Multi-rate Wireless LANs Research Articles

Stochastic approximation based on-line algorithm for fairness in multi-rate wireless LANs

It is well known that IEEE 802.11 based MAC provides max---min fairness to all nodes even in a multi-rate WLAN. However, the max---min fairness may not always be the preferred fairness criteria as it significantly reduces overall system throughput. In this paper, we explore the proportional fairness and the time fairness. First, we obtain a condition that must be satisfied by the attempt probabilities to achieve proportional fairness. Using this condition, we propose a stochastic approximation based on-line algorithm that tunes attempt probabilities to achieve proportional fairness. The proposed algorithm can be implemented in a distributed fashion, and can provide optimal performance even when node uses a rate adaptation scheme. Next, we show that the time fairness is a special case of weighted max---min fairness with the weight for a node is equal to its transmission rate. Thus, the existing algorithms to achieve weighted max---min fairness can be used to achieve time fairness as well. This exposition also demonstrates that the proportional fairness and the time fairness are not the same contrary to what was conjectured. Performance comparison of various fairness criteria is done through ns-3 simulations. Simulation results show that time fair schemes achieve the highest throughput, and the sum of logarithm of individual node's throughputs under the time fairness is close to that under a proportionally fair scheme.

RA-PSM: a rate-aware power saving mechanism in multi-rate wireless LANs

Improving energy efficiency is one of the most important issues in wireless local area networks (WLANs) and a power saving mechanism (PSM) has been proposed in IEEE 802.11 WLANs. However, the conventional PSMs do not consider multi-rate transmissions and thus the waiting time for retrieving buffered frames from the access point can be unnecessarily long and more energy can be consumed. In this paper, we propose a rate-aware power saving mechanism (RA-PSM) in multi-rate WLANs. In RA-PSM, a station sets its contention window size inversely proportional to the transmission rate and thus a station with higher transmission rate can request the buffered frames at the access point with higher priority. As a result, the overall channel waiting time can be reduced and the energy consumption can be saved accordingly. Analytical and simulation results demonstrate that RA-PSM can reduce the average channel waiting time and the energy consumption by up to 77 and 40.3 %, respectively, compared with the conventional IEEE 802.11 PSM.

Running Multiple Instances of the Distributed Coordination Function for Air-Time Fairness in Multi-Rate WLANs

Conventional multi-rate IEEE 802.11 Wireless LANs (WLANs) are associated with the so-called performance anomaly to describe the phenomenon of high bit rate nodes being dragged down by slower nodes. This anomaly is known to be an impediment to obtaining high cumulative throughputs despite the employment of effective link adaptation mechanisms. To cope with the performance anomaly, air-time fairness has been proposed as an alternative to throughput fairness, the latter being a main characteristic of the IEEE 802.11 Distributed Coordination Function (DCF). In this paper, we propose a novel distributed air-time fair MAC (Medium Access Control) without having to change the operation of the conventional DCF. In the proposed MAC, each node in the system runs multiple instances of the conventional DCF back-off algorithm where the number of DCF instances for the nodes can be chosen in a distributed manner. Both analytical and simulation-based results are provided to validate the effectiveness of the proposed air-time fair MAC.

An Estimated Delay Based Association Policy for Web Browsing in a Multirate WLAN

Station-Access Point (STA-AP) association is an important function in IEEE 802.11 Wireless LAN (WLAN) management. We obtain an association policy that can be implemented in centralized WLAN management devices, or in STAs, by taking into account explicitly two aspects of practical importance: (a) TCP-controlled short file downloads interspersed with read times (motivated by web browsing), and (b) different STAs associated with an Access Point (AP) at possibly different rates (depending on distance from the AP). Our approach is based on two steps. First, we consider an analytical model to obtain the aggregate AP throughput for long TCP-controlled file downloads when STAs are associated at k different rates r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , ..., r <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ; this extends earlier work in the literature. Second, we present a 2-node closed queueing network model to approximate the expected average-sized file download time for a user who shares the AP with other users associated at a multiplicity of rates. These analytical results motivate the proposed association policy, called the Estimated Delay based Association (EDA) policy: Associate with the AP at which the expected file download time is the least. Simulations indicate that for a web-browsing type traffic scenario, EDA performs substantially better than other policies that have been proposed earlier. Crucially, the improved performance is sustained even in realistic evaluation scenarios, where the assumptions underpinning the analytical model do not hold. To the best of our knowledge, this is the first work that proposes an association policy tailored specifically for web browsing. Apart from this, our analytical results could be of independent interest.

Modelling and Empirical Analysis of IEEE 802.11b Distributed Coordination Function (DCF) In a Multi-Rate Wireless LAN

Modelling and Empirical Analysis of IEEE 802.11b Distributed Coordination Function (DCF) In a Multi-Rate Wireless LAN

BTAC: A Busy Tone Based Cooperative MAC Protocol for Wireless Local Area Networks

Spatial diversity is an effective technique to mitigate the negative effects of fading and depends on deployment of antenna array on small mobile unit. This is infeasible due to the small size of the mobile node. In order to overcome this limitation, a new concept of diversity that has emerged called cooperative diversity is realized. In this technique, different nodes in a wireless network are allowed to share their resources and cooperate through distributed transmission, forming multiple transmission paths to the destination. In this paper, we introduce a cooperative MAC-protocol, called BTAC, for improving throughput in multi-rate wireless LANs. BTAC is compatible with IEEE wireless LANs. Our analytical and simulation results show that the proposed protocol can improve the throughput performance by at least 35% and substantially reduce system delay, comparing IEEE 802.11b MAC protocol. In addition this protocol is robust and fair.

Queuing Analysis of HCCA for Multi-Rate Wireless LANs with Truncated ARQ Protocol

In this paper, a novel queuing analytical framework is introduced to analyze the performance of the HCF controlled channel access designed as a medium access mechanism in wireless local area networks. The queuing model defines a multi-rate medium using adaptive modulation and coding which is a key point in wireless networks to increase effective transmission rate. Based on a MAP (Markovian arrival process)/PH (phase type)/1 queue with vacation and time-limited service, the model considers a limited node buffer and non-ideal channel with the automatic repeat request protocol. Using a MAP for traffic arrival process and PH distribution for packet service process, the inclusion of the non-transmission and vacation processes makes our analysis very general and comprehensive to support various types of practical traffic streams. To formulate the model, the powerful matrix-geometric method is employed, and the mutual impacts of some parameters in the MAC and the PHY layers are then evaluated to improve system performance. Finally, optimization of parameter settings to maximize throughput is discussed.

ARSM: a cross-layer auto rate selection multicast mechanism for multi-rate wireless LANs

Multicast is an efficient paradigm for transmitting data from a sender to a group of receivers. According to the IEEE 802.11 standard, the multicast service is defined as an unreliable service, that is, it does not include the use of ACK frames. Furthermore, different to the unicast service, the multicast service makes use of a single rate out of the various rates included in the basic service set defined by the IEEE 802.11 standard. Even though various proposals have recently appeared in the literature addressing these issues, none of them has come out with a structured set of control mechanisms taking into account the varying conditions characterising the wireless channels as well as the requirements of various applications. A novel cross-layer auto rate selection multicast mechanism for multi-rate wireless LANs, namely auto rate selection for multicast, capable of adapting the data transmission to the varying conditions of the channel and taking into account the characteristics of various applications, is introduced. The simulation results show that our proposal outperforms the IEEE 802.11 standard and the mechanisms recently proposed in the literature.

Cross-layer architecture for adaptive video multicast streaming over multirate wireless LANs

Multicast video streaming over multirate wireless LANs imposes strong demands on video codecs and the underlying network. It is not sufficient that only the video codec or only the underlying protocols adapt to changes in the wireless link quality. Research efforts should be applied in both and in a synchronized way. Cross layer design is a new paradigm that addresses this challenge by optimizing communication network architectures across traditional layer boundaries. This paper presents cross-layer architecture for adaptive video multicast streaming over multirate wireless LANs where layer-specific information is passed in both directions, top-down and bottom-up. The authors jointly consider three layers of the protocol stack: the application, data link and physical layers. The authors analyze the performance of the proposed architecture and extensively evaluate it via simulations. The results show that the real-time video quality of the overall system can be greatly improved by cross-layer signaling.

Contention-Based Airtime Usage Control in Multirate IEEE 802.11 Wireless LANs

In a multirate wireless LAN, wireless/mobile stations usually adapt their transmission rates to the channel condition. It is difficult to control each station's usage of network resources since the shared channel can be overused by low transmission-rate stations. To solve this problem, we propose a distributed control of stations' airtime usage which 1) always guarantees each station to receive a specified share of airtime, and 2) keeps service for individual stations unaffected by other stations' transmission rates. Such airtime control enables service differentiation or quality of service (QoS) support. Moreover, it can achieve a higher overall system throughput. The proposed airtime usage control exploits the Enhanced Distributed Channel Access (EDCA) of the IEEE 802.11e standard . Two control mechanisms are proposed: one based on controlling the station's arbitration inter-frame space (AIFS) and the other based on the contention window size. We show how the stations' airtime usage is related to the AIFS and contention window size parameters. Using this relation, two analytical models are developed to determine the optimal control parameters. Unlike the other heuristic controls or analytical models, our model provides handles or parameters for quantitative control of stations' airtime usage. Our evaluation results show that a precise airtime usage control can be achieved in a multirate wireless LAN

Temporal fairness guarantee in multi-rate wireless LANs for per-flow protection

Wireless LAN technologies such as IEEE 802.11a and 802.11b support high bandwidth and multi-rate data transmission to match the channel condition (i.e., signal to noise ratio). While some wireless packet fair queuing algorithms to achieve the per-flow throughput fairness have been proposed, they are not appropriate for guaranteeing QoS in multi-rate wireless LAN environments. We propose a wireless packet scheduling algorithm that uses the multistate (multi-rate) wireless channel model and performs packet scheduling by taking into account the channel usage time of each flow. The proposed algorithm aims at per-flow protection by providing equal channel usage time for each flow. To achieve the per-flow protection, we propose a temporally fair scheduling algorithm called Contention-Aware Temporally fair Scheduling (CATS) which provides equal channel usage time for each flow. Channel usage time is defined as the sum of the packet transmission time and the contention overhead time due to the CSMA/CA mechanism. The CATS algorithm provides per-flow protection in wireless LAN environments where the channel qualities of mobile stations are dynamic over time, and where the packet sizes are application-dependent. We also extend CATS to Decentralized-CATS (D-CATS) to provide per-flow protection in the uplink transmission. Using an NS-2 simulation, we evaluate the fairness property of both CATS and D-CATS in various scenarios. Simulation results show that the throughput of mobile stations with stable link conditions is not degraded by the mobility (or link instability) of other stations or by packet size variations. D-CATS also shows less delay and less delay jitter than FIFO. In addition, since D-CATS can coordinate the number of contending mobile stations, the overall throughput is not degraded as the number of mobile stations increases.

New frame-based network allocation vector for 802.11b multirate wireless LANs

To enhance the system capacity of the IEEE 802.11 wireless local area networks (WLANs), a frame-based adaptive multirate transmission scheme is envisaged. The medium access control (MAC) operation and the selection of parameters, such as the thresholds of request to send (RTS) and fragmentation, are discussed. The original virtual carrier sense mechanism in the IEEE 802.11 WLAN standard is no longer suitable because the transmission rate is adaptively selected such that it is prone to allocate a wrong network allocation vector (NAV) to jam or inhibit the radio transmission that degrades the performance. A modification of the NAV reservation scheme must be made to adopt this change. The authors have performed simulations to illustrate the performance improvement to the modification of the NAV reservation scheme. Simulations are based on a time-correlated multipath fading channel model and a movable mobile station assumption, where the station may be out of range from certain mobile stations.