

Minimizing energy consumption is crucial for portable wireless stations because they operate on a limited battery supply. For example, the IEEE 802.11 standard includes a mechanism called power-saving mode (PSM), which allows a network interface on a mobile station to enter a sleep state whenever possible to reduce its energy consumption. We consider a generic wireless system composed of an access point (AP) and several stations that offer a PSM to its users. Our PSM is AP-centric (i.e., gives control to the AP) to save more energy. We formulate a downlink scheduling optimization problem aimed at saving energy and propose two heuristic scheduling policies to solve it. One of these policies is non-work-conserving, and it offers an interesting tradeoff between energy consumption and user performance. We also study and show how the length of the Beacon Period (BP) has a significant impact on the energy and the delay performance of wireless stations. For each of our two scheduling policies, we derive simple approximate formulas for the length of the BP that minimizes the energy consumption and for the relationship between the delay performance and the length of the BP. Assuming the maximum allowable average packet delay is given by the users as a QoS requirement, we illustrate how to dimension the length of the BP for the two schedulers we have proposed and show that we can achieve significant energy savings while meeting the delay constraint with the non-work conserving one in many cases. Extensive simulations show that a fine-tuning of the length of the BP as well as well-designed scheduling disciplines is essential to saving energy in wireless stations.