Optimum Parameters for VoIP in IEEE e Wireless LAN

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1 Optimum Parameters for VoIP in IEEE 82.11e Wireless LAN Ryo Kitahara NTT DoCoMo 3-5 Hikari-no-oka, Yokosuka City Kanagawa Prefecture, Japan Shingo Morita Waseda University Okubo Shinjuku-ku Tokyo, JAPAN waseda.ac.jp Koichiro Doi Waseda University Okubo Shinjuku-ku Tokyo, JAPAN waseda.ac.jp Shigeki Goto Waseda University Okubo Shinjuku-ku Tokyo, JAPAN waseda.ac.jp Tomoya Iimura Waseda University Okubo Shinjuku-ku Tokyo, JAPAN waseda.ac.jp ABSTRACT IEEE 82.11e is an enhanced standard in wireless LAN which has QoS mechanisms, EDCA and HCCA. This paper analyzes the performance of IEEE 82.11e EDCA functions through working testbeds and a large-scale simulation. EDCA has three parameters: AIFS, CW and TXOP to differentiate packets. This paper shows the effectiveness of EDCA by simple experiments. Then, we try to find the optimum values for EDCA parameters. It is interesting that default parameter values do not ensure good communication quality for VoIP communications. This paper also illustrates the results of a simulation which covering a large number of nodes. Again, it is shown that the default values are not ideal for VoIP communications on a larger scale. The results of this paper are meaningful when VoIP communications have priority over other traffic in a wireless LAN environment. Categories and Subject Descriptors C.2.1 [Computer - Communication Networks]: Network Architecture and Design Wireless communication General Terms Measurement, Performance, Experimentation Keywords IEEE 82.11e, EDCA 1. INTRODUCTION IEEE standards are widely employed to access the Internet. Almost every laptop computer shipped today has an IEEE a/b/g-enabled Network Interface Card (NIC) embedded. Access points (AP) are installed into many offices and homes. Some companies provide fare-paying wireless LAN service in public places like airports, railway stations and coffee shops. In addition, there also exist free-ofcharge services. FON has started an AP sharing service over 9 countries since 25 and already has more than 5 million users worldwide [1]. People use wireless Internet access for many services such as the World Wide Web, or short messages. Wireless Internet access today is also used for video streaming and IP telephony, which requires good Quality of Service (QoS) in terms of i) low packet loss rate; ii) minimum delay time of packet arrival; iii) small jitter of delay. Traditional Media Access Control (MAC) is enough when the load is not heavy. With a large amount of traffic, however, it cannot guarantee the quality of such services because it does not classify traffic which requires high QoS from normal traffic. The main goal of simple CDMA/CA-based traditional MAC is to share the radio medium with other nodes. Traditional MAC has a single queue for outgoing frames and cannot distinguish what service the traffic is delivering. The IEEE 82.11e standard aims to provide appropriate QoS functions for traffic in accordance with their requirements. The QoS enhancement of 82.11e is called the Hybrid Coordination Function (HCF). HCF has two different MACs: HCF-controlled channel access (HCCA) and Enhanced Distributed Channel Access (EDCA). Almost all the APs labeled 82.11e enabled only implement EDCA and few implement HCCA. This paper mainly discusses EDCA. It will show the performance of VoIP traffic over a small EDCA-enabled network by experimental comparison with traditional MAC mechanism. We emphasize the fact that default EDCA parameter values are not appropriate for VoIP traffic.

2 This paper further illustrates the results of simulation for a large number of nodes which have VoIP traffic over 82.11e networks. It is hard to make a large-scale working testbed; we use simulation to show the results. Our simulation is based on DESMO-J, which is a general-purpose simulation library [2]. The result shows again that the default values are not appropriate for large-scale networks. The rest of this paper is organized as follows: In section 2, we will briefly describe the 82.11e standard and VoIP. It also introduces some related work. The experiments and the results are illustrated in section 3 followed by the conclusion in section BACKGROUND The legacy Media Access Control (MAC) mechanism included in IEEE a/b/g has two medium-sharing functions: the distributed coordination function (DCF) and the point coordination function (PCF). IEEE 82.11e is intended to add QoS functionality to legacy e provides two MAC layer protocols: the enhanced distributed channel access (EDCA) and the hybrid coordination function (HCF) controlled channel access (HCCA). The performance of 82.11e varies depending on the application. This paper mainly deals with Voice over IP (VoIP) communications over the 82.11e network Legacy Two MAC layer protocols are defined in the legacy: DCF and PCF. DCF is a contention-based medium-sharing function. On the other hand, PCF is a polling-based mediumsharing function. An Access Point (AP) decides which node should use the radio resource at every moment. DCF has a contention-based mechanism called carrier sense multiple access with collision detection (CSMA/CD), which is well-known. With DCF, wireless nodes have to wait for a fixed time (DIFS, Distributed Inter-Frame Space) before submitting frames to see if any other node is transmitting a frame to avoid collision. When collisions occur, all the nodes should stop to submit frames and wait for a random time. This mechanism is called backoff [3]. Backoff time is calculated by a formula: Backofftime = Random slottime, (1) where Random is between and Contention Window (CW ). CW is defined as follows. CW = (CW min + 1) 2 n 1, (2) where n represents the number of resubmission. DCF has two parameters: CWmin and CWmax, which are the minimum value of CW and the maximum value, respectively. PCF is a direct extension of DCF. PCF is not popular among AP products yet. PCF is not used in most EDCA functions. This paper does not deal with the details of PCF e Since IEEE 82.11e is a MAC layer extension to the legacy, 82.11e is not intended to replace 82.11a/b/g e Figure 1: SIFS Previous Frame DIFS SIFS AIFS Slot Time CW Next Frame Parameters in EDCA: AIFS, CW and doesn t specify any physical (PHY) layer protocol. A QoS mechanism is one of the new features of 82.11e. As mentioned earlier, 82.11e has two MAC mechanisms: HCCA and EDCA. HCCA is a polling-based medium access protocol, which extends the PCF of the legacy. APs allocate time slots to each node for transmission according to demand. This is called Dynamic Time Division Multiple Access. EDCA is a contention-based medium access protocol, which inherits the DCF of the legacy. Almost all the APs products today claiming to be 82.11e-enabled implement only EDCA and do not have HCCA functionality. This is the reason why we mainly discuss EDCA. The largest difference of EDCA from DCF is that EDCA has four traffic queues. It can assign as access category (AC), thus QoS settings, to each queue independently from the other queues. The four ACs are: Voice (AC V O), Video (AC V I), Best Effort (AC BE) and Background (AC BK). The judgment of AC is done by checking the type of service (TOS) field in an IP packet header. EDCA has the same two parameters as DCF to control the quality of traffic: CWmin and CWmax. EDCA has additional parameters: Arbitration Inter Frame Space (AIFS) and the Transmission Opportunity limit (TXOPlimit). AIFS determines the fixed time before a node sends a frame. An Access Point (AC) with a short AIFS value can submit frames before another AC submits, thus it can have better QoS. The TXOPlimit is time that a node can use to submit the frames exclusively without waiting for other node. A large TXOPlimit value reduces overhead and can increase throughput of the AC. The relationship between AIFS, CW, and SIFS is shown in Fig. 1. (SIFS is the Shortest Inter-Frame Space for the APs control usage.) Table 1 shows the default parameters for each access category [4]. Two T XOP limit values are given for AC V O and AC V I depending on the physical (PHY) layer. The values shown in Table 1 are those of the PHY layer of 11b [5], which is used in our experiments. 2.3 VoIP VoIP is an IP-based telephony application. VoIP software or hardware exchanges digitally coded voice data between nodes. VoIP has faced a QoS problem since the beginning. Ordinary IP networks employed by VoIP services are, to some extent, more fragile than closed Plain Standard Telephone Networks (PSTNs). Traffic may be heavy in some IP networks. In addition, LANs at each end may be overwhelmed when great numbers of people use the network simultaneously.

3 Table 1: Default EDCA parameters Access Category CW min CW max AIFSN T XOP limit AC VO (acw min + 1)/4 1 (acw min + 1)/ msec AC VI (acw min + 1)/2 1 acw min msec AC BE acw min acw max 3 msec AC BK acw min acw max 7 msec Table 2: IP telephony quality standard in Japan class delay time (ms) A > 8 < 1 B > 7 < 1 C > < 4 This paper only focuses on the QoS issue of VoIP communications over wireless LANs. We do not discuss QoS issues in general. The quality of voice exchanged over VoIP can be evaluated by scales such as: the Mean Opinion Score (MOS) and the R value. MOS is originally calculated using subjective scores obtained from human evaluators [6]. The is obtained by twenty quality indicators such as echo, delay and distortion [7]. The is more versatile than the MOS, because there is a function given by ITU-T as G.17 to convert s to MOS. Moreover, s can reflect some packet level quality indicators. We adopt the to evaluate the quality of VoIP traffic in this paper. In Japan, the Ministry of Internal Affairs and Communications (MIC) [8] defines the quality class standard (Table 2) for the together with the mean delay time as a reference for assign telephone numbers for telephone services. Telephone services with public telephone numbers must have at least C class quality, according to the standard. Readers can understand the results of this paper in terms of real VoIP service. 2.4 Related works There have been many research projects on the performance of the IEEE legacy [9][1][11], and some work has been done on 82.11e [12][13][14][15][16][17]. These papers verify that IEEE 82.11e gives better QoS to traffic categorized as AC V O and AC V I than AC BK and AC BE. Alonso-Gonzalez [14] showed by using an ns-2 simulator that the quality of high-prioritized traffic can be maintained despite background traffic saturating the channel capacity. We will confirm this result with a real testbed using the instead of packet level indicators, which do not directly represent the quality of voice. It is known that the quality varies greatly depending on the content of the MPEG-4 movies transmitted over an 82.11e network [13]. The result [13] suggests the same performance tendency in VoIP. Some adaptive method for tuning parameters is desirable to get a truly optimal set of parameters. We will give optimal parameter sets for a large number of VoIP nodes. Figure 2: Network for the first experiment Although there have been many research projects on the performance of UDP applications including VoIP and MPEG-4 video over 82.11e, the performance of TCP applications has not been investigated in detail. Thottan [15] evaluates the quality of TCP traffic with a variety of traffic patterns. The paper tries to give the best quality of service. Not only experimental researches but also analytical works have been done. Bellalta et al.[12] made an analytical model of EDCA functions. They verified that their analysis matches well the discrete simulation. 3. EXPERIMENTS We have conducted a series of experiments. It consists of three experiments, which are explained in this section. 3.1 EDCA Effectiveness We first conducted an experiment to measure the quality of VoIP packets in EDCA enabled networks [18]. It verifies the effect of EDCA for VoIP in a working network. VoIP applications require high quality for the traffic. VoIP packets should not be delayed and the jitter of the delay should be kept small Configuration We set up a network consisting of two broadcast domains (Fig. 2): the one connected by a wired switching hub and the other connected by a 82.11e-enabled Access Point (AP). The two networks are joined by a PC (software) router, which can change the TOS field value of VoIP packets in an IP header using iptables. Node VS transmits VoIP traffic to VC. We use ITU-T recommendation G723.1 [19] for the VoIP CODEC system. Nodes S and S1 submit 6 Mbps of best-effort traffic to node C and C1, respectively, using iperf [2] software. We also measured the VoIP quality in a more realistic situation where a video traffic AC V I coexists alongside voice and best-effort traffic. By changing the TOS field value of

4 Table 3: TOS not set TOS set one part, of the best-effort traffic, we classified it into AC V I at the AP. The size of the video traffic ranges from 1 Mbps to 5 Mbps. We used ASTEC Eyes for VoIP software[21] installed on VS to submit pseudo voice traffic and also on VC to receive the voice traffic from VS. ASTEC Eyes can measure voice traffic s Results As Table 3 shows, EDCA improved the voice quality over a wireless connection. With the TOS field off, the of 23.5 fell below the range of the Japanese IP telephony standard R= (Table 2). When the TOS field is on, the value of 77 almost reached A class quality 8. Without Classification. First, we disabled the iptables at the router so that all traffic is classified as background traffic. The of the VoIP traffic was 24.8 with EDCA disabled and 23.5 with EDCA enabled. This result simply confirmed that without setting the TOS field value, EDCA is just meaningless in terms of QoS classification, or even worse: It introduces some overhead to all the packets going through the AP. With Classification. The of the VoIP traffic dramatically improved (Fig. 3) when the TOS field of VoIP packets is set so that they are classified in the voice AC V O access category. Second, we replaced one part of the best-effort traffic, AC BK, with video streaming traffic categorized as AC V I, which has higher priority than AC BK traffic and is considered to introduce much more load on voice AC V O. Until the video bandwidth exceeds 3 Mbps, the voice traffic AC V O keeps the as high as that when there is no other prioritized traffic other than itself. What is more, although the of voice AC V O starts to decrease when video AC V I increases and exceeds 3 Mbps, it still keeps a high value compared with the VoIP which are not prioritized. 3.2 Default Settings Performance The effect of EDCA depends on applications because a single set of parameters may not be able to meet all the requirements for various applications. We measured the for VoIP traffic over the 82.11e network with EDCA with default parameters and with alternative settings [22] Configuration We use the same network configuration shown in Fig. 2. The tools and CODEC are the same as those of the first experiment, except that we eliminated s1 and c1 from the network to simplify it AC_VI traffic Figure 3: with AC V I traffic EDCA on EDCA off Figure 4: Effect of EDCA on background traffic Results First, we simply measured the of VoIP traffic against background traffic while changing the size. As a result (Fig. 4), EDCA has a practical effect on VoIP quality only when the bandwidth of background traffic exceeds 6 Mbps. Therefore, we will focus on the performance of EDCA for background traffic of 6 1 Mbps in this experiment. Then we performed the same experiment but changing AIF S (Fig. 5), CW (Fig. 6) and T XOP limit (Fig. 7), respectively. Each parameter set is shown in Tables 4 and 5. AIFS values shown in Fig. 5 are represented by the number of time slots, e.g., AIFS, AIFS 1. According to the results, we picked up values which apparently have good quality. We define an alternative parameter set as shown in Table 6. The s of selected settings are shown in bold lines in Fig Finally, we applied the new alternative setting to the AP and measured the performance against the default setting. As shown in Fig. 8, the alternative setting performing basically better than the default parameters. This result implies that there would be optimum parameter sets for various situations other than the default setting.

5 Table 4: CW parameter sets CW min CW max CW CW CW CW CW CW CW CW CW CW TXOP TXOP 1 TXOP 2 TXOP 3 TXOP 4 TXOP 5 Table 5: TXOP parameter sets TXOP 2 ms TXOP 1 14 ms TXOP 2 38 ms TXOP 3 16 ms TXOP ms TXOP ms Figure 7: s for different TXOP values 7 AIFS AIFS 1 AIFS 2 AIFS 3 AIFS 4 AIFS 5 AIFS 6 AIFS 7 Table 6: Alternative parameter set AIF S CW min CW max T XOP limit [ms] Figure 5: s for different AIFS values 85 8 altered default CW CW 1 CW 2 CW 3 CW 4 CW 5 CW 6 CW 7 CW 8 CW Figure 8: The performance of the alternative setting Figure 6: s for different CW values

6 3.3 Simulation for Large Scale Networks The results of the two previous experiments show that the optimum parameters of EDCA depend on the applications and the condition of the network. In this section, we conduct a simulation for a large number of nodes to seek the optimum set of parameters for large-scale networks. In this simulation, the EDCA parameters are adjusted not only at the Access Point (AP) but also at wireless nodes. In this section, up means the direction of traffic from wireless nodes to AP and down means the opposite direction. optimum CW size CWmin CWmax Configuration The structure of the simulated network is virtually the same as the one shown in Fig. 2, which is a combination of a wired LAN and wireless LAN with an AP bridging them. The wired nodes communicate with the wireless nodes across the AP. We dynamically change the number of nodes at both sides of the AP. The numbers of the nodes are 2, 22 and 24. With less than 2 nodes, the is so high that IEEE EDCA is not necessary, and with more than 24 nodes, it is difficult to achieve 9, which is required to deliver good quality of service. The physical (PHY) layer of the wireless LAN of this network is IEEE 82.11b. The size of the VoIP traffic is fixed at 64 kbps, and the payload size is 1 bytes [23]. The length of the queues at the AP and nodes are set to 2. The simulation runs for 1 seconds for each parameter. The simulator is constructed by using the DESMO-J library. DESMO-J is a Java-based simulation framework developed at the University of Hamburg, Germany [2]. We measure the performance of VoIP traffic over the EDCAenabled network with various parameter settings. In this paper, we explain some of the meaningful results from which we can learn some instructive lessons Results In Fig. 9 22, the value from to 1 in the map stands for an. Each will be shown as the average of all the values collected from the nodes. The white arrow on the map indicates the direction toward higher s, and we define optimum sets as those achieving larger than CW size. Figure 9 shows the optimum CW sizes while the number of VoIP nodes communicating through the wireless network increases. The optimum CW size increases as the number of communicating nodes increases. In general, a small CW value leads to a high possibility of collision with other nodes. If a collision happens, a node retransmits frames after waiting for only a small period of time, indicated by small CW size. This result is meaningful in designing a large-scale wireless network. If there is no other prioritized node in the network, the performance of the single prioritized node improves with smaller CW. Apparently, giving fixed parameters is inappropriate to achieving the best performance. The default The number of nodes Figure 9: The optimum CW max and CW min values TXOPlimit TXOPlimit down TXOPlimit up The number of nodes Figure 1: The optimum T XOP limit values value showed best performance only when less than four nodes coexist. T XOP limit. The result is shown in Fig. 1. This result verifies again that the default fixed parameters are only suitable for a certain network condition. T XOP limit must be increased as the number of nodes increases. T XOP limit for the AP must be greater than for client nodes because the AP communicates with many nodes while each single node only communicates with the AP alone. CW (up - down). Two trends are discovered from the result shown in Fig First, the larger the CW size for AP (down) is, the better the performance gets. Secondly, CW size for client nodes must be smaller than that of AP. T XOP limit (up - down). We adjust T XOP limit values of the AP and client nodes independently; the results are shown in Fig From these results, it is shown that to support less than 24 nodes, the T XOP limit value of the nodes can be some value not less than 3264, while the AP should

7 (Down) _ _2473_ 7 CwMin _CwMax(Up) _ _535 24_ _535 81_ _ _ _247_535 9 Figure 11: CW size (up and down) (n = 2) Figure 14: T XOP limit down (n = 2) 7-8 (Down) _ _2473_ 7 CwMin _CwMax(Up) _ _535 24_ _535 81_ _ _ _247_535 9 Figure 12: CW size (up and down) (n = 22) Figure 15: T XOP limit down (n = 22) _247 (Down) _247 CwMin _CwMax(Up) CwMin_CwMax 489_ _535 24_ _535 81_ _ _ _247_ Figure 13: CW size (up and down) (n = 24) Figure 16: T XOP limit down (n = 24)

8 _ _ _24 535_ _81 535_ _ _247535_ _ _ _ _ _ _ _ _247_ Figure 2: CW size and T XOP limit (n = 2) Figure 17: T XOP limit up (n = 2) 535_ _ _24 535_ _81 535_ _ _247535_ Figure 18: T XOP limit up (n = 22) _ _ _ _ _ _ _ _247_ Figure 21: CW size and T XOP limit (n = 22) have the largest T XOP limit possible _ _ _24 535_ _81 535_ _ _247535_ Figure 19: T XOP limit up (n = 24) CW size and T XOP limit. In Fig. 2 22, which represent configurations with 16 nodes, 2 nodes and 24 nodes, respectively. s are shown for various CW sizes and T XOP limit when T XOP limit for up and down is the same value. The result shows that T XOP limit should be maximized as much as possible, and that the pair of (127, 2) for the CW size gives the best performance. 4. CONCLUSION There have been many related studies on the performance of EDCA and their effects on VoIP packets. They have proven that EDCA improves packet-level QoS. However, two networks both having the same QoS condition do not necessarily give the same quality from the users perspective. We used well-defined to evaluate the quality of VoIP traffic instead of simple packet-level indicators such as packet delay time, jitter and packet loss rate. We performed three experiments. The first one verified that

9 _ _ _ _ _ _ _ _247_ Figure 22: CW size and T XOP limit (n = 24) EDCA mechanism of IEEE 82.11e which can prioritize the VoIP traffic against the background traffic. Secondly, we measured the of VoIP traffic over various sizes of background traffic. Then, we picked up a set of parameters and compared the performance with the default parameters set. The new set showed better performance, regardless of the size of the background traffic. This result implies that the default set of parameters is not appropriate to various network conditions. To seek for the optimum parameter set for a large-scale network, we conducted the third experiment using DESMO-J simulation and acquired many optimum parameter sets. In addition, we got results useful for modifying the EDCA parameters. We plan to add background traffic in the network simulation to get further practical results. Our results indicate some algorithmic or parameter-tuning method should be developed to adaptively change the EDCA parameters to improve Quality of Service for VoIP traffic. 5. REFERENCES [1] FON. [2] DESMO-J. [3] IEEE (R23). IEEE, New York, NY, 23. [4] IEEE e. IEEE, New York, NY, 25. [5] IEEE b-1999 (R23). IEEE, New York, NY, 23. [6] Methods for subjective determination of transmission quality. [7] Telecommunication Technology Committee. A Method for Speech Quality Assessment of IP Telephony (JJ-21.1). TTC, Minato-ku Tokyo, Japan, 27. [8] The Ministry of Internal Affairs and Communications. [9] Miroslaw Narbutt and Mark Davis. Gauging voip call quality from wlan resource usage. In International Symposium on a World of Wireless, Mobile and Multimedia Networks, 26. [1] Rovert A. Malaney, Ernesto Exposito, and Xun Wei. Seeking voip qos in physical space. In the 3rd ACM international workshop on Wireless mobile applications and services on WLAN hotspots, 25. [11] Sangki Yun, Hyogon Kim, and Inhye Kang. Squeezing 1+ voip calls out of 82.11b wlans. In International Symposium on a World of Wireless, Mobile and Multimedia Networks, 26. [12] Boris Bellalta, Cristina Cano, Miquel Oliver, and Michela Meo. Modeling the ieee 82.11e edca for mac parameter optimization. In Heterogenius Networks, Sep 26. [13] Deyun Gao, Jianfei Cai, Paul Bao, and Zhihai He. Mpeg-4 video streaming quality evaluation in ieee 82.11e wlans. IEEE International Conference on Image Processing, 1:187 2, Sept 25. [14] I. Alonso-Gonzalez, C. Ley-Bosch, and C. C. Ojeda-Guerra. Experimental evaluation of ieee 82.11e. In Proceedings of the 24th IASTED International Multi-Conference, pages 7, Feb 26. [15] Marina Thottan and Michele C. Weigle. Impact of 82.11e edca on mixed tcp-based applications. In Proceedings of the ACM WiCon 26, Aug 26. [16] Yusuke Natsume, Duoyi Yan, Koichiro Doi, Ryo Kitahara, and Shigeki Goto. Voip quality measurement in ieee 82.11e wlan. In Forum on Information Technology, 26. [17] Atsunori Noguchi, Takahiro Suzuki, and Shuji Tasaka. Effect of ieee 82.11e edca parameters on application-level qos. Technical report of IEICE. Multimedia and virtual environment, 14(635):7 12, Jan 25. [18] Shingo Morita. VoIP Quality Measurement in IEEE 82.11e Wireless LAN. Graduation Thesis of Waseda University, Shinjuku-ku Tokyo, Japan, 27. [19] ITU-T, Mar G : Dual rate speech coder for multimedia communications transmitting at 5.3 and 6.3 kbit/s. [2] Iperf - TCP/UDP Bandwidth Measurement Tool. [21] ASTEC Eyes for VoIP. [22] Tomoya Iimura. Designing of VoIP network considering load traffic in IEEE 82.11e. Graduation Thesis of Waseda University, Shinjuku-ku Tokyo, Japan, 27. [23] ITU-T, Feb 24. G.711 : Pulse code modulation (PCM) of voice frequencies. Apppendix II