The high bandwidth demand of Internet applications has recently driven the need of increasing the residential download speed. A practical solution to the problem has been proposed aggregating the bandwidth of 802.11 Access Points (APs) backhauls in range via 802.11 connections. Since 802.11 devices are usually single-radio, the communication to multiple APs on different radio-channels requires the introduction of a time-division multiple access (TDMA) policy at the client station. Current investigation in this area supposes that there is a sufficient number of TCP flows to saturate the Asymmetric Digital Subscriber Line (ADSL) behind the APs. However, this may be not guaranteed according to the user traffic pattern. As a consequence, a TDMA policy introduces additional delays in the end-to-end transmissions that will cause degradation of the TCP throughput and an under-utilization of the AP backhauls. In this paper, we first perform an in-depth experimental analysis with a customized 802.11 driver of how the usage of multi-AP TDMA affects the observed Round-Trip-Time (RTT) of TCP flows. Then, we introduce a simple analytical model that accurately predicts the TCP RTT when accessing the wireless medium with a Multi-AP TDMA policy. Based on this model, we propose a resource allocation algorithm that runs locally at the station and it greatly reduces the observed TCP RTT with a very low computational cost. Our proposed scheme can improve up to 1:5 times the aggregate throughput observed by the station compared to state-of-the-art multi-AP TDMA allocations. We also show that the throughput performance of the algorithm is very close to the theoretical upper-bound in key simulation scenarios.
Although overlay solutions are easy to be implemented, they are often sub-optimal and difficult to scale because of the overlapping and duplication of similar functionalities at different layers (e.g. in the driver and in the card firmware). The VirtualWiFi project @cite_6 proposed an architecture that abstracts a single 802.11 WLAN card to appear as multiple virtual clients to the user. Each client instance adopts standard PHY MAC protocols, but it can be separately configured at the driver level. An interesting application was the idea of connecting to multiple APs through a single radio interface. The authors rely on the 802.11 Power Save (PS) mode feature to switch among different 802.11 WLAN nodes in a time-division fashion. A station can inform the current 802.11 WLAN node that it is going into PS mode --- so that it can buffer packets directed to it --- and switch the radio-frequency to other 802.11 WLAN nodes, only to come back to the original node before the PS period expires.
@cite_6
2097164742
We propose a powerful MAC PHY cross-layer approach to measuring IEEE 802.11 transmission opportunities in WLAN networks on a per-link basis. Our estimator can operate at a single station and it is able to: 1) classify losses caused by noise, collisions, and hidden nodes; and 2) distinguish between these losses and the unfairness caused by both exposed nodes and channel capture. Our estimator provides quantitative measures of the different causes of lost transmission opportunities, requiring only local measures at the 802.11 transmitter and no modification to the 802.11 protocol or in other stations. Our approach is suited to implementation on commodity hardware, and we demonstrate our prototype implementation via experimental assessments. We finally show how our estimator can help the WLAN station to improve its local performance. Since the inception of IEEE 802.11 wireless local area networks (WLANs) in 1997, wireless networking technologies have tremendously grown in the last few decades. The fundamental IEEE 802.11 physical (PHY) and medium access control (MAC) protocols have continuously been enriched with new technologies to provide the last mile wireless broadband connectivity to end users. Consequently, several new amendments of the basic IEEE 802.11 gradually came up in the forms of IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g. More recently, IEEE 802.11n, IEEE 802.11ac, and IEEE 802.11ad are introduced with enhanced PHY and MAC layers that boost up physical data rates to the order of Gigabit per second. So, these amendments are generally known as high throughput WLANs (HT-WLANs). In HT-WLANs, PHY layer is enhanced with multiple-input multiple-output antenna technologies, channel bonding, short guard intervals, enhanced modulation and coding schemes. The MAC sublayer overhead is reduced by introducing frame aggregation and block acknowledgement technologies. However, several existing studies reveal that, many a time, the aforesaid PHY and MAC enhancements yield negative impact on various upper layer protocols, that is end-to-end transport and application layer protocols. As a consequence, a large number of researchers have focused on improving the coordination among PHY MAC and upper layer protocols. In this survey, we discuss impact of HT-WLAN PHY and MAC layer enhancements on various transport and application layer protocols. This paper also summarizes several research works that use aforesaid enhancements effectively to boost up data rate of end-to-end protocols. We also point out limitations of the existing researches and list down different open challenges that can be meaningfully explored for the development of the next generation HT-WLAN technologies. There is a vast literature on the performance modeling of the 802.11 MAC protocol, but the interplay between the TCP dynamics and the self-regulating behavior of the 802.11 CSMA CA access method has not been sufficiently investigated. In addition, it has been observed that TCP stations in a WLAN are sporadically active due to the TCP flow control mechanisms. This makes traditional saturated models of the network capacity of 802.11 WLANs not very accurate. We presented a rigorous analytical model to calculate the distribution of the number of active TCP stations in a WLAN with multiple long-lived TCP flows [R. Bruno, M. Conti, E. Gregori, Performance modelling and measurements of TCP transfer throughput in 802.11-based WLAN, in: Proceedings of the IEEE MsWiM 2006, Torremolinos, Malaga, Spain, 2006, pp. 4-11, [1]]. Starting from this analysis, in this paper we derive a simple but accurate closed-form expression of the per-connection TCP throughout as a function of the average duration of collisions, the average backoff period and the TCP packet size. We validate this formula through performance tests carried out on a real WLAN. Then, we use our model to mathematically study the optimal minimum contention window size to maximize the TCP throughput. Our analytical results indicate that the initial window size specified in the 802.11b standard is about two times larger than the optimal one. However, we also show that the TCP throughput performance is not very sensitive to changes of the minimum contention window size, and that the TCP flows significantly underutilize the channel bandwidth also in the optimal MAC operating state. Recent advances in antenna and circuit design enable radios that operate in full duplex mode on a single channel with very low residual self-interference. In this paper, the use of such full duplex radios in a wireless local area network (WLAN) is explored. Different scenarios in which the full duplex transmission can be exploited are studied. A distributed full duplex MAC design based on IEEE 802.11 DCF that adopts to the traffic conditions is proposed. The proposed MAC design works for both ad hoc and infrastructure modes of WLAN and takes into consideration new interference and contention during full duplex transmissions. OPNET simulations comparing the performance of the proposed MAC with traditional half duplex based IEEE 802.11 DCF show that the new MAC protocol provides up to 88 throughput gain in a heavily loaded network. Due to its extreme simplicity and flexibility, the IEEE 802.11 standard is the dominant technology to implement both infrastructure-based WLANs and single-hop ad hoc networks. In spite of its popularity, there is a vast literature demonstrating the shortcomings of using the 802.11 technology in such environments, such as dramatic degradation of network capacity as contention increases and vulnerability to external interferences. Therefore, the design of enhancements and optimizations for the original 802.11 MAC protocol has been a very active research area in the last years. However, all these modifications to the 802.11 MAC protocol were validated only through simulations and or analytical investigations. In this paper, we present a very unique work as we have designed a flexible hardware software platform, fully compatible with current implementations of the IEEE 802.11 technology, which we have used to concretely implement and test an enhanced 802.11 backoff algorithm. Our experimental results clearly show that the enhanced mechanism outperforms the standard 802.11 MAC protocol in real scenarios. To satisfy the needs of wireless data networking, study group 802.11 was formed under IEEE project 802 to recommend an international standard for Wireless Local Area Networks (WLANs). A key part of standard are the Medium Access Control (MAC) protocol needed to support asynchronous and time bounded delivery of data frames. It has been proposed that unslotted Carrier Sense Multiple Access with Collision Avoidance (CSMA CA) be the basis for the IEEE 802.11 WLAN MAC protocols. We conduct performance evaluation of the asynchronous data transfer protocols that are a part of the proposed IEEE 802.11 standard taking into account the decentralized nature of communication between stations, the possibility of “capture”, and presence of “hidden” stations. We compute system throughput and evaluate fairness properties of the proposed MAC protocols. Further, the impact of spatial characteristics on the performance of the system and that observed by individual stations is determined. A comprehensive comparison of the access methods provided by the 802.11 MAC protocol is done and observations are made as to when each should be employed. Extensive numerical and simulation results are presented to help understand the issues involved. With the increasing development of IEEE 802.11 based wireless local area network (WLAN) devices, large-scale multi-cell WLANs with a high density of users and access points (APs) have emerged widely in various hotspots. Providing resilient data transmission has been a primary challenge for scaling the WLANs because the high density of users and APs results in too many collisions. In this paper, we analyze and point out the defect of the single association mechanism defined in IEEE 802.11 on transmission reliability from a network perspective. Then, we propose a "multi-AP" architecture with which a MAC layer device called an AP controller (AC) is employed to enable each user to associate and cooperate with multiple APs. In this way, the users can benefit from the diversity effect of multipaths with independent collisions and transmission errors. This paper concentrates on the theoretical analysis of performance comparison between the proposed ldquoMulti-APrdquo architecture and that in IEEE 802.11. Extensive simulation results show that the proposed ldquomulti-APrdquo architecture can obtain much better performance in terms of the throughput per user and the total throughput, and the performance gain is position dependent. Moreover, the unfairness issue in traditional WLANs due to capture effect can be alleviated properly in the ldquomulti-APrdquo framework. Quality-of-service (QoS) is a key problem of today's IP networks. Many frameworks (ImServ, DiffServ, MPLS etc.) have been proposed to provide service differentiation in the Internet. At the same time, the Internet is becoming more and more heterogeneous due to the recent explosion of wireless networks. In wireless environments, bandwidth is scarce and channel conditions are time-varying and sometimes highly lossy. Many previous research works show that what works well in a wired network cannot be directly applied in the wireless environment. Although IEEE 802.11 wireless LAN (WLAN) is the most widely used IEEE 802.11 wireless LAN (WLAN) standard today, it cannot provide QoS support for the increasing number of multimedia applications. Thus. a large number of 802.11 QoS enhancement schemes have been proposed, each one focusing on a particular mode. This paper summarizes all these schemes and presents a survey of current research activities. First, we analyze the QoS limitations of IEEE 802.11 wireless MAC layers. Then, different QoS enhancement techniques proposed for 802.11 WLAN are described and classified along with their advantages drawbacks. Finally, the upcoming IEEE 802.1 le QoS enhancement standard is introduced and studied in detail. 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. We propose a novel power saving strategy called Sleep during Neighbor-Addressed Frame (SNAF) for improving energy efficiency of IEEE 802.11 based wireless networks. IEEE 802.11 (Wi-Fi) radios that employ SNAF mode can turn OFF their wireless transceivers (i.e., put radios in sleep mode) within specific periods of neighbor-addressed frames while they are being received. The sleep duration of transceivers is easy to determine with no loss of packet's critical control information. The proposed SNAF mode operation does not have any negative effect on network throughput and even complements Power Saving Mode (PSM) available in 802.11 standard. We further propose GreenFrame format for next generation wireless networks. In experiments conducted in wireless LAN scenarios, we observed savings as much as 57.8 when we implement SNAF mode in 802.11 standard and up to 49.5 when we implement SNAF mode in 802.11 PSM. This paper analyzes the performance of a duty-cycled polling-based access mechanism that exploits the Transmission Opportunity Power Save Mode (TXOP PSM) defined in the IEEE 802.11ac to improve the energy efficiency of Wireless Local Area Networks (WLANs) based on the IEEE 802.11. The basic idea behind the proposed approach, named GreenPoll, is to enable contention free periods, based on polling with beacons, during which wireless stations can save energy by turning off their radio transceivers after exchanging data with the access point. The closed expression of energy efficiency of GreenPoll is formulated in this paper and is used to evaluate the performance of GreenPoll considering important parameters like the traffic load, packet length, data rate, and number of stations in the network. Both analytical and simulation results show the high energy efficiency of GreenPoll with gains of up to 330 and 110 when compared to the legacy Distributed Coordination Function (DCF) and the Point Coordination Function (PCF) defined in the IEEE 802.11, respectively. Many studies on measurement and characterization of wireless LANs (WLANs) have been performed recently. Most of these measurements have been conducted from the wired portion of the network based on wired monitoring (e.g. sniffer at some wired point) or SNMP statistics. More recently, wireless monitoring, the traffic measurement from a wireless vantage point, is also widely adopted in both wireless research and commercial WLAN management product development. Wireless monitoring technique can provide detailed PHY MAC information on wireless medium. For the network diagnosis purpose (e.g. anomaly detection and security monitoring) such detailed wireless information is more useful than the information provided by SNMP or wired monitoring. In this paper we have explored various issues in implementing the wireless monitoring system for an IEEE 802.11 based wireless network. We identify the pitfalls that such system needs to be aware of, and then provide feasible solutions to avoid those pitfalls. We implement an actual wireless monitoring system and demonstrate its effectiveness by characterizing a typical computer science department WLAN traffic. Our characterization reveals rich information about the PHY MAC layers of the IEEE 802.11 protocol such as the typical traffic mix of different frame types, their temporal characteristics and correlation with the user activities. Moreover, we identify various anomalies in protocol and security of the IEEE 802.11 MAC. Regarding the security, we identify malicious usages of WLAN, such as email worm and network scanning. Our results also show excessive retransmissions of some management frame types reducing the useful throughput of the wireless network. The Point Coordination Function (PCF) of the IEEE 802.11 standard represents a well-known Medium Access Control (MAC) protocol providing Quality-of Service guarantees in Wireless Local Area Networks (WLANs). However, with the currently employed polling mechanism WLANs consume a significant amount of the energy resources from battery-powered user devices. To provide energy saving, an improved MAC protocol is presented in this paper, where bidirectional transmissions of fixed duration are incorporated into PCF in order to enable dynamic scheduling of real-time traffic. Based on this new strategy, wireless access points (APs) can estimate the proper duration of the Contention Free Period (CFP), in order to allow mobile stations to acknowledge any received data packet with a data packet equal to the received packet in size. Having this information, a mobile station, following the data exchange with the AP, can determine its wake-up timer and activate the sleep mode for the rest of the CFP interval. Comprehensive computer-based simulations demonstrate the feasibility of the proposed MAC improvements to achieve energy efficiency with negligible impact on packet delivery delay. We consider the following two views of an IEEE 802.11 based Wireless LAN: (i) as seen by the WLAN medium and (ii) as seen by a user. In the system centric view, we model the WLAN medium as a server that allocates its resources to users in a round Robin manner. This resource sharing model not only provides a simple model for the system, it also enables us to derive the channel service rate and the total delay incurred in transmitting a packet. For Poisson arrivals, the resource sharing model takes the form of an M G 1 PS system with the mean delay being the same as that in an equivalent M M 1 system. We then take a user centric view and model each user's queue as a separate G G 1 queue. We derive the probability distributions for the different delay sources, i.e., random back-off time, random number of collisions and random number of successful transmissions from other users. This user centric model can provide insights into understanding access and queuing delays in 802.11 DCF. Finally, we discuss the utility of these models for functions such as capacity analysis, admission control and QoS enforcement. 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. In wireless LANs (WLANs), the medium access control (MAC) protocol is the main element that determines the efficiency in sharing the limited communication bandwidth of the wireless channel. In this paper we focus on the efficiency of the IEEE 802.11 standard for WLANs. Specifically, we analytically derive the average size of the contention window that maximizes the throughput, hereafter theoretical throughput limit, and we show that: 1) depending on the network configuration, the standard can operate very far from the theoretical throughput limit; and 2) an appropriate tuning of the backoff algorithm can drive the IEEE 802.11 protocol close to the theoretical throughput limit. Hence we propose a distributed algorithm that enables each station to tune its backoff algorithm at run-time. The performances of the IEEE 802.11 protocol, enhanced with our algorithm, are extensively investigated by simulation. Specifically, we investigate the sensitiveness of our algorithm to some network configuration parameters (number of active stations, presence of hidden terminals). Our results indicate that the capacity of the enhanced protocol is very close to the theoretical upper bound in all the configurations analyzed. Spatial multiple access holds the promise to boost the capacity of wireless networks when an access point has multiple antennas. Due to the asynchronous and uncontrolled nature of wireless LANs, conventional MIMO technology does not work efficiently when concurrent transmissions from multiple stations are uncoordinated. In this paper, we present the design and implementation of a crosslayer system, called SAM, that addresses the challenges of enabling spatial multiple access for multiple devices in a random access network like WLAN. SAM uses a chain-decoding technique to reliably recover the channel parameters for each device, and iteratively decode concurrent frames with misaligned symbol timings and frequency offsets. We propose a new MAC protocol, called CCMA, to enable concurrent transmissions by different mobile stations while remaining backward compatible with 802.11. Finally, we implement the PHY and MAC layer of SAM using the Sora high-performance software radio platform. Our evaluation results under real wireless conditions show that SAM can improve network uplink throughput by 70 with two antennas over 802.11. In WLAN the medium access control (MAC) protocol is the main element for determining the efficiency in sharing the limited communication bandwidth of the wireless channel. This paper focuses on the efficiency of the IEEE 802.11 standard for wireless LANs. Specifically, we derive an analytical formula for the protocol capacity. From this analysis we found (i) the theoretical upper bound of the IEEE 802.11 protocol capacity; (ii) that the standard can operate very far from the theoretical limits depending on the network configuration; (iii) that an appropriate tuning of the backoff algorithm can drive the IEEE 802.11 protocol close to its theoretical limits. Hence we propose a distributed algorithm which enables each station to tune its backoff algorithm at run-time. The performances of the IEEE 802.11 protocol, enhanced with our algorithm, are investigated via simulation. The results indicate that the enhanced protocol is very close to the maximum theoretical efficiency. Perpetually rising energy costs are forcing research communities to focus their efforts on a reduction of the global CO2 footprint, and since Information and Communication Technologies (ICT) account for a significant percentage of the overall global energy consumption, energy efficiency is becoming increasingly important in the operation of ICT infrastructure, especially in enterprise and data center networks. Simultaneously, the proliferation of devices such as smartphones and tablets, which have to use battery powered wireless radio adapters, and whose battery life is dependent on the power consumption of the radio adapters used for wireless communication indicates that efficient radio power saving strategies are needed to increase the battery life of such devices. This paper extends an earlier proposal by [1], and examines the performance of the same on IEEE 802.11n networks. It also extrapolates the results to estimate performance on 802.11ac WLANs, the planned successor to 802.11n WLAN in 5 GHz unlicensed band. The results indicate that this approach can be used to save over 59 of the RX energy (i.e., energy consumed by Wi-Fi radio while operating in receive mode) without any loss whatsoever in throughput unlike 802.11 power save mode and that this approach is completely backward compatible. For the battery-powered wireless stations, power saving is one of the significant issues in IEEE 802.11 wireless local area network (WLAN). Recently, Lei and Nilsson investigated the power save mode in IEEE 802.11 infrastructure mode by an M G 1 queue with bulk service. They obtained the average packet delay and lower and upper bounds of the average Percentage of Time a station stays in the Doze state (PTD). In this paper, the further investigation of power save mode is done; simple derivation of the average and the variance of packet delay and the exact value of PTD are obtained. Numerical results show that our analytic results for PTD match quite well with simulation results. Using our performance analysis, we can find the maximal listen interval which minimizes the power consumption of a station while satisfying the required quality of service (QoS) on the average and the variance of packet delay.