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Distributed Multimedia Systems
Introduction
Introducing Multimedia Systems
● Example target applications networked video libraries, Internet telephony and videoconferencing
● Realtime 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)
Relationship to Realtime Systems
● Widely studied● Many successful Realtime Systems have been
developed● Highly specialized● Not generally integrated into more generalpurpose
operating systems and networks (which is the problem)
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
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
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
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
Example Distributed Multimedia System
Wide area gateway Videoserver
DigitalTV/radioserver
Video cameraand mike
Local network Local network
Example Distributed Multimedia Applications● (Note: today's computing and network environments are QoSless and
besteffort)● Webbased multimedia besteffort 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 roundtrip delays
● Videoondemand services digitized video; large online storage systems; requires dedicated network bandwidth and dedicated stations; extensive buffering at destination
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)
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, besteffort delivery; IP routing introduces unavoidable latencies; gateways to conventional telephone systems is underway, as is standardization (e.g., SIP)
● Multiuser videoconferencing limited by bandwidth and latency constraints
● Multisite 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
Distributed Multimedia Requirements
● Lowlatency communications in order to appear synchronous, delays need to be small (100300ms)
● 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
Key Point
Distributed Multimedia Applications will only run successfully when rigorous QoS management systems
are deployed
The Window of Scarcity
1980 1990
remotelogin
networkfile access
highqualityaudio
interactivevideo
insufficientresources
scarceresources
abundantresources
2000
Characteristics of Multimedia Data
● The term "continuous" refers to the enduser'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 timebased 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
Typical Multimedia Streams
Data rate(approximate)
Sample or frame size frequency
Telephone speech 64 kbps 8 bits 8000/secCDquality sound 1.4 Mbps 16 bits 44,000/secStandard TV video(uncompressed)
120 Mbps up to 640 x 480pixels x 16 bits
24/sec
Standard TV video (MPEG1 compressed)
1.5 Mbps variable 24/sec
HDTV video(uncompressed)
1000–3000 Mbps up to 1920 x 1080pixels x 24 bits
24–60/sec
HDTV videoMPEG2 compressed)
10–30 Mbps variable 24–60/sec
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)
● Specialpurpose 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
