A system of distributing video and/or audio information employs digital signal processing to achieve high rates of data compression. The compressed and encoded audio and/or video information is sent over standard telephone, cable or satellite broadcast channels to a receiver specified by a subscriber of the service, preferably in less than real time, for later playback and optional recording on standard audio and/or video tape.

Audio and video transmission and receiving system

A multi-user collaborative system in which the contents as well as the current status of other user activity of a shared structured data object representing one or more related structured data objects in the form of data entries can be concurrently accessed by different users respectively at different workstations connected to a common link. The WYSIWIS user interface representation of the shared structured data object includes an ordered listing of such entries that are maintained by the structured data object and various attributes of each listed entry, inter alia, the type and class of entry; the revision number of the shared structured data object; the number of pages and revision number of each structured data object entry; the date of creation and last revision of each such entry; whether an entry can be accessed by a user and, if not, who has prevented such access; whether a local instance of an entry is present on a user's system; and a provision for miscellaneous notes or comments relative to each entry for view by other users. Means is provided for maintaining current information relative to the shared structured object and its entries on a user initiated demand updated basis invoked by a user operation, which operation requires updated information to properly implement the operation. Further, means is provided for locking up one or more data entries by an individual user and thereby prevent access of the locked entries by other users to prevent concurrent editing and other changes to the same entries by two or more users. In this connection, editing or modification cannot be performed by a user until the shared structured data object entry or entries have been locked up. Visual indication as to the locked state of entries and other information relative to the locking user and the time of lock is updated and displayed in the shared structured data object representation present at user workstations when a use invokes a user operation on the shared structured data object or its contents.

Updating local copy of shared data in a collaborative system

This paper presents the design, implementation, and performance of a reliable multicast transport protocol (RMTP). The RMTP is based on a hierarchical structure in which receivers are grouped into local regions or domains and in each domain there is a special receiver called a designated receiver (DR) which is responsible for sending acknowledgments periodically to the sender, for processing acknowledgment from receivers in its domain, and for retransmitting lost packets to the corresponding receivers. Since lost packets are recovered by local retransmissions as opposed to retransmissions from the original sender, end-to-end latency is significantly reduced, and the overall throughput is improved as well. Also, since only the DRs send their acknowledgments to the sender, instead of all receivers sending their acknowledgments to the sender, a single acknowledgment is generated per local region, and this prevents acknowledgment implosion. Receivers in RMTP send their acknowledgments to the DRs periodically, thereby simplifying error recovery. In addition, lost packets are recovered by selective repeat retransmissions, leading to improved throughput at the cost of minimal additional buffering at the receivers. This paper also describes the implementation of RMTP and its performance on the Internet.

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Reliable multicast transport protocol (RMTP)

The publish/subscribe (or pub/sub) paradigm is an increasingly popular model for interconnecting applications in a distributed environment. Many existing pub/sub systems are based on pre-defined subjects, and hence are able to exploit multicast technologies to provide scalability and availability. An emerging alternative to subject-based systems, known as content-based systems, allow information consumers to request events based on the content of published events. This model is considerably more flexible than subject-based pub/sub. However, it was previously not known how to efficiently multicast published events to interested content-based subscribers within a large and geographically distributed network of broker (or router) machines. We develop and evaluate a novel and efficient distributed algorithm for this purpose, called -link matching". Link matching performs just enough computation at each node to determine the subset of links to which an event should be forwarded. We show via simulations that: link matching yields higher throughput than flooding when subscriptions are selective; and the overall CPU utilization of link matching is comparable to that of centralized matching.

An efficient multicast protocol for content-based publish-subscribe systems

A method for use in a multimedia conferencing arrangement to control multiple concurrent calls where each call comprises one or more channels. A first call among a first set of user stations and a second call among a second set of user stations are merged into a single call comprising a plurality of channels among at least three user stations from the first and second sets. The plurality of channels of the single, merged call corresponds to a combination of channels of the first and second calls and may include a signalling channel, a voice channel, and one data channel for each data channel of the first and second calls. The method is also applicable to calls including image or video channels.

Multiple call control method in a multimedia conferencing system

We present a solution to the problem of multidestination routing in internetworks. The component subnets of these internets share a common datagram internet layer, and the gateways and hosts can determine the next gateway en route to a foreign net. Our datagram routing offers high resilience to network failures, major reductions in network traffic, and no changes whatsoever to the subnetwork routing. The routing follows “shortest” paths as defined by the distance criteria of an internet. We intend to use the algorithm as an option of the DoD Internet Protocol with only minor changes to IP while preserving interoperability with IP modules not supporting multidestination.

Datagram routing for internet multicasting

We present an object-oriented architecture for a computer-based, real-time, multimedia conferencing system. This architecture divides the system into five functional areas: a multimedia shared workspace, a user interface, conference management, communications, and an information base. The structure and operation of the first four areas are modeled with object-based concepts that address design requirements identified during the development of a proof-of-concept prototype, that preceded the architecture specification.The shared workspace, the most important component, is thoroughly discussed; the other components support its realization. The modeling of workspace entities emphasizes their aggregation into composed entities and the homogeneous handling of several data media. The user interface manages uniformly the man-machine interaction for both local and remote user actions. Conference management deals with session establishment, participation, and control of multiple media floors. The communication functions replicate user actions over workspace objects in all hosts participating in a conference, matching traffic types with transmission services in the process.

An architecture for a multimedia teleconferencing system

This article describes the requirements for a graphical format on which a graphical on-line communications protocol can be based. It is argued that on-line graphical communications is similar to graphical session capture, and thus we propose an Interactive Graphical Communications Format using the GKSM session metafile. The discussion includes items that complement the GKSM metafile as a format for on-line interactive exchanges. One key application area of such a format is multimedia on-line conferencing. Therefore, we present a conferencing software architecture for processing the proposed format. This format specification is made available to those planning multimedia conferencing systems, as a contribution toward the development of a graphical communication protocol that will permit the interoperation of these systems.

Lorenzo Aguilar

Papers

Datagram routing for internet multicasting

An architecture for a multimedia teleconferencing system

A Format for a Graphical Communications Protocol