Agrafiotis, D

Bull, DR

Chiew, T-K

Ferre, PL

Nix, AR

English

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Multimedia Transmission over IEEE 802.11g WLANs: Practical Issues and Considerations

Multimedia transmission is widely available over wired networks. With the advent of low-cost WLAN devices, the wireless delivery of multimedia content is highly desirable. However, for media requiring low end-to-end latency, the use of WLAN technology introduces many significant challenges. These challenges are further enhanced if multicast/broadcast transmission is employed to serve a wide range of wireless terminals. This paper provides an understanding of the practical issues associated with WLAN multimedia transmission. A cross-layer measurement programme is performed to identify design issues for low-cost off-the-shelf WLAN multimedia systems. Problems identified include i) broadcast/multicast transmission using the slowest link-speed, ii) common link adaptation mechanisms for all clients, iii) lack of a call admission policy, and iv) irreducible PER even in good channel conditions

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                          Ferre, P. L., Agrafiotis, D., Chiew, T-K., Nix, A. R., & Bull, D. R. (2007).Multimedia transmission over IEEE 802.11g WLANs: practical issues andconsiderations. In International Conference on Consumer Electronics, LasVegas, USA. (pp. 1 - 2). Institute of Electrical and Electronics Engineers(IEEE). 10.1109/ICCE.2007.341314Link to published version (if available):10.1109/ICCE.2007.341314Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.Multimedia Transmission over IEEE 802.1 1g WLANs:Practical Issues and ConsiderationsP. Ferre', D. Agrafiotis', T.K. Chiew2, A.R. Nix"13 and D.R. Bull"13'University of Bristol, UK, 2Institute for Infocomm Research, Singapore, ProVision Communications, UKAbstract: Multimedia transmission is widely available overwired networks. With the advent of low-cost WLAN devices,the wireless delivery of multimedia content is highly desirable.However, for media requiring low end-to-end latency, the useof WLAN technology introduces many significant challenges.These challenges are further enhanced if multicast/broadcasttransmission is employed to serve a wide range of wirelessterminals. This paper provides an understanding of thepractical issues associated with WLAN multimediatransmission. A cross-layer measurement programme isperformed to identify design issues for low-cost off-the-shelfWLAN multimedia systems. Problems identified include i)broadcast/multicast transmission using the slowest link-speed,ii) common link adaptation mechanisms for all clients, iii) lackof a call admission policy, and iv) irreducible PER even in goodchannel conditions.I. INTRODUCTIONThere is growing interest in wireless multimedia deliveryfor the home and corporate (e.g. hotels and hospitals)environment. Wireless Local Area Network (WLAN) [1]extensions to existing wired infrastructure are growingrapidly, often in the form of hot-spots. The cost of WLANequipment is falling fast and most portable terminals(laptops, PDAs and even cell phones) include a WLANmodem as standard. The use ofWLAN technologies [2]-[3]for data communication is well established. Here theTransport Control Protocol (TCP) is used on top of theInternet Protocol (IP) to correct for missing or corrupteddata packets. The transmission of video files via TCP-IPover WLAN allows the non-real-time playback of videofiles at the client. Near real-time video streaming via TCP-IP allows video playback without the need to store a localcopy of the file. Since TCP cannot offer a time-boundedservice, for low-latency video applications the video packetsare sent using the User Datagram Protocol (UDP). WhileUDP offers a time bounded service, it does not guaranteethe delivery of the video packets. To use this protocolsuccessfully, the video codec must support strong errorresilience and concealment. Furthermore, some videoapplications require transmission to a large number ofremote terminals. Due to a lack of radio bandwidth, in thesesituations a dedicated point-to-point UDP-IP transmissioncannot be accommodated with each remote client. Instead,Broadcast (or Multicast) transmission must be used at theWLAN. For these types of link there is absolutely noretransmission (ARQ) of errored or missing data packets(either at the WLAN MAC or the transport layer).As discussed above, wireless UDP based unicast andmulticast/broadcast transmissions pose the problem ofpacket erasures. The lossy nature of the transmissionmedium can lead to unacceptable video quality at the client.As a result, it is important to consider the channel behaviourI This work was performed as part of the EU FP6 WCAM projectin terms of packet delay, packet loss rate and datathroughput. Moreover, for low-latency multimedia there arepractical issues that can affect the performance and usabilityof the system. This paper illustrates these issues byanalysing the cross-layer performance of a number ofWLAN links. Observations are made after collecting andprocessing large sets of measurement data'. The paperprovides an insight into the critical problems that arise insuch scenarios and suggests a number of possible solutions.II. PLATFORM AND MEASUREMENT DESCRIPTIONTo collect measurement data, a platform consisting of across-layer enabled client/server software pair running onWindows XP-based laptops was developed [4]. This allowsus to log and analyse a large range of parameters (across thetransmission and protocol stacks) from the application layerto the physical (PHY) layer. The software enables us tocharacterise key issues that affect the radio performance,and that potentially degrade or prevent video delivery. Thelaptops use IEEE 802.1 1g Network Interface Cards (NICs),while the server is connected to a BelkinR IEEE 802.1 IgAccess Point (AP) (High-Speed Mode Wireless G Router).Three Unicast UDP links were configured at the AP witha load that increases over time. Using a fixed link-speed,each client makes the same requests to the server in terms ofpacket size and bit rate. This is illustrated in figure 1 for atarget rate of 8 Mbits/s and a packet size of 300 and 1200bytes. This example highlights the strong impact of packetlength on the radio performance, and therefore its possibleimpact on video performance. The data shows (figure 1 -left) that with small packets lengths, the network can onlysupport a single client at 8Mbits/s. When clients 2 and 3join the network, thus increasing the load to 16 and24Mbits/s respectively, the packet loss rate was seen toincrease to 50% and 80% respectively. The target rate of8Mbits/s cannot be maintained and drops to around5Mbits/s and 1.8Mbits/s respectively for each client. Weconclude that the network cannot support 3 links at such ahigh data rate when small packets are used for transmission.III. KEY ISSUESThe following key issues have been identified:i) Broadcast transmission: Figure 2 shows broadcasttransmission for a packet size of 600 bytes and a targetvideo rate of 700kbits/s and 1 Mbits/s. In broadcast, theWLAN can only support application data rates of700kbits/s. Only a small amount of transmission error isseen at the lower rate. These errors can be consideredinsignificant when error resilient video codecs are used.1-4244-0763-X/07/$20.00 ©2007 IEEE7.4-2Figure 1: UDP Static Unicast - Channel behaviour - Video Bit AFigure 2.Brodcastt Cha behaIS1vioU7 11 P9acke Siz700_(°400300_2000 0 20 30 40 500.0150.01 r a r0.005__0( 0 20 30 4050Figure 2: Broadcast Static- Channel behaviour - Packet SizeAt IMbits/s the performance is unacceptable due to a lackof broadcast bandwidth at the AP. This occurs since the APuses the lowest available link-speed (1 Mb/s) to optimise thebroadcast coverage area. This severely limits the quality andquantity of broadcast video that can be sent from the AP.ii) Near-Far problem 1: In this scenario a static client ispositioned close to the AP, while a mobile client moves tothe edge of the hot-spot. As the mobile client moves awayfrom the AP, reception is seen to deteriorate. Moresurprisingly, the static client close to the AP starts toexperience poor reception. This is explained by the fact thatmany low-cost APs uses a common link-speed on thedownlink for all connected clients, and thus adapt their link-speed using an average of the packet statistics from all links.If one client experiences poor channel conditions then thiswill degrade the overall set of statistics. As a result, the APreduces the link-speed for all associated clients. Toovercome this problem it is necessary for the AP toimplement link-adaptation on a 'per peer-station' basis. Aset of statistics must be maintained at the AP for each peer-station (client). Link adaptation can then occur optimally foreach client.iii) Near-Far problem 2 (lack of "call admission'): In asimilar scenario to that described above, the client far fromthe AP experiences poor channel conditions and thus adaptsto the slowest, but most reliable, link-speed. UDP and TCPWLAN links make strong use of MAC level ARQ. Hence,given the poor channel conditions, the far client consumesconsiderable resource from the AP (since the link-speed islow and many packets must be resent). The far terminaleffectively steals bandwidth from the high quality clientsnear to the AP. Packets intended for 'near' clients cannot besent until the transmission cycle to the 'far' client iscomplete (which could require in excess of 30retransmissions). One solution to overcome this 'bandwidthstarvation' phenomenon is to implement a Call Admissionpolicy that rejects users that require the use of the lowest?ate = 8Mbits/s, Packet Size = 300 bytes(left) 1200 bytes(right).^^BCT(=600)@10OOOKBPS VIDEO=0. STARTS: TUE JUN 07 13:54:21 2005200 _1000_800_400be200__0010 20 30 40500M4 T . T .0.3 1 1 i l 1l 1l 10 10 20 30 40 50t(-e)-600 bytes - Video Bit Rate =700kbits/s (left), lMbits/s (right).link-speed. Alternatively, users that make excessive use ofMAC layer ARQ could be rejected. This would ensure thatonly terminals with good channel conditions cancommunicate with the AP. Without this procedure, a singlepoor user will degrade the performance to all other usersconnected to the AP.v) Irreducible PER: For all our WLAN UDP andbroadcast/multicast measurements a residual applicationlayer PER was observed. This occurred despite MAC levelARQ, and even for terminals in good channel conditions.These errors were related to 1) interference from other co-existing networks; this leads to congestion and collision,and 2) packet loss due to low battery levels and loadfluctuations on the host laptop (operating system calls, anti-virus software, WLAN drivers and utilities). This residualPER is variable and was found to lie at around 0.5% to 2%.IV. CONCLUSIONThis paper has presented a number of practical issues thataffect UDP and multicast/broadcast video transmission overWLANs. Observations were made after post-processingcross-layer data from a number of trials. Key issues fromthis study include i) multicast/broadcast transmission usingthe slowest mode, ii) common link adaptation mechanismsfor all clients at the AP, iii) a lack of 'call admission', andiv) irreducible PER in good channel conditions.[1][2][3][4]REFERENCESIEEE Std 802.11 - 1999, Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) specifications.A. Doufexi, S. Armour, P. Karlsson, M. Butler, A. Nix and D. Bull,"A comparison of the HIPERLAN/2 and IEEE 802.1 1a WirelessLAN Standards", IEEE Com Magazine, May 2002.P. Ferre, A. Doufexi, A. Nix and D. Bull, "Throughput Analysis ofthe IEEE 802.1 1 and IEEE 802.1 le MAC", IEEE WCNC '04.T.K. Chiew, P. Ferre, D. Agrafiotis, A. Molina, A, Nix, and D. Bull,"Cross-Layer WLAN Measurement and Link Analysis for LowLatency Error Resilient Wireless Video Transmission." in Proc. ofIEEE Int. Conf. on Consumer Electronics (ICCE), Jan. 2005.r I00-1Cli.,,t 2 J.i,,i,,g/ --20 30 4Ol/ 10 ,0 70 80 9000000000Cli-t 2 J.i,,i,,g01 0 0 20 30 10 / 10 10 70 80 60-l IT

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Multimedia transmission over IEEE 802.11g WLANs: practical issues and considerations

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Multimedia transmission over IEEE 802.11g WLANs: practical issues and considerations

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