Experimental Assessment of Media Synchronization Quality in IEEE 802.11b under Bluetooth Interference Masami KATO, Hirotsugu OKURA, Kiyoshige ITO and Shuji TASAKA Digital Systems Development Center BU, Department of Computer Science and Engineering, SANYO Electric Co., Ltd., Gifu 503-0195, Japan Nagoya Institute of Technology, Nagoya 466-8555, Japan {kato, ookura}@gf.hm.rd.sanyo.co.jp {sekureto, tasaka}@inl.elcom.nitech.ac.jp Abstract This paper assesses the media synchronization quality of audio-video transmission over the IEEE 802.11b wireless LAN under Bluetooth interference by experiment. In this situation, the temporal constraints of audio and video may be disturbed, since the Bluetooth interference cause delay jitter due to retransmission and carrier sensing in the MAC layer of IEEE 802.11b. In the experiment, a media server transfers stored video and audio streams to a mobile terminal over an IEEE 802.11b wireless LAN; as radio interference from Bluetooth, two other terminals transmit data over Bluetooth at the same time. We examine the influence of Bluetooth on the audio-video transmission over IEEE 802.11b in terms of the Bluetooth signal level and its data load. We also apply a media synchronization control scheme and confirm its effectiveness. Keywords; Media Synchronization, IEEE 802.11b, Bluetooth, Interference, Continuous Media transmission I. INTRODUCTION Personal wireless networks such as wireless local area networks (WLANs) and wireless personal area networks (WPANs) come into wide use. The WLAN can provide highspeed wireless access to the Internet at home, at office and in hot spot outdoor areas. The WPAN can offer the connectivity between mobile terminals and their peripheral equipments by short-range wireless links. These two different personal wireless networks are required to realize various kinds of applications in ubiquitous networking environments. There are two major personal wireless network technologies using the Industrial, Scientific and Medical (ISM) band: IEEE 802.11b WLAN [1] and Bluetooth [2], which is a famous one of WPANs. These two systems adopt different media access control (MAC) protocols and different spread spectrum techniques from each other. IEEE 802.11b is based on carrier sense multiple access with collision avoidance (CSMA/CA) with a direct sequence spread spectrum (DSSS) technique. On the other hand, Bluetooth operates under a time division duplex (TDD) polling scheme with a frequency hopping spread spectrum (FHSS) technique. However, the interference between the two can occur because both operate in the same frequency band of 2.4 GHz. The interference may cause delay jitter due to retransmission in each MAC layer. Moreover, Bluetooth may degrade the performance of IEEE 802.11b WLANs, since IEEE 802.11b devices sense radio carriers of Bluetooth and postpone their transmissions; this leads to delay jitter. In the case of continuous media transmission (such as audio-video transmission) over the IEEE 802.11b WLAN, the delay jitter due to the interference from Bluetooth can disturb the temporal constraints of audio and video. In order to obtain excellent quality of the media at the destination, we need to keep the temporal constraints of these streams. That is, we must achieve two types of media synchronization: intra-stream synchronization and inter-stream synchronization [3]-[5]. The former is for preservation of the temporal constraints within each media stream, while the latter is for keeping the temporal relationships among multiple media streams. Lip synch, which adjusts the output timing between spoken voice and the movement of the speaker's lips, is a typical example. A variety of studies on media synchronization control schemes have already been reported [3]. Among them, the Virtual Time Rendering () algorithm [5] is an effective one that is applicable to various network environments. We can find several researches on the interference problem between IEEE 802.11b WLANs and Bluetooth in the literature [6]-[9]. In [6], the influence of Bluetooth on the throughput in the WLAN is examined only by simulation. In [7] through [9], the interference between the two is evaluated by experiment. However, these researches used performance measures only at packet-level such as throughput, delay and error rate; they did not assess the media synchronization quality. The media synchronization quality of continuous media transmission over WLANs or Bluetooth has already been evaluated. In [] through [12], Tasaka et al. examine the media synchronization quality in WLANs, but they do not discuss radio interference of any other wireless networks. In [13] and [14], Okura et al. evaluate the media synchronization quality in Bluetooth with white noise, which simulates interference from DSSS systems; however, they do not assess the quality in the IEEE 802.11b WLAN under Bluetooth interference. In this paper, we assess the media synchronization quality of continuous media transmission over the IEEE 802.11b WLAN under Bluetooth interference by experiment. In the experiment, a media server transfers stored video and audio streams to a mobile terminal over an IEEE 802.11b WLAN. As radio interference from Bluetooth, two other terminals transmit data over Bluetooth at the same time. We examine the influence of the signal level of Bluetooth, as well as the interference data load over Bluetooth, on the audio-video transmission. We also apply the algorithm as a media synchronization scheme and confirm its effectiveness. The rest of the paper is organized as follows. Section II describes the specifications of the IEEE 802.11b WLAN and Bluetooth. It also presents the media synchronization scheme used in the experiment. Section III explains the experimental
methodology. Section IV shows and discusses experimental results. II. IEEE 802.11B WLAN AND BLUETOOTH A. IEEE 802.11b WLAN The IEEE 802.11b WLAN is based on CSMA/CA with a DSSS technique. Fourteen RF channels with the bandwidth of 22 MHz each are placed at between 2.401 GHz and 2.495 GHz. Therefore, IEEE 802.11b can be affected by other wireless network systems and microwave ovens using the same ISM band. IEEE 802.11b has multiple transmission rate capabilities; 1, 2, 5.5 and 11 Mb/s, which employ different modulation schemes. It allows the implementation to perform dynamic rate switching with the objective of improving performance. The algorithm for performing the rate switching is outside the scope of the standard. The MAC sublayer defines the distributed coordination function (DCF) and the point coordination function (PCF). DCF is a fundamental access method, which is well known as CSMA/CA. PCF is an optional one, which is based on a polling scheme. In this paper, only DCF is treated. DCF enables automatic medium sharing through the use of the CSMA/CA and a random backoff time following a busy medium condition. In addition, immediate positive acknowledgment (ACK) is used; the sender schedules retransmission, if no ACK is received. B. Bluetooth Bluetooth operates under a TDD polling scheme with a FHSS technique; the time slot duration is 625 µs, and 79 RF hopping channels are placed at between 2.402 GHz and 2.480 GHz with 1 MHz step. Bluetooth attempts to avoid interference by hopping to a new frequency after transmitting or receiving a packet. It should be noted that Bluetooth does not support carrier sensing. Then, Bluetooth may affect other wireless network systems with the same ISM band. Two or more Bluetooth units participating in the same piconet are time- and hop-synchronized; one Bluetooth unit acts as the master of the piconet and controls the communications, whereas the other units behaves as slaves. The master and slaves alternately transmit the packets using either one, three or five time slots. Bluetooth specifies the Asynchronous Connectionless (ACL) link for data transfer. Seven kinds of packets in the ACL link are defined: DM1, DH1, DM3, DH3, DM5, DH5 and AUX1. DM and DH stand for Data Medium rate and Data High rate, respectively. They differ from each other in the kind of error control applied and the number of time slots occupied by a packet. In the experiment in this paper, only the DM1 packet is used. The maximum data rate of the DM1 packet is 8.8 kb/s. C. Basic concept of the algorithm We next present the basic concept of the algorithm, which is applied as a media synchronization scheme in this paper. The temporal structure of continuous media can be disturbed by various causes; in best-effort networks like the Internet and DCF based IEEE 802.11 WLANs, network delay jitter is a dominant one. In this case, we can achieve media synchronization by absorbing the jitter at the destination. This is carried out by buffering the information unit such as a video frame or a voice packet, which is referred to as an MU (Media Unit), for an appropriate period of time. It is clear that the period of time should be the maximum delay jitter. However, we cannot necessarily set the buffering time to this value, because getting the exact value in the best-effort networks is very hard, and even if we can know it, setting the value may destroy the real time property. The media synchronization algorithm [5] assumes no exact knowledge of the network delay jitter and adaptively changes the buffering time according to the amount of delay jitter of MUs received at the destination and MU loss. Initially, the buffering time is set to a rough estimate of the maximum delay jitter, which is denoted by J max ; after the first MU is received, it can be changed by the modification of the target output time, which is the time when the destination should output an MU. The application form of the modification depends on the kind of media treated, i.e., stored or live. In the case of stored media, the target output time is put backward only; this means increase in the buffering time. On the other hand, live media need both forward and backward movement, since the real time property must be preserved. For live media, we can set the maximum allowable delay al so that the modification of the target output time does not make MU delay exceed this limit. In this paper, we transfer stored audio and video over an IEEE 802.11b WLAN. A video frame is defined as a video MU, and an audio packet consisting of a constant number of audio samples as an audio MU. Audio is selected as the master stream and video as the slave stream, which is synchronized to the master. This is because audio is more sensitive to intra stream synchronization error than video. Only the master stream can modify the target output time for itself, and accordingly the slave stream modifies it by the same amount at the same time. III. EXPERIMENTAL METHODOLOGIES This section describes the configuration of the experimental system, experimental conditions and performance measures used in the experiment. A. Experimental System We developed an experimental system shown in Fig. 1; it is composed of a media server, an IEEE 802.11b access point, a client terminal with an IEEE 802.11b PC card, two data terminals (DT1 and DT2) each with a Bluetooth unit, and a simulated wireless environment.
As for the IEEE 802.11b WLAN system, the media server (Pentium IV 1.7GHz, RedHat Linux7.3) stores a MPEG-1 video file and the corresponding G.711 µ-law audio file; it sends out the video and audio streams to the client terminal (Pentium III 1GHz, RedHat Linux7.3) on demand by UDP. The media server is connected to the IEEE 802.11b access point (MELCO Inc., WLA-L11G) by a 0BASE-T Ethernet. The access point and the client terminal with the PC card (Agere Systems Inc., ORiNOCO Gold PC Card) communicate with each other over the IEEE 802.11b WLAN. From among the 14 RF channels, we have selected channel 1, whose center frequency is 2.412 GHz. The nominal output power is 15 dbm, and no power control scheme is applied. As for the Bluetooth system, DT1 (Celeron 700MHz, RedHat Linux7.1) sends a data load as interference traffic to DT2 (Pentium IV 1.5GHz, RedHat Linux7.3) by only DM1 packets over a Bluetooth link. Note that no IP protocol is applied in this link; the Bluetooth link emulates a serial cable connection. DT1 and DT2 are connected to individual Bluetooth units each by a serial interface, whose speed is 115.2 kb/s. The Bluetooth unit has a Bluetooth module (ERICSSON, ROK 1 007), whose maximum output power is 4 dbm. The simulated wireless environment consists of power dividers/combiners (PD/C) and variable attenuators, all of which are connected together with coaxial cables. The output of the access point is input to a variable attenuator that can change the attenuation by db in the range from 0 to 121. Also, the output of Bluetooth unit with DT1 is input to another Media Server IEEE 802.11b Access Point DT1 Ethernet Simulated Wireless Environment Serial Interface Figure 1. TABLE I. item coding scheme image size Picture pattern average MU size Bluetooth Unit original average MU rate original average bit rate original recording time Variable ATT Power Divider / Combiner Variable ATT Power Divider / Combiner Data : Coaxial Cable ATT : Attenuator Client Terminal IEEE 802.11b PCMCIA Card Bluetooth Unit SPECIFICATION OF AUDIO AND VIDEO. audio ITU-T G.711 µ-law - - 00 bytes 8 MU/s 64 kb/s Audio & Video Spectrum Analyzer Protocol Analyzer Configuration of the experimental system. 90 s video MPEG-1 320 x 240 pixels DT2 IBBPBBPBBPBBPBB 5000 bytes 20 MU/s 533 kb/s variable attenuator. The output signals of the two attenuators are combined at a PD/C. The mixed signal is divided at another PD/C among the client terminal, DT2, a protocol analyzer (WildPackets Inc., AiroPeek NX) and a spectrum analyzer. The protocol analyzer captures packets over IEEE 802.11b and displays some kinds of statistics such as the number of packets with CRC error, the number of packets sent with each transmission rate, and so on. The spectrum analyzer measures signal levels of IEEE 802.11b and Bluetooth. In the experiment, we adjusted the two variable attenuators. In order to control the received signal level of Bluetooth at the client terminal, we changed the value of the attenuator connected to the Bluetooth unit. We also adjusted the other attenuator so that the received signal level of IEEE 802.11b at the client terminal can be high enough for its internal thermal noise to be negligible. Thus we have set the received peak signal level of IEEE 802.11b to about 45.6 dbm in the center frequency. It should be noted that owing to the configuration of the PD/C, the client terminal can sense signals transmitted by DT1 as interference, while it cannot receive signals sent by DT2. In the same way, the access point perceives interference signals from DT2, while it does not notice signals from DT1. B. Conditions of the experiment We assessed the media synchronization quality of the audio-video transmission over the IEEE 802.11b WLAN under Bluetooth interference by changing the interference signal level of Bluetooth or the interference data load over Bluetooth. In theses experiments, we used a lady s voice and her head view video as the audio stream and video stream, respectively. Table I shows the specifications of the audio and video. In the case of applying the algorithm for media synchronization control, we set the value of the initial buffering time J max to 0 ms. We also set the value of the maximum allowable delay al to infinity because we treat stored media in the experiment. C. Performance Measures In order to assess the quality of the media synchronization, we employ measures used in previous studies on media synchronization [4], []-[14]. For the quality assessment of intra stream synchronization for audio or video, we use the mean square error of intra stream synchronization, which is the average square of the difference between the output time of each MU and the target output time of the MU. The error represents how accurately the temporal structure of each stream is preserved. For the inter stream synchronization quality, we also use the mean square error of inter stream synchronization, which denotes the average square of the difference between the output time of each slave MU and that of the corresponding master MU plus the relative generation time of the slave MU to the master MU. We also adopt the MU loss rate, which is defined as the ratio of the number of MUs lost somewhere to the total number of MUs.
In addition, we employ the occupancy ratio of the transmission rate and the number of packets with CRC error as performance measures in lower layers. The former is defined as the ratio of the number of packets sent with the designated transmission rate to the total number of transmitted packets. IV. EXPERIMENTAL RESULTS As mentioned earlier, we examine the influence of the interference signal level of Bluetooth and that of the interference data load over Bluetooth at the client terminal. In this section, we present the experimental results. A. Influence of the interference signal level of Bluetooth on IEEE 802.11b We first evaluate the influence of the interference signal level of Bluetooth on the audio-video transmission from a media synchronization point of view. In each figure presented below, the interference signal level of Bluetooth, which is denoted by S BT in this paper, is defined as the average of three peak signal levels which were measured by the spectrum analyzer on three hopping channels; the lowest, the middle and the highest frequency channels (i.e., 2.402, 2.441 and 2.480 GHz). The data load over Bluetooth was set to 90 kb/s, which implies a heavy load in the case of using only DM1 packets. We conducted the experiment five times for each symbol and plotted the average. Figures 2 and 3 display the mean square error of interstream synchronization and that of intra-stream synchronization for audio and video, respectively, as a function of S BT. In these figures, and denote the result of the system with the algorithm and the corresponding result of the system with no control, respectively. Fig. 4 shows the occupancy ratio, i.e., 11, 5.5, 2 and 1 Mb/s, which are plotted by square, triangle, circle and cross symbols, respectively. In addition, Figs. 5 and 6 present the number of packets with CRC error and the MU loss rate of video, respectively. We first notice in Figs. 2 and 3 that the mean square error of is much smaller than that of for all the values of S BT. This means that the algorithm is effective in achieving good quality of media synchronization. To examine the quality of inter-stream synchronization in more detail, we utilize Steinmetz's report on human perception of jitter [15]: A skew of less than 80 ms between audio and video (i.e., a mean square error less than 6400 ms 2 ) attains good quality of interstream synchronization, while a time difference beyond ±160 ms (a mean square error larger than 25600 ms 2 ) corresponds to asynchrony. The criteria tell us that the algorithm can attain excellent quality of inter-stream synchronization even if the value of S BT is large. In the following discussion, we focus only on ; most of the following consideration for is applicable to the case of. Note that Bluetooth interference may cause the retransmission due to bit errors, the fallback of the transmission rate and MU loss. We explain the details below. We see in Figs. 2 and 3 that the mean square error of increases as the value of S BT becomes larger. This implies that the Bluetooth interference degrades the media synchronization quality when the value of S BT becomes large. This is because the Bluetooth interference causes bit errors, which are detected as CRC errors at the client terminal (see Fig. 5). Then, the packet retransmissions due to the CRC error disturb the temporal relations. Note that in Fig. 5 the number packets with CRC error has local peaks around 52 dbm and 46 dbm, where the occupancy ratio is radically changed (see Fig. 4) as mentioned later. We also find that the two kinds of mean square errors rise up largely at two values of S BT ; one is approximately 46 dbm and the other is about 40 dbm. We first examine the system s behavior around 46 dbm. We find in Fig. 4 that the occupancy ratio of the transmission rate is radically changed around this value; the transmission rate of IEEE 802.11b falls down to 1 Mb/s from 5.5 Mb/s. Since the difference between the transmission rate of 1 Mb/s and the sum of the average bit rates for audio and video, which is approximately 600 kb/s, is comparatively small, the MU delay is often affected by the MU size. Then, MU delay jitter becomes large and disturbs the temporal relations. It should be noted that the occupancy ratio of the transmission rate is also radically changed in Fig. 4 when the value of S BT is around 52 dbm; the transmission rate of IEEE 802.11b falls down to 5.5 Mb/s from 11 Mb/s. However, the degradation of the media synchronization quality is low in this case. This is because the transmission rate is sufficiently large compared to the sum of the average bit rates for audio and video. We next consider the interval where the value of S BT is 40 dbm or more. In this situation, the media synchronization quality is largely degraded, since the MU loss occurs frequently (see Fig. 6). The reason for the MU loss is as follows. The large values of S BT force the fallback of the transmission rate to 1 Mb/s (see Fig. 4). Since the high interference signal level also makes high bit error rate, many CRC errors are detected at the client terminal (see Fig. 5); therefore, many packets are retransmitted over IEEE 802.11b. Consequently, the buffer overflow at the access point may occur. Of course, the access point may sense the Bluetooth interference signals and delay the transmissions of packets. In this configuration of the experimental system, however, note that the access point can sense the interference signal only from DT2; it cannot sense that from DT1. That is, the carrier sense at the access point is independent of the value of S BT ; only the fixed signal level from DT2 affects the carrier sense at the access point. The evaluation of the influence of the interference signal level from DT2 on the carrier sense at the access point is one of our future studies. B. Effect of the interference data load over Bluetooth on IEEE 802.11b We next evaluate the influence of the interference data load over Bluetooth. In the experiment, we varied the value of the interference data load between kb/s and 90 kb/s with kb/s step. We measured the performance for three different values
of S BT, 47, 46 and 45 dbm, which are plotted by circle, triangle and asterisk symbols, respectively, in each figure presented below. We have selected these three values since they are around 46 dbm where the occupancy ratio of the transmission rate is radically changed between 5.5 Mb/s and 1 Mb/s (see Fig. 4). We measured ten times for each symbol and plotted the average. We show the results of only because of limitations of space. Figures 7 and 8 show the mean square error of inter-stream synchronization and that of intra-stream synchronization for audio and video, respectively, as a function of the interference data load. Fig. 9 presents the occupancy ratio of the transmission rate over IEEE 802.11b for the three values of S BT. We see that Figs. 7 and 8 both have the same tendency for the mean square errors to increase according to the increment of the data load; the increasing rate of the error is large for S BT = 46 dbm, while it is small for S BT = 45 dbm and 47 dbm. We will explain the reason later. This result means that the interference data load causes bit errors at the client terminal and that the packet retransmission due to the CRC error disturbs the temporal relations. In addition, the access point may delay the transmissions to degrade the media synchronization quality, since it senses the interference signal from DT2 more frequently as the data load becomes heavier; DT2 sends DM1 packets with acknowledgement to DT1. We also notice in Fig. 9 that the ratio of 1 Mb/s becomes higher as the data load increases when S BT = 45 dbm and 46 dbm. This result means that the high interference data loads cause the fallback to 1 Mb/s frequently. Referring to Figs.7 through 9, we observe that when S BT = 46 dbm, not only the mean square error but also the ratio is changed largely as the data load becomes larger; the ratio of 5.5 Mb/s decreases, while the ratio of 1 Mb/s increases. On the other hand, both increase of the error and change of the ratio are small when S BT = 45 dbm and S BT = 47 dbm; the major transmission rates for S BT = 45 dbm and that for S BT = 47 dbm are 1 Mb/s and 5.5 Mb/s, respectively, for the entire data loads. The above observations indicate that the media synchronization quality largely depends on the selected transmission rates. V. COLUSIONS We assessed the media synchronization quality of audiovideo transmission over an IEEE 802.11b WLAN under Bluetooth interference by experiment. We examined the influence of the signal level of Bluetooth and that of the interference data load over Bluetooth at the client terminal. We confirmed that the Bluetooth interference degrades the media synchronization quality. The Bluetooth interference brings about the retransmission due to the CRC error, which disturb the temporal relations of audio and video. Moreover, the interference causes the fallback in IEEE 802.11b. Especially, the use of 1 Mb/s transmission rate degraded the media synchronization quality largely. Also, very strong interference signals were a major cause of the quality degradation because of the buffer overflow at the access point. We saw that the application of the algorithm is effective in achieving good quality of media synchronization under Bluetooth interference. We also observed that the interference data load over Bluetooth affects the media synchronization quality. The media synchronization quality largely depends on the selected transmission rates. The interference signal may cause the access point to delay the transmission because of carrier sensing. The systematic evaluation of the influence of the interference on the carrier sense is one of our future studies. REFEREES [1] IEEE standard 802.11b, Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, higher-speed physical layer extension in the 2.4 GHz band, Jan. 2000. 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IEEE PIMRC 2001, pp. D-64 D-70, Sept. 2001. [14] H. Okura, M. Kato and S. Tasaka, The effect of segmentation mismatch on quality of continuous media transmission by Bluetooth, in Proc. IEEE PIMRC 2002, pp. 702 709, Sept. 2002. [15] R. Steinmetz, Human perception of jitter and media synchronization, IEEE J. Sel. Areas in Commun., vol. 14, no. 1, pp. 61 72, Jan. 1996. Figure 1..
Mean square error of inter-stream synchronization [ms 2 ] synchronization [ms 2 ] (video) synchronization [ms 2 ] (audio) 1E+0 Figure 2. Mean square error of inter-stream synchronization 1E+5 1E+0 1E+5 (a) Video Figure 4. over IEEE 802.11b Number of packets with CRC error MU loss rate [%] (video) 800 700 600 500 400 300 200 0 0 Figure 5. Number of packets with CRC error in IEEE 802.11b 60 50 40 30 20 1E+0 (b) Audio Figure 3. synchronization 0 Figure 6. MU loss rate of video
Mean square error of inter-stream synchronization [ms 2 ] synchronization [ms 2 ] (video) 45dBm 46dBm 47dBm 20 30 40 50 60 70 80 90 S BT Figure 7. Mean square error of inter-stream synchronization versus data load over Bluetooth. S BT 45dBm 46dBm 47dBm 20 30 40 50 60 70 80 90 (a) Video S BT = 45 dbm 20 30 40 50 60 70 80 (a) S BT = 45 dbm 20 30 40 50 60 70 80 (b) S BT = 46 dbm S BT = 46 dbm 90 90 synchronization [ms 2 ] (audio) S BT 45dBm 46dBm 47dBm 20 30 40 50 60 70 80 90 (b) Audio Figure 8. synchronization versus data load over Bluetooth. S BT = 47 dbm 20 30 40 50 60 70 (c) S BT = 47 dbm Figure 9. over IEEE 802.11b versus data load over Bluetooth. 80 90