RealMedia Streaming Performance on an IEEE 802.11b Wireless LAN T. Huang and C. Williamson Proceedings of IASTED Wireless and Optical Communications (WOC) Conference Banff, AB, Canada, July 2002 Presented by Feng Li lif@cs.wpi.edu CS577 Spring 2005

Introduction Three fast-growing Internet technologies World-Wide Web – TCP/IP to the masses Multimedia streaming – real-time, on-demand audio/video to the home Wireless networks – freedom from physical constraints of wires (anything, anytime, anywhere)  All available and relative low cost This paper explores the convergence of the 3 Focus on Real Media (popular) Focus on IEEE 802.11b (popular) 11/19/2018 CS577 Spring 2005

Objectives Characterize network traffic by Real Media Useful for capacity planning Useful for building simulations/models Relationship between wireless channel (error rate, delay, etc) and user quality Use wireless “sniffer”, correlate with application Ascertain impact of streaming on competing (ie- TCP) traffic Impact of streaming on Internet traffic of interest 11/19/2018 CS577 Spring 2005

Outline Introduction (done) Background Methodology Results Related Work Conclusions 11/19/2018 CS577 Spring 2005

IEEE 802.11b Wireless LAN (1 of 2) High speed (up to 11 Mbps, 11g up to 54) Specifies physical layer and MAC layer Physical layer allows 1, 2, 5.5, 11 Mbps Higher rates achieved by using sophisticated modulation Header transmitted at 1 Mbps with clocking information (so payload can be transmitted faster) Physical layer has loss, fading and interference Result in corrupted packets, especially at high rates So, dynamically adjust rates based on channel error rate 11/19/2018 CS577 Spring 2005

IEEE 802.11b Wireless LAN (2 of 2) Is shared broadcast, so MAC layer regulates access Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) or Distributed Coordination Function (DCF) If station wants to send, senses channel If idle for frame time, send packet Otherwise, wait until idle + another frame time + random (double random time) Data sent requires ACK. No ACK, then resend. Give up after 4 tries. Receiver ignores if CRC error. Can be Infrastructure mode (AP) or ad-hoc mode (peer-to-peer) 11/19/2018 CS577 Spring 2005

Real Networks Streaming Media (1 of 2) Server RTSP Data: TCP or UDP Buffering Sure Stream Scalable Video Technology Repair Server-Player protocol RTSP for control UDP or TCP for data Progressive Networks Architecture (PNA) for backward compatibility HTTP if going through a firewall Buffering (also called preroll) for smoothness in the face of bandwidth changes SureStream different bandwidth encodings of the same clip can switch on fly Scalable video technology Data and frame rate controlled by client speed Audio data gets priority. If bwdith left, then video. Repair Forward Error Correction Interleaving (at least for audio) 11/19/2018 CS577 Spring 2005

Real Networks Streaming Media (1 of 2) Codec, server, client Reliable or unreliable Live or on-demand Header identifies “Key frames”, decide to retransmit Streaming rate RTSP for communication Control in TCP, data UDP Parameters during session 11/19/2018 CS577 Spring 2005

Outline Introduction (done) Background (done) Methodology Results Related Work Conclusions 11/19/2018 CS577 Spring 2005

Experimental Environment (1 of 2) Real Server 8.0, Linux, 1.8 GHz P-4, 10 Mbps NIC RealPlayer 8.0, 800 MHz P-3, Cisco Aironet 350 NIC AP lucent RG-1000 WAP, Retransmit limit set to 4 11/19/2018 CS577 Spring 2005

Experimental Environment (2 of 2) Video of a rock concert Target rate about 200 kbps above modem, below broadband Short clip 11/19/2018 CS577 Spring 2005

Experimental Design Streaming with and without TCP/IP traffic Classify wireless Based on OS status meter TCP background gener- ated from server to client Three traces per experiment Trace at server using tcpdump Trace close to AP using sniffer Trace at client using tcpdump Get wireless and higher layers 11/19/2018 CS577 Spring 2005

Outline Introduction (done) Background (done) Methodology (done) Results Related Work Conclusions 11/19/2018 CS577 Spring 2005

Baseline Throughput Results Use netperf for 60-seconds, 84 KB receive socket buffer, 8 times Weaker signal, lower throughput Maximum observed, 4.6 Mbps, less than 11 10 Mbps Ethernet not bottleneck Only Poor has too low a throughput 11/19/2018 CS577 Spring 2005

Subjective Assessment Playback very smooth for Excellent and Good * For Fair, playback was jerky (lost frames?), but visual quality was good Audio was good for Fair-Excellent For Poor, playback was jerky, some pictures blurry and truncated, audio deteriorated In some cases, setup failed 11/19/2018 CS577 Spring 2005

Effect of Wireless Channel (1 of 2) 11/19/2018 CS577 Spring 2005

Effect of Wireless Channel (2 of 2) - App has different view of channel - Mostly, expects to be static Bursty loss Still residual errors 11/19/2018 CS577 Spring 2005

Application Layer Streaming Rate (1 of 2) Initial phase (10-20 sec) is higher rate (about 3x) Audio always meets target rate (Real favors audio) Excellent and Good similar, meet target video Fair and Poor well below target rate - 17.5 kbps, 12.1 kbps 11/19/2018 CS577 Spring 2005

Application Layer Streaming Rate (2 of 2) Excellent and Good similar, meet target video Fair and Poor well below target rate - 17.5 kbps, 12.1 kbps 11/19/2018 CS577 Spring 2005

Application-Layer Retransmission NACK based approach reasonable for lost packets Excellent does not lose any Raw loss: - Good has 0.3% - Fair has 10% - Poor has 30% Effective loss: - Excellent and Good have none - Fair has 0.2% audio, 1.3% video (it looked good) - Poor had 7% audio, 28% video (deteriorating) 11/19/2018 CS577 Spring 2005

Is That True? One statement: “In our experiment, the only packets that miss the deadline are retransmitted packets.” page 6, left column. So I doubt this statements: Because some retransmitted packets may meet the deadline. I think the number of retransmitted packets should be greater than what they listed in their paper. 11/19/2018 CS577 Spring 2005

Streaming with Competing Traffic Excellent channel 10, 20, 30 ,40, 50 competing bulk-TCP Should be 460, 230, 150, 115, 92 kbps Asks for more than fair share so not TCP-Friendly 11/19/2018 CS577 Spring 2005

Outline Introduction (done) Background (done) Methodology (done) Results (done) Related Work Conclusions 11/19/2018 CS577 Spring 2005

Related Work No wireless streaming (“To the best of our knowledge…”) Mena et al RealAudio [11] Non-TCP friendly, periodic Wang et al RealVideo [19] Average 10 fps, little full-motion video Loguinov et al MPEG-4 emulation [10] Modem, jitter, asymmetry Chesire at al University workload (Levy) 11/19/2018 CS577 Spring 2005

Conclusions Wireless channel has bursty loss but MAC layer retransmission can hide Application layer takes care of most of rest Good and Excellent fine for some streaming Fair and Poor have degraded quality With TCP traffic, RealPlayer not fair 11/19/2018 CS577 Spring 2005

Discussion: Shortcomings of their experiments? Subjective Assessment of Streaming Quality. Qualitative Characterization of wireless conditions, based on the Link Status Meter on the Cisco Aironet 350 devices. (eyeball tests)? 68 secs video and low encoding bitrate.. However, in figure 5. From figure 5, the play back duration should be greater than 90 secs with poor signal strength. So I am asking one experiment is enough ? ( variability in throughput, and scaling?) 11/19/2018 CS577 Spring 2005

Future Work? 11/19/2018 CS577 Spring 2005

Future Work Larger scale study (more videos, encodings, …) Effects of mobility Effects on other users on WAP Fragmentation to reduce loss Other technologies (WSM …) Estimating capacity 11/19/2018 CS577 Spring 2005

References Mark Claypool, slides for CS529 http://www.cs.wpi.edu/~cs529/f04/slides/KW02.ppt 11/19/2018 CS577 Spring 2005