Distributed Multimedia Systems. Introduction

Transcription

1 Distributed Multimedia Systems Introduction

2 Introducing Multimedia Systems Example target applications networked video libraries, Internet telephony and video conferencing Real time systems performing tasks and delivering results according to a schedule that is externally determined Quality of Service is a big issue (if not "the" big issue)

3 Relationship to Real time Systems Widely studied Many successful Real time Systems have been developed Highly specialized Not generally integrated into more general purpose operating systems and networks (which is the problem)

4 Quality of Service Management The planned allocation and scheduling of resources to meet the needs of multimedia and other applications Most current operating systems and networks do not include the QoS management facilities needed to support multimedia applications

5 Some Characteristics Failure to meet deadlines in multimedia applications can be serious Multimedia applications are often highly distributed and operate on standard platforms, competing with other applications and services for resources Multimedia resource requirements are often very dynamic (think about the content) Users often wish to balance the resource cost of multimedia applications with other "mainstream" applications they do not want to purchase specialist equipment

6 QoS Management Systems QoS Management systems are designed to meet the needs of distributed multimedia applications Requirement managing the available resources dynamically Requirement Varying the current allocations in response to changing demands and priorities

7 More on QoS Management Managing all of the computing and communication resources needed to acquire, process and transmit multimedia data streams, especially where the resources are shared between applications Needed in order to guarantee that applications will be able to obtain the necessary quantity of resources at the required times, even when other applications are competing for resources

8 Example Distributed Multimedia System Video camera and mike Local network Local network Wide area gateway Video server Digital TV/radio server

9 Example Distributed Multimedia Applications (Note: today's computing and network environments are QoS less and best effort) Web based multimedia best effort access to audio and video streams via the WWW; performance constrained by limited bandwidth and variable latencies; no real support from current OSes; extensive use of buffering at destination; delays can reach several seconds Network phone and audio conferencing low bandwidth requirements; compression can be applied quite successfully; demands low round trip delays Video on demand services digitized video; large on line storage systems; requires dedicated network bandwidth and dedicated stations; extensive buffering at destination

10 Examples of Current Problems Highly interactive applications cannot be supported by today's environments Multimedia applications tend to be cooperative (many users) and synchronous (many coordinated users)

11 Examples of Current Problem Applications Internet Telephony VoIP; current Internet not designed for this type of traffic; as backbone runs at over 40Gbps, telephony can be supported to some degree; UDP used; QoS ignored, best effort delivery; IP routing introduces unavoidable latencies; gateways to conventional telephone systems is underway, as is standardization (e.g., SIP) Multiuser video conferencing limited by bandwidth and latency constraints Multi site music rehearsal synchronization constraints are very tight; all performers must "see" and "hear" the other musicians as if they were all in the same room

12 Distributed Multimedia Requirements Low latency communications in order to appear synchronous, delays need to be small ( ms) Synchronized distributed state what/when one user sees/does, they all see/do Media synchronization 'lip sync' is important; e.g., the distributed ensemble needs delays of less than 50ms External synchronization updates to "live content" needs to appear to be instantaneous

13 Key Point Distributed Multimedia Applications will only run successfully when rigorous QoS management systems are deployed

14 The Window of Scarcity interactive video high quality audio insufficient resources scarce resources network file access abundant resources remote login

15 Characteristics of Multimedia Data The term "continuous" refers to the end user's view or perception of the data Continuous media are represented as sequences of discrete values that replace each other over time Multimedia systems are said to be time based or "isochronous" timed data elements in the audio and video streams define the semantics or content of the stream Obviously, multimedia applications need to preserve the timing data/information

16 Typical Multimedia Streams Data rate (approximate) Sample or frame 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 /sec (uncompressed) pixels x 16 bits Standard TV video (MPEG 1 compressed) 1.5 Mbps variable 24/sec HDTV video (uncompressed) Mbps up to 1920 x /sec pixels x 24 bits HDTV video MPEG 2 compressed) Mbps variable 24 60/sec

17 Importance of Compression Obviously, compression is an important technology as far as Multimedia applications are concerned Bandwidth can be reduced from factors of 10 to 100 However, compression introduces sometimes substantial processing overhead at both the source and destination endpoints (which may or may not be tolerable) Special purpose hardware can help, but software codecs are more common/popular/flexible Example compression scheme MPEG video compression is asymmetric; source algorithm is slow and complex (but good), destination algorithm is fast and simple