1. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

2. Distributed multimedia applications –Networked video library, Internet telephony, videoconference Characteristics of multimedia applications –Timely delivery of streams of multimedia data to end-users Audio sample, video frame To meet the timing requirements –QoS( quality of service) Characteristics of distributed multimedia applications

3. Traditional real-time system –E.g. telephone switching –Small quantities of data, strict time requirement –QoS management: fixed schedule that ensures worst- case requirements are always met Different requirements of multimedia app. –General environment Compete with other distributed app. for network bandwidth, computing resource –Dynamic resource requirements E.g. the number of participants of a video conference may vary –User participant in the control of resource consumption QoS management

4. Web-based multimedia –Extensive buffering –Best-effort Network phone and audio conference –Relatively low bandwidth –Efficient compression techniques –High interactive latency Video-on-demand services –Require sufficient dedicated network bandwidth Existing distributed multimedia Apps. without QoS

5. Examples –Videoconference –Distributed online ensemble Requirements –Low-latency communication round trip delays < 100 ms –Synchronous distributed state If one user stops a video on a given frame, the other users should see it stopped at the same frame –Media synchronization All participants in a music performance should hear the performance at approximately the same time –External synchronization Sometime, other information need to be synchronized with the time- based multimedia streams Expecting rigorous QoS management Highly interactive applications

6. A history of computer systems that support distributed data access The window of scarcity

7. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

8. Continuous –Refer to the user’s view of the data –Video: a image array is replaced 25 times per second –Audio: the amplitude value is replaced 8000 times per second Time-based –The delivery delay for each element is bounded in a value –Bulky multimedia streamBulky multimedia stream –Data compression Reduce bandwidth requirements by factors between 10 and 100 E.g. GIF, TIFF, JPEG, MPEG-1, MPEG-2, MPEG-4 Characteristics of multimedia data

9. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

10. Multimedia App. compete with other App. for –Processor cycles, bus cycles, buffer capacity –Physical transmission links, switches, gateways Best-effort policies –Multi-task OS: round-robin scheduling Each task is allocated so few cycles when there are so many tasks –Ethernet: conflict detecting Much collisions when the network is heavily loaded Best-effort policies are not fit to multimedia apps. Traditional computer systems

11. Architecture of a typical system –Source Stream processors –Connections Network connection In-memory transfer –Target Each process must be allocated adequate CPU time, memory capacity and network bandwidth Resource requirement QoS management for distributed multimedia apps.

12. The OoS manager’s task –Quality of service negotiation Apps. specify the resource requirements QoS manager evaluates the feasibility Give a positive or negative response –Admission control Applications run under a resource contract Recycle the released resource The QoS manager’s task

13. Resource requirements specification –bandwidth The rate at which a multimedia stream flows –Latency The time required for an individual data element to move through a stream from the source to the destination –Loss rate Data loss due to unmet resource requirements A rate of data loss that can be accepted. E.g., 1% Quality of service negotiation

14. Describe a multimedia stream –Describe the characteristics of a multimedia stream in a particular environment –E.g. a video conference Bandwidth: 1.5mbps; delay: 150ms, loss rate: 1% Describe the resources –Describe the capabilities of resources to transport a stream –E.g. a network may provide Bandwidth: 64kbps; delay: 10ms; loss rate: 1/1000 The usage of resource requirements spec.

15. Bandwidth –Specified as minimum-maximum value or average value Required bandwidth varies according to the compression rate of the video. E.g., 1:50 - 1:100 of MPEG video –Specify burstiness Different traffic patterns of streams with the same average bandwidth LBAP model: Rt + B, where R is the rate, B is the maximum size of burst Specify the QoS parameters for streams

16. Latency –The frames of a stream should be processed with the same rate at which frames arrive –No human perception E.g. 150ms for interactive apps, 500ms for demand on video –No jitter Jitter: the variation in the period between the delivery of two adjacent frames Loss rate –Typically be expressed as a probability Be calculated based on worst-cast assumptions or on standard distributions Specify the QoS parameters for streams (2)

17. Traffic shaping –Output buffering to smooth the flow of data elements The leaky bucket algorithm –Eliminate burst –R A stream will never flow with a rate higher than R –B Size of the buffer Bound the time for which an element will remain in the buffer Traffic shaping

18. The token bucket algorithm –Allow larger burst –Token is generated at a fixed rate of R –the tokens are collected in a bucket of size B –Data of size S can be sent only if at least S tokens are in the bucket –The sender process removes these S tokens –Ensure: over any interval t, the amount of data sent is not larger than Rt+B, An implementation of the LBAP model Traffic shaping (2)

19. Bandwidth –The maximum transmission unit and maximum transmission rate –The burstiness of the stream The token bucket size and rate Delay –The minimum delay that an application can notice, the maximum jitter it can accept Loss rate –The total acceptable number of losses over a certain interval –The maximum number of consecutive losses Flow specification – RFC 1363RFC 1363

20. Simple approach –Follow the flow of data along each stream from the source to the target The source send out a flow spec to local QoS manager The manager check against its database of available resources whether the requested QoS can be provided The flow spec is forwarded to the next node until the last node The result is passed from the target to the source Transactional QoS negotiation procedure –Deal with concurrent QoS negotiations Negotiation procedures

21. Avoid resource overload Bandwidth reservation –Reserve maximum bandwidth exclusively Used for applications that cannot adapt to different QoS levels, e.g. x-ray video Statistical multiplexing –Reserve minimum or average bandwidth –Handle burst that cause some service drop level occasionally –Hypothesis a large number of streams the aggregate bandwidth required remains nearly constant regardless of the bandwidth of individual streams Admission control

22. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

23. Fair scheduling –Round-robin Packet-by-packet Bit-by-bit –Practically, the scheduler calculates the time in which each packet should be sent out on the base of bit-by- bit round-robin, and then sends the packet in time Real-time scheduling –Earliest-deadline-first (EDF) Each media element is assigned a deadline by which it must be sent out The scheduler send media elements according to their deadline Resource scheduling

24. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

25. Stream adaptation –An application adapt to changing QoS levels when a certain QoS cannot be guaranteed Drop pieces of information –Audio stream Drop can be noticed immediately by the listener –Video stream Motion JPEG: easy since frames are independent MPEG: difficult since frames are interdependent Increase delay –Acceptable for non-interactive applications Stream adaptation

26. When to perform scaling –Adapt a stream to the bandwidth available in the system before it enters a bottleneck resource Scaling approach –Subsample E.g. reduce the rate of audio sample E.g. drop a channel in a stereo transmission –Implementation A monitor process at the target A scaler process at the source Scaling

27. Temporal scaling –Decrease the number of video frames transmitted within an interval Spatial scaling –Reduce the number of pixels of each image in an video stream Frequency scaling –Modify the compression algorithm applied to an image Amplitudinal scaling –Reduce the color depths for each image pixel Color space scaling –Reduce the number of entries in the color space, e.g., from color to grey-scale presentation Different scaling methods

28. Scaling is not suitable to a stream that has several receivers –Since scaling is conducted at the source, a scale-down message will degrade the quality of all streams Filtering –A stream is partitioned into a set of hierarchical sub-streams –The capacity of nodes on a path determines the number of sub-streams a target receives Filtering

29. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

30. Video-on-demand for a large number of users –A large stored digital movie library –Delay of receiving the first frame is within a few seconds –Users can perform pause, rewind, fast-forward Quality of service –Constant rate –a maximum jitter and low loss rate Scalable and distributed –Support up to 10000 clients simultaneously Low-cost hardware –Constructed by commodity PC Fault tolerant –Tolerant to the failure of any single server or disk Design goals

31. One controller –Connect with each server by low-bandwidth network Cubs – the server group –Each cub is attached by a number of disks ( 2-4) –Cubs are connected to clients by ATM System architecture

32. Stripping –A movie is divided into blocks –The blocks of a movie are stored on disks attached to different cubs in a sequence of the disk number –Deliver a movie: deliver the blocks of the movie from different disks in the sequence number –Load-balance when delivering hotspot movies Mirroring –Each block is divided into several portions (secondaries) –The secondaries are stored in the successors If a block is on a disk i, then the secondaries are stored on disks i+1 to i+d –Fault-tolerance for single cub or disk failure Storage organization

33. Slot –The work to be done to play one block of a movie Deliver a stream –Deliver the blocks of the stream disk by disk –Can be viewed as a slot moving along disks step by step Deliver multiple streams –Multiple slots moving along disks step by step Viewer state –The address of the client computer –The identity of the file being played –The viewer’s position in the file –The viewer’s sequence number –Some bookkeeping information Distributed schedule

34. Block play time - TBlock play time - T –The time that will be required for a viewer to display a block on the client computer –Typically about 1 second for all streams –The next block of a stream must begin to be delivered T time after the current block begin to be delivered Block service time – t ( a slot )Block service time – t ( a slot ) –Read the next block into buffer –Deliver it to the client –Update viewer state in the schedule and pass the updated slot to the next cub –T / t typically result in a value > 4 The maximum streams the Tiger system can support simultaneously –T/t * the number of disks Distributed schedule (2)

35. Initial prototype [1994] –5 133Mhz Pentium PCs(48M RAM, 2G SCSI disk, Windows NT) 68 simultaneous streams with perfect quality One cub failed, the loss rate is 0.02% 14 cubs, 56 disks [1997] –602 simultaneous streams ( 2Mbps) 1000 cubs –30000-40000 simultaneous viewers Microsoft NetShow Theater Server Performance and scalability

36. Introduction Characteristics of multimedia data Quality of service management Resource management Stream adaptation Case study: the Tiger video file server Summary Chapter 15: Distributed Multimedia Systems

37. Characteristics of distributed multimedia apps. –Large volumes of time-dependent data QoS management –QoS negotiation Bandwidth, latency, loss rate Traffic shapping: bucket leaky algorithm, token bucket leaky algorithm –Admission control Schedule –Fair schedule –Real-time schedule Summary

38. Stream adaption –Scale –Filter The Tiger video file servers –Date layout Striping Mirroring –Distributed schedule slots Summary

39. A typical distributed multimedia system

40. Characteristics of typical multimedia streams Data rate (approximate) Sample or frame size frequency Telephone speech64 kbps8 bits8000/sec CD-quality sound1.4 Mbps16 bits44,000/sec Standard TV video (uncompressed) 120 Mbpsup to 640x 480 pixelsx 16 bits 24/sec Standard TV video (MPEG-1 compressed) 1.5 Mbpsvariable24/sec HDTV video (uncompressed) 1000–3000 Mbpsup to 1920x 1080 pixelsx 24 bits 24–60/sec HDTV video MPEG-2 compressed) 10–30 Mbpsvariable24–60/sec

41. Typical infrastructure components for multimedia applications

42. QoS specifications for components of the application shown in Figure 15.4 ComponentBandwidthLatencyLoss rateResources required Camera Out:10 frames/sec, raw video 640x480x16 bits Zero ACodecIn: Out: 10 frames/sec, raw video MPEG-1 stream InteractiveLow10 ms CPU each 100 ms; 10 Mbytes RAM BMixerIn: Out: 2  44 kbps audio 1  44 kbps audio InteractiveVery low1 ms CPU each 100 ms; 1 Mbytes RAM HWindow system In: Out: various 50 frame/sec framebuffer InteractiveLow5 ms CPU each 100 ms; 5 Mbytes RAM KNetwork connection In/Out:MPEG-1 stream, approx. 1.5 Mbps InteractiveLow1.5 Mbps, low-loss stream protocol LNetwork connection In/Out:Audio 44 kbpsInteractiveVery low44 kbps, very low-loss stream protocol

43. The QoS manager’s task

44. Traffic shaping algorithm Token generator (a) Leaky bucket(b) Token bucket

45. The RFC 1363 flow Spec Protocol version Maximum transmission unit Token bucket rate Token bucket size Maximum transmission rate Minimum delay noticed Maximum delay variation Loss sensitivity Burst loss sensitivity Loss interval Quality of guarantee Bandwidth: Delay: Loss:

46. Filtering Source Targets High bandwidth Medium bandwidth Low bandwidth

47. Tiger schedule 012 slot 0 viewer 4 slot 1 free slot 2 free slot 3 viewer 0 slot 4 viewer 3 slot 5 viewer 2 slot 6 free slot 7 viewer 1 block play time T block service time t state