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ISBN 978-952-5726-09-1 (Print) Proceedings of the Second International Symposium on Networking and Network Security (ISNNS ’10) Jinggangshan, P. R. China, 2-4, April. 2010, pp.218-222 An Adaptive Scheduling for QoS Enhancement Mechanism in IEEE 802.11 WLAN Aiyun Zhan, Aihan Yin, Yueli Jiao, and Qingmiao Zhang School of Information Engineering, East China Jiaotong University Nanchang, China 330013 E-mail: yinaihan@126.com Abstract-For the problem of QoS in the IEEE802.11DCF, proposed an enhanced adaptive scheduling distributed EASDCF mechanism. Through the largest contention window and retry times, this mechanism provides differentiated services and guarantees the real-time service priority access to the channel with higher probability. Besides, it provides a reliable guarantee for the packet drop rate sensitive service. Simulation results indicate that the classification method adopted by the EASDCF has a good effect, enhancing the overall throughput performance of the WLAN and making reasonable use of system resources. Index Terms-largest contention window; QoS; distributed scheduling mechanism; throughput; packet drop rate I. INTRODUCTION Wireless communication technology has brought us more and more convenient communication services. However, with the development of technology, the system performances could not meet the demand of requirements for the increasing services from us. As is well-known, the wireless channel is a limited resource, it is a problem needed to be resolved in MAC (Medium Access Control) layer that how to share and distribute the channel fairly, effectively and reliably. CSMA/CA (Carrier Sense Multiple Access with Collision Back-off) mechanism, as the elementary access way in IEEE802.11 protocol [1] , adapts to the change of station numbers and the communication traffic. Due to CSMA/CA lack of effective QoS (Quality of Service) mechanism, makes it hard to support the services with high requirements of delay performance [2] . A basic application of CSMA/CA is the IEEE802.11DCF (Distributed Coordination Function) mode. Based on DCF model, the paper analyzes the traditional back-off mechanisms of 802.11 protocol and compares EDCF (Enhanced DCF) with frequently used DCF, then put forward a performanceenhanced mode---EASDCF (Enhanced Adaptive SDCF). In this way, we execute the network simulations under NS-2, whose results have certain guiding significance in the transmission of real time services, like voice in the wireless local area network. II. DCF AND EDCF ACCESS MODE DCF is the basic media access control mechanism of 802.11, it adopts CSMA/CA and binary exponential back-off mechanism. DCF access mode is shown in figure 1. In the IEEE802.11 model, each station accesses channels through competition [3] . The initial size of © 2010 ACADEMY PUBLISHER AP-PROC-CS-10CN006 218 contention window is CWmin. When the transmission fails, it doubles its size and chooses another random back-off time in the new window. Otherwise, the contention window returns to minimum size CWmin directly. Checking again whether there are still data to be transmitted in the station, if so, after a period of DIFS (DCF Interframe Space), reenter the back-off waiting state again until it gets the authorization for media access [4] . The way which DCF adopted to allocate channels is simple and effective. Particularly when collisions increase, it can enlarge CW (Contention Window) promptly. The larger the contention window is, the more powerful the system is to solve collisions. Figure 1 Basic DCF access model EDCF model is the improvement of MAC protocol proposed by IEEE802.11e work group, and the program introduced the Enhanced Distributed Coordination Function [5] . The support of QoS in the EDCF is realized by the introduction of TC (Traffic Categories).The priority of channel access is differentiated by the QoS parameters of TC. After detecting the channel idle time reaches an AIFS (Arbitration InterFrame Space), the mobile station starts back-off. The relationship between various IFS which defined by EDCF is shown in figure 2. AIFS is at least a DIFS time. The back-off counter range is [0, CW-1]. Figure 2 EDCF access model
EDCF in comparison with DCF, has some important differences, when detecting that the channel is busy in AIFS period, the back-off counter reduce 1 at the last time slot of AIFS. However in the DCF, the back-off counter reduces 1 at the first time slot after DIFS. When the transmission fails, EDCF makes use of the PF (Persistent Factor) to control the selection of CW, CWnew [TC]=PF×CWold [TC]. However in the DCF, after the failing transmission, the CW always doubles the size, that is PF = 2. In summary, EDCF provides different access priorities by modifying three control parameters of DCF competitive mechanism: 1) InterFrame Space; 2) Minimum contention window; 3) Persistent Factor. III. PERFORMANCE ENHANCEMENT SCHEME EASDCF A. Improved back-off mechanism Figure 3Improved back-off mechanism The improved back-off mechanism as shown in figure 3, we set a variable whose stage is stage_k in our scheme, to some extent, the stage_k could reflect the system state. While the stage number is high, it is indicated that the number of stations is many and the congestion of the network is serious. While in a lower stage shows a better network condition. Because the stage of each station is different before a successful transmission, so we can use the different stages of stations to control different backoff methods. Different schemes can be chosen through controlling the stage_k. With fewer stations, adopting the DCF backoff mechanism because of it’s simple and easy to achieve. If there are many stations, we adopt the Slow CW Decrease scheme [6] . For the successful transmission, don’t return to the CWmin to select back-off counter, otherwise results in unfairness and collision problems. Setting a threshold value K, when stage_k K. If previous back-off stage is big enough, stage_k>K, it adopts stage_k-g back-off window before the next transmission (g is a constant, the value is 1 or 2), the window descends after each successful transmission. When stage_k CWjmin j>i Each grade has the same size of maximal back-off stage, that is: mi= mj=m=5 , i, j∈[0,M] According to the above principles, the minimal contention window of each grade is divided in table 1 as following: Table 1: The sizes of contention window of each grade Level 1 2 3 4 5 6 7 8 102 204 CWmin 32 64 128 256 512 4096 4 8 102 204 409 819 163 327 655 1310 CWmax 4 8 6 2 84 68 36 72 Besides, packet drop rate also take into consideration, we divide the frame into N grades. The relationship of maximal retry limit among each grade is: mq> mp , q>p , p, q∈[0,N].The division method is as following in table 2: Table 2: Maximal retry limit of each grade leveli 1 2 3 4 5 6 7 8 Shot frame 4 6 8 10 12 14 16 18 Long frame 7 9 11 13 15 17 19 21 Delay and packet drop rate belong to two aspects of evaluation network QoS performance. Through the combination of them, from Table 1 and Table 2, we know that EASDCF obtain 64 different QoS classification services in all. IV. SIMULATION AUTHENTICATION NS2 is a simulation tool for optimizing real network traffic transmission by establishing network devices and links [10] . In the process of simulation, adopting right scale to evaluate the QoS of the WLAN is a key point. It 219
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Proceedings The Second Internationa
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Table of Contents Message from the
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Zuming Xiao, Zhan Guo, Bin Tan, and
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Message from the Symposium Chairs T
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Second International Symposium on N
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model that can deal with time serie
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training for the Wushu competition
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information, called weak uncertain
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Student side Student side Student s
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If we define element of student as
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QoS, each frame data is divided int
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for Imaging Two and Three Phase Flo
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Research and Realization about Conv
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special services. SOAP is used to b
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detection methods of DDoS attacks m
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Ⅳ. EVALUATION OF BLENDED LEARNING
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Liping Chen .......................
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