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Lecture 4:  Multimedia Communication Systems

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Overview Cooperating computing Application Sharing approach Audio/video conferencing Transport Subsystems

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It deals with the transfer, the protocols, services and mechanisms of discrete media data (such as text and graphics) and continuous media data (like audio and video) in/over digital networks.  Such a communication requires all involved components to be capable of handling a well-defined quality of service. The most important quality of service parameters are used to request  the required capacities of the involved resources, compliance to end-to-end delay and jitter as timing restrictions, and  restriction of the loss characteristics.

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The OSI (Open Systems Interconnection) model was created by the ISO to help standardize communication between computer systems.  It divides communications into seven different layers, which each include multiple hardware standards, protocols, or other types of services.

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The OSI Model
Layer 7 — Application
Layer 6 — Presentation
Layer 5 — Session
Layer 4 —
Transport
Layer 3 — Network
Layer 2 — Data Link
Layer 1 Physical

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In Multimedia communication systems issues are discussed related to above Link layer. In OSI model, multimedia communication systems are divided into two subsystems. Application systems Transport systems

7 Application
6 Presentation
5 Session
4 Transport
3 Net
2 Link
1 physical
Appl.
Transp.

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Application system is responsible for management and services issues for group cooperation and supporting large scale of multimedia applications. Transport system (here transport and network layer protocols for multimedia applications are presented.

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Some multimedia applications:
- Multimedia Mail
- Virtual Reality Applications
- Video Conferencing
- CSCW (Computer Supported Cooperative Work)
Communication Requirements:
- Highspeed networks with high transfer rates
e.g.: FDDI, Switched Ethernet, Fast Ethernet,
Gigabit Ethernet, DQDB, ATM,
- High performance transport protocols
e.g.: TCP, XTP, RTP,.
Application Subsystems
Transport Subsystems

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Cooperative computing is also known as Computer Supported Cooperative Work (CSCW). Tools for cooperative computing: Electronic mail Bulletin boards Screen sharing tools Application sharing Text based conferencing systems Video conferencing systems ( Mbone  tools,  Proshare  from Intel, Picture Tel,  Teles  online, NetMeeting  from Microsoft)

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Computer Supported Cooperation can be categorized  according to the following parameters; Time -  Asynchronous cooperative work - Synchronous cooperative work User scale - Single users, two users (dialogue, point to point, direct cooperation) or groups with more than two users.) - Static and dynamic groups if either the member are pre-determined or not. Control - Centralized i.e. controlled by a chairman - Distributed i.e. control protocol provide consistent cooperation Locality Cooperation at the same place Tele-cooperation of users at the different places.

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Cooperation-transparent systems : existing applications are extended for cooperation. Single user document editors expanded for simultaneous editing of shared document among several users  eg . Some text processor Cooperation-aware systems:  Dedicated software application for CSCW  eg . Lotus Notes, Conferencing Systems.

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Group Communication Agent
Application
Group
Sharing
Rendezvous
Conferencing
Communication
(Transport) Support
Multicast
Communication
Network
Group Communication Agent
Application
Group
Sharing
Rendezvous
Conferencing
Communication
(Transport) Support
Group Communication Agent
Application
Group
Sharing
Rendezvous
Conferencing
Communication
(Transport) Support
Acti

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Centralized A single copy of the shared application are available and runs at one site only. All input are forwarded to the local site. Output (shared document) is distributed to all sites . Replicated A copy of the shared application runs locally at each site Input distributed to all sites

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Application Sharing Approach Architecture

cent
Floor Holder
Shared Window
Input
Network
Output
Shared Application Bottleneck risk
((single site)
Floor Holder
in
Shared Window
replicated
ut
Network
Shared Window
Floor Holder
Shared Window
Input
Output
Input
Output
Shared Application
Shared Application
consistency has to be maintained

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Audio/video conferencing is management service which control simultaneous face-face communication between multiple users using multiple media (video, audio, text etc.) Video: Conference participants (all, speaker or moderator only). Large video walls with multiple high resolution screens may be used  especially for Conference with more than four participants Display view-graphs, images, animations etc.   Audio Used for discussion Important for clarifying visual information.

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Requirements: High bandwidth (for data intensive-media) Lower latency (for user interactivity) Distributed messaging of data and control information Conferencing control: Management of conferences (Establishing, Closing, Adding/removing Users ) Providing information about conferences (conference name, duration etc.) Centralized or distributed control: Centralized control is easy to implement Distributed is much more complicated but less sensitive to failure.

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Multimedia applications demand high requirements on network protocol. High data throughput Deliver data as much as possible. Fast data forwarding Deliver data as fast as possible Service guarantees Deliver data with the negotiated policies. Multicasting   1:n and m:n point communication [ a:b  denotes: a senders and b receivers

CD 321 Multimedia Technology II

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