Performance Enhancement of EDCA Access Mechanism of IEEE ...

Performance Enhancement of EDCA Access Mechanism of IEEE 802.11e Wireless LAN Danda B. Rawat, Dimitrie C. Popescu, and Min Song Dept. of Electrical and Computer Engineering, Old Dominion University, Norfolk, VA 23529, USA Email: {drawa001, dpopescu, msong}@odu.edu Abstract — IEEE 802.11e has a new coordination function, which has both contention-based and contentionfree medium access methods, in order to support applications with Quality of Service (QoS) requirements. The contentionbased MAC protocol, Enhanced Distributed Channel Access (EDCA) supports QoS; however, it cannot guarantee strict QoS required by real-time applications without proper tuning of QoS parameters and network control mechanisms. In this paper we adjust QoS parameters trading off between call admission control and rate control to enhance the EDCA access mechanism of IEEE 802.11e wireless LAN. Performance of the proposed scheme is analyzed using an OPNET modeler 12.0. Index Terms — IEEE 802.11e, EDCA, QoS.

The organization of the paper is as follows. In Section II we briefly describe overview of EDCA access mechanism. In Section III we describe CAC and RC schemes, and then in IV we propose new scheme making tradeoff between CAC and RC scheme. The performance of the proposed scheme is analyzed using an OPNET Modeler 12.0 [5] in V. Finally; we conclude the paper in section VI. II. OVERVIEW OF IEEE 802.11E AND PROBLEM STATEMENT In IEEE 802.11e, the hybrid coordination function (HCF), combines aspects of both the contention-based access method EDCA and the contention free access mechanism HCF Control Channel Access (HCCA). In EDCA, QoS support is provided by the introduction of access categories (AC) with their independent backoff entities. According to [1] QoS station (QSTA) implements four access categories. For each AC, there is a set of EDCA parameters associated with it. Those parameter include arbitration interframe space AIFS[AC], contention window (CW) with its minimum and maximum value CWmin[AC] and CWmax[AC] respectively. The set of parameters is announced by the access point (AP) via beacon frame and can be modified over time. Each AC from every station competes with others ACs so as to gain a transmission opportunity (TXOP) and therefore it starts independently a backoff timer after detecting that the channel is idle for an AIFS[AC] interval. The backoff period of each AC is chosen according to a uniform distribution over [0, CW[AC]]. The CW size is initially assigned CWmin value and doubles when transmission fails up to the CWmax. Basically the smaller the CWmin[AC] the shorter the channel access delay for the corresponding priority and hence the better chance to access the media for a given traffic condition. When an application is admitted, it will be attached with QoS parameters. If two or more backoff timers within the same station finish backoff at the same time there will be virtual collision. The station’s internal scheduler solves the virtual collision. EDCA support QoS; however, it cannot guarantee strict QoS required by real-time services without proper network control mechanisms.

I. INTRODUCTION IEEE 802.11 wireless LAN (WLAN) has been highly successful and became the edge network of choice because of its simplicity, scalability, flexibility, cost effective, faster deployment and connectivity. In recent years, it is gaining popularity at an unprecedented rate and is being increasingly deployed. As this type of network is an integral part of the infrastructure they are increasingly used for multimedia applications. In order to support applications with QoS requirements, the IEEE 802.11e standard [1] enhances the original IEEE 802.11 MAC protocol by introducing a new coordination function, which has both contention-based and contentionfree medium access methods. The contention-based MAC protocol, EDCA supports QoS; however, it cannot guarantee strict QoS required by real-time applications without proper tuning of QoS parameters and network control mechanisms. The channel utilization and delay factors are considered as control metrics for call admission control (CAC) and rate control (RC) in [2] in unsaturated condition. In [3] extensive survey is done for different types of CAC algorithms. Overall delay and throughput are considered as a control metrics for CAC in [4]. In this paper, we propose a tradeoff between CAC and RC with proper adjustment of QoS parameters to enhance the IEEE 802.11e EDCA access mechanism. CAC is used for real time traffic, while the rate control is used for best effort traffic to get prompt residual channel access left by real time traffic.

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Where Rbe_new and Rbe_old are updated and old data rates of best effort traffic, cumax is maximum channel utilization and rbr is the contribution from the real time traffic to channel busyness ratio rb.

III. TRAFFIC CONTROL MECHANISM In this section, the traffic control mechanism, call admission control for real time applications and rate control for non real time applications, are described. A. Call Admission Control

IV. IMPROVED MECHANISM

The CAC is a suitable traffic control mechanism for real time traffic since it is not greedy in terms of bandwidth usage, and more importantly, has strict delay requirements. In [2] Chen et al. implemented CAC algorithm taking channel utilization and delay as a performance metrics for QoS required by real time traffic. The algorithm works by monitoring channel busyness ratio of the real time applications. Before initiating a real-time flow of priority i (i = 2 or 3), a node must send an ADDTS (add traffic stream) request to the coordinator. The ADDTS contains the traffic priority and the traffic specification (TSPEC) corresponding to the specific application, and the TSPEC specifies Rmean, Rpeak, and PKl (i.e. the nominal MSDU size). Upon receiving the ADDTS, the coordinator associates the flow with the appropriate ACi and meanchannel utilization (cui, mean) is evaluated as Rmean cui , mean = (Tsuc ) (1) PKl Where Tsuc is transmission successful time in RTS/CTS mode, and Rmean is mean data rate of real time application. The coordinator or AP records the total channel utilization due to all admitted real-time flows into cu (channel utilization) parameter. New flow gets admitted if channel utilization is fully satisfied. Otherwise new flow is rejected so that already connected real time traffic doesn’t suffer to get required QoS. The channel utilization is equal to the channel busyness ratio for unsaturated case [2]. B. Rate Control For non-real-time data traffic, which can tolerate delay ranging from seconds to minutes but is greedy in terms of bandwidth usage, rate control is more appropriate. In [2], Chen et al. proposed a rate control algorithm taking control metric as channel busyness ratio. The transmission rate of the best effort traffic is controlled based on two criteria. First, the best effort traffic should not affect the QoS level of the admitted real-time traffic. Second, the best effort traffic should be able to promptly access the residual bandwidth left by real-time traffic in order to efficiently utilize the channel. For rate control scheme, each node needs to monitor the channel busyness ratio rb during the period of Trb. Rbe _ new = Rbe _ old

(cumax - rbr) . (rb - rbr)

(2)

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The traffic control mechanism mentioned in section-III is not considering the greedy station problem, where the higher priority station always gets opportunity for packet transmission. This paper proposes an adjustment method for QoS parameter that trades off between CAC and RC. The proposed scheme adjusts the QoS parameters at AP in BSS (Basic Service Set) mode incorporating the tradeoff between CAC and RC. The AP distributes the changed QoS parameters in wireless LAN within transmission range. Two-way handshaking (basic access) mode of EDCA is better for real time traffic, however the RTS/CTS helps resolve the hidden station problem. The time to get channel access and the delay reduce significantly for real time traffic when we consider the two-way hand shaking and “no acknowledgement” policy only at layer-two for real time traffic. Proper tuning of maximum CW value of real time traffic makes efficient contention to access the medium for high priority ACs and also the non real time application gets chances to access residual channel to transmit. In addition to that, there might be only one nonreal time application in the WLAN and might be treated as background traffic having lowest AC. So, the mapping of AC for non-real time traffic to the higher one results better opportunity to get media access. Pseudo code for proposed scheme to address above issues is as following: While (1) { if (no new flow request) { Break } elseif (a new flow request exist) { Calculate cu, data rate using (1) and (2) if (flow is real time traffic) if (cuA,mean+cui,mean< cumax ) Accept new call If new data rate is less than the previous one Adjust CW value for voice and video else default parameters. end else Reject new call elseif (flow is non real time traffic) Accept the BET with new data rate Map the AC if applicable end }

V. SIMULATION RESULTS AND DISCUSSION

adjustment of AC and QoS parameter. In addition to lowering the number of data drops in new scheme, the voice traffic that could be successfully delivered from source to destination is higher. Fig. 5 shows that received voice packets are also more numerous in the new scheme than in the default EDCA scheme.

The proposed scheme is implemented at AP to adjust the QoS parameter taking account of CAC and RC. A. Simulation Setup We have used the OPNET Modeler 12.0 [5] to evaluate the performance. The simulator natively supports IEEE 802.11e EDCA functionality. We have created two scenarios: one with default EDCA QoS parameters specified in IEEE 802.11e in [1] and set in [5], and the other for proposed scheme at AP in infrastructure mode. An 802.11e based wireless LAN with 20 mobile nodes (MNs) is simulated. The MNs start to contend and transmit their corresponding traffics between 50 to 55 seconds in our simulation setup. We consider the voice and video conferencing as real time traffic and the HTTP browsing application as the best effort traffic. The cu for real time services as in [2] is 80% and the remaining for non real time services. Physical Characteristics of 802.11g with data rate having 6 Mbps and buffer size 1024k are considered. The voice, video and http-traffic correspond to AC(3), AC(2) and AC(0) respectively. For the proposed scheme, we had configured no acknowledgement policy at layer-two for real time traffic at basic access mode. AP adjusts the QoS parameters as CW3, 0 = 15, CW2, 0 = 31, and changes the AC(0) to AC(1) for HTTP application at run time. In such a setting, it is clear that the real time traffic still has the highest priority and the HTTP traffic has the lowest priority in terms of channel access. However, because of such setup, the greediness nature of higher priority station will significantly be reduced because of proper contention to get channel access. During the simulation, as the time increases, the traffic load increases gradually.

Fig. 1.

WLAN average and voice end-to-end delay

Fig. 2.

Delay variation for voice packet

Fig. 3.

Jitter for voice traffic

Fig. 4.

Data dropped in WLAN

B. Results and Discussion With the increase of simulation time, more stations (voice, video and http MNs) become active and contend to get channel access. We evaluated the performance of above simulation scenarios observing throughput, average end-to-end delay, jitter, packet-dropping probability, traffic sent and received in the network for 200 seconds. Fig. 1 shows that the significant decrease in average delay in wireless LAN and end-to-end delay of voice traffic. Fig. 2 and 3 show the improvement of delay variation and jitter respectively of voice packet in the new scheme. The new scheme, with the adjustment of related WLAN 802.11e parameters, has increased quality of voice across the network by reducing overall delay, end-to-end delay, and jitter. In Fig. 4, the new scheme has drastically less data drop than in default case. The http application also gets prompt access to the residual channel because of

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Fig. 5.

Voice traffic sent and received

Fig. 6.

HTTP traffic sent and received

Fig. 7.

Fig. 6 shows that, in default case, after nearly 60 seconds, the http-application browsing activity is less because of the presence of large number of active high priority MNs in the network. But http-application traffic is not blocked by higher priority traffic because of change of AC for HTTP from AC(0) to AC(1) and the change of CW for higher priority service. So, best effort traffic (http) gets prompt access to residual bandwidth left by real time services. Fig. 7 depicts that the decrease of average delay of video packet in the new scheme, however for few time there is almost the same delay in both schemes. Fig. 8 shows that almost all video packets are received in new scheme. As a result of QoS parameter and AC adjustment in new scheme, there is less delay, jitter, end-to-end delay, delay variation and frame dropping probabilities. Hence, the overall throughput of the system is increased significantly in the new scheme in Fig. 9. VI. CONCLUSION This paper proposes a new method for performance enhancement of EDCA access mechanism of IEEE 802.11e by incorporating tradeoff between admission control and rate control by changing QoS parameters with the help of simulations. The results show that default EDCA scheme alone doesn't significantly improve the delay and jitter performance of real-time traffic; instead it potentially results in increased significant data drops. Simulation results have shown that adjusting the AC and QoS parameter according to the network conditions and by making tradeoff between CAC and RC, the better throughput can be obtained with guaranteed QoS for real time traffic the same physical layer setup.

Average delay of Video Packets

REFERENCES

Fig. 8.

Average video load and throughput

Fig. 9.

Overall throughout of WLAN

[1] Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements, IEEE Std. 802.11e, 2005. [2] X. Chen, H. Zhai, X Tian, and Y. Fang, “Supporting QoS in IEEE 802.11e Wireless LANs”, IEEE Transactions on Wireless Communications, Vol. 5, Issue 8, Page(s): 2217 2227, August 2006. [3] D. Gao, J. Cai, and K. N. Ngan, “Admission Control in IEEE 802.11e Wireless LANs”, IEEE Transactions on Network, Volume 19, Issue 4, Page(s): 6 – 13, July-Aug. 2005. [4] J. Zhu and O. Abraham, “A New Call Admission Control Method for Providing Desired Throughput and Delay Performance in IEEE 802.11e Wireless LANs”, IEEE Transactions on Wireless Communications, Vol. 6, Issue 2, Page(s): 701-709, Feb. 2007.

[5] OPNET modeler 12.0, http://www.opnet.com.

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