II. IEEE802.11e EDCA OVERVIEW

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1 The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'7) CACITY IMPROVEMENT OF WIRELESS LAN VOIP USING DISTRIBUTED TRANSMISSION SCHEDULING Kei Igarashi, Akira Yamada and Tomoyuki Ohya Research Laboratories, NTT DoCoMo, Inc. 3-5, Hikarino-oka, Yokosuka-shi, Kanagawa, , JAN {igarashike, yamadaakir, ABSTRACT This paper proposes a Medium Access Control (MAC) protocol that provides Quality of Service (QoS) in Voice over IP (VoIP) over Wireless Local Area Networks (WLANs). Our proposed protocol minimizes packet collision and improves voice communication quality in both single-cell and multiple-cell cases. Introduction of the proposed protocol requires only VoIP STA software modification, i.e. replacement of Access Points (s) is not necessary. Our protocol also keeps backward compatibility with the legacy devices that do not implement the proposed method. We evaluate the performance of the proposed protocol and show that it can increase the number of accommodated VoIP calls by approximately 5%. I. INTRODUCTION The Wireless Local Area Network (WLAN) based on IEEE82.11 [1] has become increasingly popular due to its low cost, flexibly, and ease of use. WLAN devices have recently been implemented in consumer electronic devices as well as laptop computers. In addition to non-real-time applications such as HTTP and FTP, the WLAN is expected to support real-time applications such as Voice over IP (VoIP), video streaming and other delay-sensitive applications. For real-time WLAN applications, Quality of Service (QoS) technologies for the WLANs represent a key challenge to supporting such kinds of applications. Many performance analysis studies have examined the application of IEEE82.11 WLAN to real-time services such as VoIP and Video. Most of them have shown the difficulties in QoS [2-4]. In order to solve these problems, IEEE82.11e adopted Enhanced Distributed Coordination Access () [5]. defines four access categories (ACs) that provide packet prioritization for the delivery of traffic. Priority control on Carrier Sense Multiple Access with Collision Avoidance (CSMA / CA) is realized by setting the parameters to meet the QoS demands of application. However provides only the prioritized QoS for different ACs and does not take into consideration the QoS of traffics which belong to the same AC. Hence it is difficult to meet the QoS demands of each application using only static parameters. When only is applied to VoIP, packet collision probability increases as the number of simultaneous calls increases and this results in voice quality degradation [6-8]. Therefore, new solutions are needed to realize adequate quality for real-time applications. This study proposes a Medium Access Control (MAC) protocol that provides QoS guarantees for VoIP over WLAN. Our proposed protocol is based on a feasible and flexible scheduling method that takes advantage of the feature that VoIP packets are generated at intervals of voice codec period. The proposed protocol can not only suppress packet collision but also minimize the costs associated with development and introduction. The paper is organized as follows. Section II describes schemes and their problems. We propose a MAC protocol for VoIP over WLAN in Section III. Its performance evaluations are shown in Section IV. We conclude the paper in Section V. II. IEEE82.11e OVERVIEW A. Enhanced Distributed Coordination Access Enhanced Distributed Coordination Access () supports a prioritized mechanism on CSMA/CA. differentiates packets from an upper layer into four different ACs according to the applications and their QoS characteristics. An overview of the mechanism is illustrated in Figure 1. A station (STA) with packets to transmit and senses the medium before initiating a transmission. If the medium is sensed busy, the STA defers its transmission to a later time. If the medium is sensed idle for a prespecified time, i.e., Arbitration Inter Frame Space (AIFS), then the STA generates a random backoff period for an additional deferral time before initiating transmission. The AIFS is given as AIFS = AIFSN aslottime + SIFS (1) where aslottime is the value of a slot time and SIFS is the value of a Short IFS. AIFSN is an integer value which is set according to the AC. The higher prioritized AC has the smaller AIFSN value as described in Figure 2. The random backoff period is derived as BackoffTim e = Random() aslottime (2) where Random() is the pseudo-random integer drawn from a uniform distribution over the interval [,CW]. CW (Contention Window) is an integer within the range of values of the PHY characteristics, acwmin /7/$ IEEE

2 The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'7) Transmitter Receiver AIFS Backoff SlotTime SIFS Figure 1: Access scheme in AIFS[BK] AIFS[BE] Backoff[BE] Backoff[VI] AIFS[VI] Backoff[VO] AIFS[VO] Backoff[BK] BK: Background BE: Best effort VI: Video VO: Voice Figure 2: Prioritization in CW acwmax. The CW is set equal to acwmin at the first transmission attempt, and it increases in the following manner, until it reaches acwmax. n CW = 2 ( CWmin + 1) 1 (3) where n is the number of retransmissions. The CWmin/CWmax values also vary for different ACs. For higher prioritized ACs, CWmin and CWmax are set to smaller values. B. Problems on As described above, provides prioritized QoS for different ACs. However does not take into consideration the QoS of traffics which belong to the same AC. Figure 3 shows the problems that occur in. The packet collision probability increases as the number of calls increases due to simultaneous transmission. Additionally the delay tends to increase if more than one STA are in backoff states. These problems result in voice quality degradation. Therefore, it is necessary to restrict the number of VoIP calls over a WLAN system. Instead of, IEEE82.11e also provides the polling based MAC protocol called Hybrid coordination function Controlled Channel Access (HCCA). However HCCA proposes no concrete scheduling algorithms. In addition, collision of polling packets from more than one access point () may always occur in multiple-cell environment because VoIP application generates packets to transmit at a constant interval. III. PROPOSED TECHNIQUES A. Overview of Scheme We propose a MAC protocol to provide effective QoS guarantees for VoIP over WLAN. Our proposed AIFS Collision Backoff Delay Figure 3: Problems on protocol follows three policies: (1) No modification of access points, (2) No hardware (H/W) modification of VoIP terminals, and (3) Maintain backward compatibility. The first policy enables users to implement the proposed method on existing s. The second one minimizes the impact on implementing the proposed method in VoIP terminals. The third one prevents the proposed method from blocking communications among terminals that are equipped with only conventional techniques. The flow chart of behaviour of STAs equipped with our proposed protocol is shown in Figure 4. After each STA connects to, each STA decides its own transmission timing using the distributed transmission scheduling. We call this stage the observation phase. After that, the scheduled transmission phase follows. In the scheduled transmission phase, each STA transmits packets using the dynamic priority setting. It is also possible to re-schedule transmission timing if necessary. Our proposed techniques are premised on s and STAs which host Unscheduled Automatic Power Save Delivery (U-SD), which is defined in the IEEE82.11e. The U-SD is intermittent reception mechanism that transmits a packet upon receiving a packet from an STA. techniques Association (conventional methods) Observation phase: Distributed Transmission Scheduling Scheduled transmission phase: Dynamic Priority Setting Yes Re-scheduling No Figure 4: Flow chart of STA equipped with our proposed protocol Table 1: An example of scheduling table Order : MAC address xx:xx:xx:1:1:3 xx:xx:xx:3:2:6 xx:xx:xx:7:9:2 :

3 The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'7) B. Distributed Transmission Scheduling Because VoIP application generates packets to transmit at a constant interval, each STA has only to decide a schedule in a certain VoIP codec period. Each STA in single-cell environment proceeds as follows. First of all, in order to detect other STAs, each STA observes down-link packets from and reads the destination address field in the MAC header. This enables each STA to recognize others. Secondly, each STA makes a list of MAC addresses of STAs which belong to the same. The final step in this phase is to sort the MAC addresses in ascending order and make a scheduling table such as Table 1. These enable all the STAs in the same cell to share a scheduling table. During this phase, STAs transmit packets using. C. Dynamic Priority Setting After the scheduling table is made, each STA creates a virtual slot periodically. Each STA sets the priority of its virtual slot according to the scheduling table. Specifically, each STA dynamically changes its AIFSN, CWmin and CWmax. These inverted numbers correspond to the priority of proposed technique and (solid line) and (dotted line) in Figure 5. This makes it possible to realize the scheduled sequences shown in Figure 6. The Best Effort STA in Figure 5 and 6 is not equipped with the proposed techniques. The virtual slot of a VoIP STA is shifted to the next VoIP STA in the length of the time required for transmission of up-link and down-link packets. Moreover, the duration of a virtual slot is longer than the length of the time required for transmission of up-link and down-link packets. Therefore, the virtual slot of a VoIP STA partially overlaps with that of the next STA. This leads to flexible scheduling which prevents the Best Effort Voice Codec Period Tx Priority Figure 5: Dynamic priority setting scheduled sequences from collapsing due to interrupts from the previous VoIP STA or STA equipped only with conventional techniques. If the scheduled sequences collapse due to join of new STA, each STA adopts any of the following methods. One way is for the new STA to transmit a re-scheduling request packet in a broadcast manner. The other is for any STA connected already to detect frequent failures of reception of an acknowledgement (ACK) from and then transmit a re-scheduling request packet in a broadcast manner. D. Application to Multiple-Cell Environment The number of the available frequency channels is very limited in IEEE82.11 WLAN. This is why it is inevitable that more than one cell with the same frequency channel will overlap. In this section we extend the proposed techniques to implement the distributed transmission scheduling for multiple-cell environment. We consider the scenario in Figure 7 as our example. 1 and 2 use the same frequency. VoIP A, B and C are connected to 1 and VoIP D and E are connected to 2 and lies in the overlapping area of 1 and 2. If the VoIP STAs are equipped with only the technique described in Section III.B, there is a possibility that the virtual slot of will overlap with that of VoIP D and E. This leads to packet collision between and VoIP D/E due to the transmission at the same time. Hence the virtual slots of the VoIP STAs in an overlapping area need to be set in different timing from those of VoIP STAs in not only the same cell but also neighbour cells. The technique proposed to solve the problem is described using Figure 7 and 8. Figure 8 shows the transmission sequences in the proposed technique. At the beginning, each VoIP STA behaves in the manner of Section III.B and makes the scheduling table of VoIP STAs which belong to the same. After that, each VoIP STA determines if packets from other s are being received. This enables to find itself in an overlapping area. searches the virtual slots which are not occupied by the VoIP D/E in neighbour cell. Then transmits a slot request packet including the virtual slot number which is not occupied by the VoIP D/E in a broadcast manner. This leads for and B to receive the necessary information and make the scheduling table under the multiple-cell environment. 1 2 Best Effort Figure 6: Sequences by proposed techniques VoIP D VoIP E Figure 7: Multiple-cell environment

4 The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'7) Virtual Slot A B B.C. C A B A A (xx:xx:xx:1:1:3) (xx:xx:xx:3:2:6) B B.C. B C C (xx:xx:xx:7:9:2) VoIP D (xx:xx:xx:2:3:11) D collision D xx:xx:xx:7:9:2 VoIP E Request:slot1 E E (xx:xx:xx:3:1:12) 2 D E D E Virtual Slot Fig. 8 Multiple-cell scheduling IV. PERFORMANCE EVALUATION A. Simulation Scenarios We evaluated the proposed scheme using the OPNET simulator. The simulation parameters are listed in Table 2. Voice signals are encoded with the G.711 speech codec [9], which generates 16 bytepackets every 2ms with constant bit rate. The simulations were performed in three specific network scenarios, (i) VoIP STAs with the proposed techniques and a Best Effort traffic STA with only in a single cell (left side of Figure 9), (ii) two cells with eight VoIP STAs in each cell (middle of Figure 9) and (iii) three cells with five VoIP STAs in each cell (right side of Figure 9). Let us consider the theoretical number of VoIP STAs that a cell can accommodate if no packet collisions occur. The length of the time required for a set of successful up-link and down-link transmission is given as AIFS STA + BackoffTimeSTA + TVoIP + SIFS + T (4) + AIFS + BackoffTime + TVoIP + SIFS + T where TVoIP is the transmission duration of a VoIP packet and T is the transmission duration of an packet. For IEEE82.11b, aslottime is 2 μs and SIFS is 1 μs. Therefore AIFSSTA and AIFS are, respectively, 5 μs and 3 μs from Equation (1) and Table 2. The average BackoffTime is derived from CW / 2 aslottime, so the average values of min PHY model Voice Payload Retry Limit AIFSN CWmin CWmax Table 2: Simulation condition Voice Codec Period Distribution of Best Effort traffic MAC payload size of a Best Effort packet IEEE82.11b 11Mbps Short preamble 16byte 2ms 7 : 1 STA (in virtual slot): 2 STA (others): 3 : 1 STA (in virtual slot): 7 STA (others): Exponential 15 VoIP STAs & Best Effort STA 8 VoIP STAs 16 VoIP STAs 5 VoIP STAs 8 VoIP STAs Overlapping area 15 VoIP STAs Figure 9: Simulation scenarios 5 VoIP STAs 5 VoIP STAs BackoffTimeSTA and BackoffTime are, respectively, 7 μs and 1 μs. TVoIP is given as T VoIP = PLCP preamble + PLCP header / PLCP rate + ( MAC header + LLC header + IP header + UDP header + header + Voice Payload + FCS) / rate = 268μs T = PLCP preamble + PLCP header / PLCP rate + ( + FCS) / rate = 152μs Assigning the values above to Equation (4), the length of the time required for a set of successful uplink and down-link transmission is 19 μs. Hence the theoretical number of VoIP STAs which can be accommodated in a voice codec period (2ms) is as follows. 3 floor {2 1 /119} = 19 (5) B. Simulation Results and Discussions Figures 1 and 11 depict the packet loss rate and the average delay of VoIP traffic, respectively, under the environment that both VoIP STAs and Best Effort STA (offered load =, 2 and 4Mbps) coexist in a single cell. The packet loss rate and the delay of the proposed techniques in solid lines are lower than those of in dotted lines. The packet loss rate of is higher than 3 % which is a target value defined by ITU-T G.11 [1] when the number of VoIP STAs is higher than 1 STAs. The proposed techniques can support approximately 16 VoIP STAs that is near the theoretical number obtained by Equation (5). This shows that the proposed techniques can effectively suppress packet collision. Figure 12 illustrates the throughput of Best Effort traffic when the number of VoIP STAs is 5, 1 and 15. As can be shown from these results, the throughput of Best Effort STA, which is not equipped with the proposed techniques, is also improved. Figure 13 and 14 shows the packet loss rate and the average delay under two-cell environment of scenario (ii) and threecell environment of scenario (iii), respectively. As is clear from these figures, both the packet loss rate and the delay of the proposed scheduling techniques are lower than those of. Moreover, the multiplecell scheduling is more effective than the single-cell scheduling. Hence, the proposed techniques improve the performance under the multiple-cell environment

5 The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'7) (OL=2M) (OL=4M) (OL=2M) (OL=4M).8.6 (PLR) Single-cell scheduling(plr) Multiple-cell scheduling(plr) (Delay) Single-cell scheduling(delay) Multiple-cell scheduling(delay) Number of VoIP STAs Figure 1: Packet Loss Rate of VoIP in scenario (i) (OL=2M) (OL=4M) (OL=2M) (OL=4M) Number of VoIP STAs in overlapping area Figure 13: Packet Loss Rate and Average Delay in scenario (ii) (PLR) Single-cell schduling(plr) Multiple-cell scheduling(plr) (Delay) Single-cell scheduling(delay) Multiple-cell scheduling(delay) Number of VoIP STAs Figure 11: Average Delay of VoIP in scenario (i) (VoIP=5STAs) (VoIP=1STAs) (VoIP=15STAs) (VoIP=5STAs) (VoIP=1STAs) (VoIP=15STAs) Offered Load [Mbps] Figure 12: Throughput of Best Effort in scenario (i) as well. V. CONCLUSIONS This paper presented a MAC protocol that provides effective QoS for VoIP over WLAN. The characteristics of our proposed protocol are (1) No modification of access points, (2) No H/W modification of VoIP STAs and (3) Backward compatibility in order to minimize the costs of development and introduction. Simulation showed that our proposed protocol can increase the number of accommodated VoIP calls by approximately 5%. As future works, we will implement our proposed protocol in a testbed and evaluate its performance Number of VoIP STAs in overlapping area Figure 14: Packet Loss Rate and Average Delay in scenario (iii) REFERENCES [1] IEEE, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, Reference number ISO/IEC :1999(E), IEEE Std 82.11, 1999 edition, [2] F. Anjum, M. Elaoud, D. Famolari, A. Ghosh, R. Vaidyanathan, A.Dutta, and P. Agrawal, Voice Performance in WLAN Networks - An Experimental Study, Globecom23, pp , 23. [3] M. Elaoud, D. Famolari and A. Ghopsh, Experimental apacity Measurements for 82.11b WLANs, CCNC 25, Jan. 25. [4] K. Medepalli, P. Gopalakrishnan, D. Famolari and T. Kodama, Voice Capacity of IEEE 82.11b, 82.11a and 82.11g Wireless LANs, Globecom24, pp , 24. [5] IEEE standard for information technology - specific requirements part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications: Amendment 8: Medium access control (MAC) quality of service enhancements. IEEE Standard 82.11E-25, 25. [6] S. Mangold, S. Choi, G. Hiertz, O. Klein and B. Walke, Analysis of IEEE 82.11E for QoS Support in Wireless LANs, IEEE Wireless Communications, vol. 1, no. 6, pp. 4-5, Dec. 23. [7] P. Engelstad and O. Osterbo, Non-saturation and Saturation Analysis of IEEE 82.11e with Starvation Prediction, with Starvation Prediction 8th ACM international symposium on Modeling, analysis and simulation of wireless and mobile systems, 25. [8] Y. Lin and V. Wong, Saturation Throughput of IEEE 82.11e Based on Mean Value Analysis, In Proceedings of the IEEE Wireless Communications and Networking Conference 26, Las Vegas, USA, April 26. [9] ITU-T Recommendation, Pulse Code Modulation (EM) of Voice Frequencies, ITU-T G.711, [1] ITU-T G.11, 21.