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Tele-service ITU-T Rec. ETSI Broadband Video Telephony Services F.722 DE/NA-010029 Broadband Videotex Services F.310 - Broadband Video Conference Services F.732 DE/NA-010030 Broadband TV Distribution Services F.821 - Broadband HDTV Distribution Services F.822 - Multimedia Distribution Services F.MDS - Multimedia Delivery Services F.MDV -The specifications of these services within ITU-T and ETSI are at their very early stages and still incomplete. A number of new activities have also been started within ETSI/NA5 and ETSI/NA1 in order to derive a specification for a Best Effort Service (DE/NA-010031) and a Video on Demand (VOD) service. The Best Effort Service is mainly aimed at applications running over interconnected LANs with a variable bit rate.
The BERKOM Multimedia Collaboration service [Altenhofen93] allows audio-visual conversions that consist of directory service, conference management and applications sharing. The BERKOM Multimedia teleservice [Blum93] supports multimedia message transfers in a heterogeneous environment. In particular, it provides a framework for the presentation of multimedia messages and their compositions, sending and receptions.
JVTOS has been developed in order to share information between users using X Window applications. It allows for a distributed display of graphical data as well as remote control for , say, joint editing. In addition, JVTOS offers the facility of a picturephone for direct audio and video communication which can be used independently of X-Windows applications.
The Multimedia Messaging Service is built around X 400 and X 500 which have been extended to support multimedia data (such as audio and video data). In contrast to JVTOS this service has been developed as stand alone application and is available for a restricted set of platforms.
A service session model provides the concepts for establishing, using and releasing sessions. A session in this framework is not unlike a call in the telecommunications world. The service management model deals with subscriptions, fault, performance, accounting, security, and configuration. To provide multimedia teleservices developers will need to use TINA-C Distributed Processing Environments. It is, as yet, unclear which of the existing technologies, ANSAware, OSF DCE/DME, OMG CORBA, are to be used to build this support environment.
The Sequoia network provides services to the California Department of Water Services, UC Davis, UC Berkeley, UC Santa Barbara, UCLA, the San Diego Supercomputer Center, UC San Diego, and the Scripps institute of Oceanography. The infrastructure consists of FDDI rings for local distribution with private T1 (1.54 Mbps) leased lines for wide-area services. A number of DECstations 5000/240 general-purpose workstations interconnect the FDDI and T1 links and serve as network routers. The researchers use scientific workstations to load, browse and query objects such as satellite weather maps and global climate modelling data. Additionally, many Sequoia workstations support network transmission of digitally-encoded audio and video streams. Several workstations use DEC s J-Video and J300 hardware compression/decompression cards with live video and audio capture features. These cards are linked to cameras, speakers and microphones to cater for multimedia requirements.
Again just as the OSI stack has been found to be inadequate, the Internet protocols, the most widely used protocols in research environments, have also been found to be incapable of catering for some of the real-time requirements of multimedia data. This realisation has led to the development of a new suite of protocols called TENET after the TENET group [Ferrari92] of the University of California at Berkeley. Thus the Internet Protocols (IP, UDP, TCP) are used for best effort data delivery, while the Tenet protocols are under investigation as possible solution for real-time requirements.
The TENET suite of protocols provides real-time or guaranteed performance communication services in an internetworking environment and adopts a connection oriented and reservation-based architecture. The Tenet suite of protocols is divided into data delivery and control protocols. The data delivery protocols include the Real-Time Internet Protocol (RTIP) [Zhang92] at the network layer, and the Real-Time Message Transport Protocol (RMTP) [Zhang93] and the Continuous Media Transport Protocol (CMTP) [Wolfinger91] at the transport layer. The control protocol is called the Real-Time Channel Administration Protocol (RCAP) [Banerjea91], which performs the channel establishment, status reporting, and closing down.
It has been found that the Sequoia infrastructure with it T1 links is prone to congestion due to its competitive multimedia workloads leading to the degradation in the audio and video quality. These T1 links are due to be updated to T3 (45 Mbps) links, however, it is anticipated that the new infrastructure will also be congested due to the rapid growth of the offered load on the Sequoia network.
The NCIH services [Grovenstein] include ATM Cell Relay, SMDS, and Circuit Emulation. The ATM Cell Relay is a connection oriented service that provides high-speed and low delay transfer. All other services in the NCIH are built on top of ATM Cell Relay. Initially, only permanent virtual circuits but with future plans for ATM switched virtual circuits. In addition, a constant bit rate (CBR) and variable bit rate (VBR) are also supported. The Switched Multimegabit Data Service (SMDS) is a connectionless data service with packets of up to 9,188 bytes in addition to a header that contains a source and destination address. The header information is used to segment it into ATM cells for transport. The ATM Adaptation Layer Type 3/4 (AAL 3/4) that occupies 4 bytes out of 48 bytes ATM cell. The circuit emulation is needed to provide backwards compatibility to existing telecommunications network that operate at the rates of 64 kbs/s 1.5 Mb/s and 45 Mb/s.
The typical NCIH architecture consists of :A number of applications have been specifically designed to exploit the high bandwidth offered the data highway. The first one to implemented was that of distance learning. A number of configurations are possible but the most demanding is that of video links between classrooms. Other trials included the VISTAnet and MICA (Medical Information Communications Application) [Bruwer94] which enabled real-time multi-dimensional imaging for health care applications. For example, radiation therapy and planning through the provision of real-time X-ray consultation and teleconferencing.
The Medusa project at Olivetti research aims to provide a networked multimedia environment in which many streams of multimedia data are active simultaneously. Medusa software environment consists of a number of active objects or modules that represent cameras, displays, format converters etc. Applications are then build on top by grouping together these objects. This approach results in a heavy biased and architecture that is based on the underlying hardware. The Medusa hardware is made of a collection of ATM direct peripherals, cameras, audio, systems, multimedia storage servers, LCD displays and televisions. The authors refer to the different components as bricks to convey the analogy with building and how they are built. Each of the direct peripherals are build around an ARM processor from Advanced Risc Machines in Cambridge, which directly connected to an input or output device of the ATM network. These processors a microkernel which has been specifically designed fro networked, embedded, real-time systems. This is different from a traditional workstation where there is no direct connection for the separate components.
The Medusa integration of networked components is taken further by the VuNet approach. VuNet desk area expands throughout the network with small switches are distributed all over. This leads to the multimedia components usually found in the workstation to be spread out over the network. The authors claim that through this approach real-time processing is enhanced. This processing include real-time stationary filtering, motion detection, shot change detection, edge filtering, and blue-screening. One of the unknown in this approach is the intensity of computation in these distributed resources, in addition to the complex management of many devices. The management in synchronisation for cooperation could lead to performance problems for example in trying to achieve a consistent state.
Another effort at the development of new protocols is that of the Real-Time Transport Protocol (RTP) [Schulzrinne93]. RTP combines the tasks of application, presentation, session and transport layer in a single protocol. This results in an improved performance but at the expense of the layering principles that have been established by the OSI Model and standardisation processes. RTP has been used to provide some MBone service such as vat and ivs.
An area which has received much research interest is that of multicasting. The ability to multicast is the basis for collaborative work and thus one of the main applications of the multimedia capability. Multicast Transport Protocol (MTP) [Armstrong92] provides a reliable transport service on top of the network layer and with a multicast facility. In the case of ST-II [Topolcic90] we have a guaranteed end to end bandwidth and delay (thus suited for multimedia) together with multicast support. The contention for resources and the wide area coverage of the potential applications, for example, a conference can lead to congestion and thus the need for routing. One solution has been the Distance Vector Multicast Routing Protocol (DVMRP) [Waitzman88]. DVMRP is an experimental routing protocol for internet multicasting. It forms the basis for MBone and as such has achieved some popularity. The problems in efficiency due to for, example to truncated broadcasting has resulted in a new protocol: Protocol Independent Multicast (PIM) [Deering94]. PIM approach to multicasting routing is to operate in two different modes: dense and sparse mode to cater for situations where nodes are clustered closely together or widely and sparsely distributed.
Contents
Graphics Multimedia Virtual Environments Visualisation Contents