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Lecture 14: Distributed Multimedia Systems

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  1. Lecture 14:Distributed Multimedia Systems Haibin Zhu, PhD. Assistant Professor Department of Computer Science Nipissing University © 2002

  2. Contents • Introduction • Characteristics of multimedia data • Quality of service management • Resource management • Stream adaptation • Summary 2

  3. Learning objectives • To understand the nature of multimedia data and the scheduling and resource issues associated with it. • To become familiar with the components and design of distributed multimedia applications. • To understand the nature of quality of service and the system support that it requires. • To explore the design of a state-of-the-art, scalable video file service; illustrating a radically novel design approach for quality of service. * 3

  4. Video camera and mike Local network Local network Wide area gateway Video Digital server TV/radio server A distributed multimedia system Figure 15.1 • Applications: • non-interactive: net radio and TV, video-on-demand, e-learning, ... • interactive: voice &video conference, interactive TV, tele-medicine, multi-user games, live music, ... * 4

  5. Multimedia in a mobile environment • Applications: • Emergency response systems, mobile commerce, phone service, entertainment, games, ... Global System for Mobile Communications * 5

  6. At the right timeand in the right quantities Characteristics of multimedia applications • Large quantities of continuous data • Timely and smooth delivery is critical • deadlines • throughput and response time guarantees • Interactive MM applications require low round-trip delays • Need to co-exist with other applications • must not hog resources • Reconfiguration is a common occurrence • varying resource requirements • Resources required: • Processor cycles in workstations • and servers • Network bandwidth (+ latency) • Dedicated memory • Disk bandwidth (for stored media) * 6

  7. Application requirements • Network phone and audio conferencing • relatively low bandwidth (~ 64 Kbits/sec), but delay times must be short ( < 250 ms round-trip) • Video on demand services • High bandwidth (~ 10 Mbits/s), critical deadlines, latency not critical • Simple video conference • Many high-bandwidth streams to each node (~1.5 Mbits/s each), high bandwidth, low latency ( < 100 ms round-trip), synchronised states. • Music rehearsal and performance facility • high bandwidth (~1.4 Mbits/s), very low latency (< 100 ms round trip), highly synchronised media (sound and video < 50 ms). http://www.topsavings.net/dedicated-t-1.html * 7

  8. System support issues and requirements • Scheduling and resource allocation in most current OS’s divides the resources equally amongst all comers (processes) • no limit on load • \ can’t guarantee throughput or response time • MM and other time-critical applications require resource allocation and scheduling to meet deadlines • Quality of Service (QoS) management • Admission control: controls demand • QoS negotiation: enables applications to negotiate admission and reconfigurations • Resource management: guarantees availability of resources for admitted applications • real-time processor and other resource scheduling * 8

  9. Data rate Sample or frame (approximate) frequency size Telephone speech 64 kbps 8 bits 8000/sec CD-quality sound 1.4 Mbps 16 bits 44,000/sec Standard TV video 120 Mbps up to 640 x 480 24/sec (uncompressed) pixels x 16 bits Standard TV video 1.5 Mbps variable 24/sec (MPEG-1 compressed) HDTV video 1000–3000 Mbps up to 1920 x 1080 24–60/sec (uncompressed) pixels x 24 bits HDTV video and DVD 10–30 Mbps variable 24–60/sec MPEG-2 compressed) Characteristics of typical multimedia streams Figure 15.3 * 10

  10. PC/workstation PC/workstation Windowsystem Camera Component Bandwidth Latency Loss rate Resources required K H G A Codec Codec Out: 10 frames/sec, raw video Zero Camera L B Microphones 640x480x16 bits Mixer Network connections A Codec In: 10 frames/sec, raw video Interactive Low 10 ms CPU each 100 ms; Screen C Video file system Video store M D Out: MPEG-1 stream 10 Mbytes RAM Codec : multimedia stream Window system White boxes represent media processing components, many of which are implemented in software, including: B Mixer In: 2 44 kbps audio Interactive Very low 1 ms CPU each 100 ms; Out: 1 44 kbps audio 1 Mbytes RAM codec: coding/decoding filter mixer: sound-mixing component H Window In: various Interactive Low 5 ms CPU each 100 ms; system Out: 50 frame/sec framebuffer 5 Mbytes RAM K Network In/Out: MPEG-1 stream, approx. Interactive Low 1.5 Mbps, low-loss connection 1.5 Mbps stream protocol L Network In/Out: Audio 44 kbps Interactive Very low 44 kbps, very low-loss stream protocol connection Typical infrastructure components for multimedia applications Figures 15.4 & 15.5 • This application involves multiple concurrent processes in the PCs • Other applications may also be running concurrently on the same computers • They all share processing and network resources * 11

  11. Quality of service management • Allocate resources to application processes • according to their needs in order to achieve the desired quality of multimedia delivery • Scheduling and resource allocation in most current OS’s divides the resources equally amongst all processes • no limit on load • \ can’t guarantee throughput or response time * 12

  12. Elements of Quality of Service (QoS) management • Admission control: controls demand • QoS negotiation: enables applications to negotiate admission and reconfigurations • Resource management: guarantees availability of resources for admitted applications • real-time processor and other resource scheduling 13

  13. Component Bandwidth Latency Loss rate Resources required Out: 10 frames/sec, raw video Zero Camera 640x480x16 bits A Codec In: 10 frames/sec, raw video Interactive Low 10 ms CPU each 100 ms; Out: MPEG-1 stream 10 Mbytes RAM B Mixer In: 2 44 kbps audio Interactive Very low 1 ms CPU each 100 ms; Out: 1 44 kbps audio 1 Mbytes RAM H Window In: various Interactive Low 5 ms CPU each 100 ms; system Out: 50 frame/sec framebuffer 5 Mbytes RAM K Network In/Out: MPEG-1 stream, approx. Interactive Low 1.5 Mbps, low-loss connection 1.5 Mbps stream protocol L Network In/Out: Audio 44 kbps Interactive Very low 44 kbps, very low-loss connection stream protocol Figure 15.5QoS specifications for components of the application shown in Figure 15.4 14

  14. The QoS manager’s task Figure 15.6 * 15 *

  15. QoS Parameters Bandwidth • rate of flow of multimedia data Latency • time required for the end-to-end transmission of a single data element Jitter • variation in latency :– dL/dt Loss rate • the proportion of data elements that can be dropped or delivered late * 16

  16. Managing the flow of multimedia data • Flows are variable • video compression methods such as MPEG (1-4) are based on similarities between consecutive frames • can produce large variations in data rate • Burstiness • Linear bounded arrival process (LBAP) model: • maximum flow per interval t = Rt + B (R = average rate, B = max. burst) • buffer requirements are determined by burstiness • Latency and jitter are affected (buffers introduce additional delays) • Traffic shaping • method for scheduling the way a buffer is emptied * 17

  17. Protocol version Maximum transmission unit Figure 15.8 The RFC 1363 Flow Spec Token bucket rate Token bucket size Bandwidth: burstiness Maximum transmission rate maximum rate Delay: Minimum delay noticed acceptable latency acceptable jitter Maximum delay variation percentage per T Loss: Loss sensitivity maximum consec-utive loss T Burst loss sensitivity value Loss interval Quality of guarantee 50Mbps 150ms <1/1000 18

  18. (a) Leaky bucket Traffic shaping algorithms – leaky bucket algorithm Figure 15.7 analogue of leaky bucket: • process 1 places data into a buffer in bursts • process 2 in scheduled to remove data regularly in smaller amounts • size of buffer, B determines: • maximum permissible burst without loss • maximum delay process 1 process 2 * 19

  19. (b) Token bucket Token generator Traffic shaping algorithms – token bucket algorithm Figure 15.7 Implements LBAP • process 1 delivers data in bursts • process 2 generates tokens at a fixed rate • process 3 receives tokens and exploits them to deliver output as quickly as it gets data from process 1 Result: bursts in output can occur when some tokens have accumulated process 1 tokens: permits to place x bytes into output buffer process 2 process 3 * 20

  20. Admission control Admission control delivers a contract to the application guaranteeing: For each computer: • cpu time, available at specific intervals • memory Before admission, it must assess resource requirements and reserve them for the application • Flow specs provide some information for admission control, but not all - assessment procedures are needed • there is an optimisation problem: • clients don't use all of the resources that they requested • flow specs may permit a range of qualities • Admission controller must negotiate with applications to produce an acceptable result • For each network connection: • bandwidth • latency • For disks, etc.: • bandwifth • latency * 21

  21. Resource management • e.g. for each computer: • cpu time, available at specific intervals • memory • Scheduling of resources to meet the existing guarantees: Fair scheduling allows all processes some portion of the resources based on fairness: • E.g. round-robin scheduling (equal turns), fair queuing (keep queue lengths equal) • not appropriate for real-time MM because there are deadlines for the delivery of data Real-time scheduling traditionally used in special OS for system control applications - e.g. avionics. RT schedulers must ensure that tasks are completed by a scheduled time. Real-time MM requires real-time scheduling with very frequent deadlines. Suitable types of scheduling are: Earliest deadline first (EDF) Rate-monotonic * 22

  22. EDF(Earliest Deadline First) scheduling Each task specifies a deadline T and CPU seconds S to the scheduler for each work item (e.g. video frame). EDF scheduler schedules the task to run at least S seconds before T (and pre-empts it after S if it hasn't yielded). It has been shown that EDF will find a schedule that meets the deadlines, if one exists. (But for MM, S is likely to be a millisecond or so, and there is a danger that the scheduler may have to run so frequently that it hogs the cpu). Rate-monotonic scheduling assigns priorities to tasks according to their rate of data throughput (or workload). Uses less CPU for scheduling decisions. Has been shown to work well where total workload is < 69% of CPU. 23

  23. Source Targets High bandwidth Medium bandwidth Low bandwidth Stream adaptation: Scaling and filtering Figure 15.9 • Scaling reduces flow rate at source • temporal: skip frames or audio samples • spatial: reduce frame size or audio sample quality • Filtering reduces flow at intermediate points • RSVP is a QoS negotiation protocol that negotiates the rate at each intermediate node, working from targets to the source. • The Principle of BitTorrent (http://download.bitcomet.com/doc/principle.htm) * 24

  24. QoS and the Internet • Very little QoS in the Internet at present • New protocols to support QoS have been developed, but their implementation raises some difficult issues about the management of resources in the Internet. • RSVP(http://www.isi.edu/div7/rsvp/rsvp.html) • Network resource reservation • Doesn’t ensure enforcement of reservations • RTP (http://www.cs.columbia.edu/~hgs/rtp/) • Real time data transmission over IP • need to avoid adding undesirable complexity to the Internet • IPv6 has some hooks for it 25

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  26. Video conferencing • Video conferencing applications can also support: • Text chat • Document sharing (exchanging files) • PowerPoint • Application sharing (running the same program, viewing at the same content) • frequently there is a way for any participant to control the program • Electronic white board – everyone can view what • someone writes and draws • The word “Collaboration” appears a lot in docs 27

  27. Video conferencing • Desktop (computer) videoconferencing can be donethrough applications such as • Netmeeting (Windows 2000 and XP) –http://www.microsoft.com/windows/netmeeting/ • Windows Messenger (Windows XP) –http://www.microsoft.com/windowsxp/windowsmessenger/ • MSN Messenger (Windows) - http://messenger.msn.com/iChat (Apple) - http://www.apple.com/ichat/ 28

  28. Video conferencing • the clients we have mentioned are mainly used for “Instant Messaging” • IM is an exchange of text between two or more people who are online at the same time • supports group interaction • a conversation by typing instead of speaking • this is great for short conversations but doesn’t support extended discussion well • moving from text to audio and video makes the interaction much more natural 29

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  30. Video conferencing • Can buy videoconferencing appliances that plug into your computer. They come with: • camera • microphone • speaker • encoding of video and audio streams done in hardware • software to drive it all • load on your PC for encoding is then quite small 31

  31. Video conferencing • Video conferencing is a technology in which video and audio streams are transmitted among the various geographically separated participants in a meeting. • Typically this is done through a room which has been set up by a telephone company • booking the room also books an operator from the phone company to run the meeting for you 32

  32. Summary • MM applications and systems require new system mechanisms to handle large volumes of time-dependent data in real time (media streams). • The most important mechanism is QoS management, which includes resource negotiation, admission control, resource reservation and resource management. • Negotiation and admission control ensure that resources are not over-allocated, resource management ensures that admitted tasks receive the resources they were allocated. • Video conferences 33