Distributed Multimedia Systems Introduction Introducing Multimedia - - PowerPoint PPT Presentation

Distributed Multimedia Systems

Introduction

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)

Relationship to Real-time Systems

• Widely studied
• Many successful Real-time Systems have been

developed

• Highly specialized
• Not generally integrated into more general-purpose
• perating 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
• perate 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 Video server Digital TV/radio server Video camera and mike Local network Local network

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

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, 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

Distributed Multimedia Requirements

• Low-latency communications - in order to appear

synchronous, delays need to be small (100-300ms)

• 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 remote login network file access high-quality audio interactive video insufficient resources scarce resources abundant resources 2000

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

Typical Multimedia Streams

Data rate (approximate) Sample or frame size frequency Telephone speech 64 kbps 8 bits 8000/sec CD-quality sound 1.4 Mbps 16 bits 44,000/sec Standard TV video (uncompressed) 120 Mbps up to 640 x 480 pixels x 16 bits 24/sec Standard TV video (MPEG-1 compressed) 1.5 Mbps variable 24/sec HDTV video (uncompressed) 1000–3000 Mbps up to 1920 x 1080 pixels x 24 bits 24–60/sec HDTV video MPEG-2 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 end- points (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