A Distributed Fair Scheduling Scheme With a New Analysis Model in IEEE 802.11 Wireless LANs (original) (raw)
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IEEE 802.11 is a widely used standard for MAC and PHY layers of WLANs. Unfortunately, the access methods offered in this standard cannot support QoS (Quality of Service) for real-time traffics. Using multimedia applications over WLANs is increasing and, on the other hand, it seems that the access methods employed in this standard causes high variations in delay or jitter and wastes bandwidth due to collisions. There are many methods to enable DCFbasic access method in 802.11-with service differentiation and QoS. The difficulty in majority of these methods is unfair bandwidth allocation among low and high priority traffics at high loads resulting starvation for low priority traffics. In this paper, we modify the way that the CW (Contention Window) size is calculated after a successful transmission and study the effect of the CW size on performance and fairness. Results of our simulations show that the performance of DCF with this modification is better, specially, for traffics in which throughput is the most important parameter.
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In future wireless networks, different traffic classes will exhibit a large variety of characteristics and QoS requirements, such as transmission rate, maximum tolerable bit error rate and timeout specifications. However, currently there is no standard way of guaranteeing QoS in wireless access networks like Wireless LAN based on IEEE 802.11. In this paper, we propose medium access control protocol enhancements and a distributed scheduler for QoS guarantees and fairness over IEEE 802.11 WLAN. The proposed scheme uses distributed scheduling at both the access point and the user terminals to schedule the transmission of packets according to their delay requirements. The algorithms used for both scheduling are the same. A flexible and fair resource allocation method among the traffic classes and the user terminals is provided by this scheme. Its performance has been evaluated using UCB/LBNL/VINT Network Simulator and an implementation in the Linux kernel.
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The IEEE 802.11e Medium Access Control (MAC) is an emerging standard to support Quality of Service (QoS). Some recent works prove that 802.11e Hybrid Coordination Function (HCF) can improve significantly the QoS support in 802.11 networks. A simple HCF scheduler has been proposed in the 802.11e which takes the QoS requirements of flows into account and allocates time to stations on the basis of the mean sending rate. As we show in this paper, this HCF scheduling algorithm is only efficient for flows with strict Constant Bit Rate (CBR) characteristics. However, a lot of real-time applications, such as videoconferencing, have small variations in their packet sizes, sending rates or even have Variable Bit Rate (VBR) characteristics. In this paper, we propose a new HCF scheduling algorithm, FHCF, that aims to be fair for both CBR and VBR flows. The FHCF scheme uses queue length estimations to tune its time allocation to stations. We present a set of simulations and provide performance comparisons with other schemes. Our performance study indicates that FHCF provides good fairness while supporting bandwidth and delay requirements for a large range of network loads.
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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. This paper presents and evaluates a distributed mechanism for the 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 in the paper via simulation. Simulation results indicate that our mechanism is very effective, robust and has traffic differentiation potentialities. Keywords: Wireless LAN (WLAN), IEEE 802.11, multiple access protocol (MAC), protocol capacity, performance analysis 2 access at a competitive price. A major factor in achieving this goal is the availability of appropriate networking standards. IEEE Std 802.11 defines a Medium Access Control (MAC) and Physical Layer (PHY) specification for a Wireless Local Area (WLAN) network to provide wireless connectivity for fixed, portable, and moving stations within a local area [IEE97].