Software mechanisms that make use of analog transmission and storage of video for computer-controlled video communication and file storage are described. The authors extended the Etherphone voice management system to support video conferencing and file storage. The software mechanisms for conference management consist of a central connection manager, and video and user interface agents running on client workstations. A conferencing architecture that supports multiplicity in the number of conferences and media is presented. The video file server stores and retrieves full-motion video information within conferences. It uses optical disks to store video frames and builds a high-level file and directory interface. The user interface to the directory interface is a multimedia document editor that permits video annotations to be freely interspersed with text. The performance of the system, which spans multiple media networks, has been highly satisfactory.>

Thevoice managerin the Etherphone system provides facilities for recording, editing, and playing stored voice in a distributed personal-computing environment. It provides the basis for applications such as voice mail, annotation of multimedia documents, and voice editing using standard text-editing techniques. To facilitate sharing, the voice manager stores voice on a special voice file server that is accessible via the local internet. Operations for editing a passage of recorded voice simply build persistent data structures to represent the edited voice. These data structures, implementing an abstraction calledvoice ropes, are stored in a server database and consist of lists of intervals within voice files. Clients refer to voice ropes solely by reference.Interests, additional persistent data structures maintained by the server, serve two purposes: First, they provide a sort of directory service for managing the voice ropes that have been created. More importantly, they provide a reliable reference-counting mechanism, permitting the garbage collection of voice ropes that are no longer needed. These interests are grouped into classes; for some important classes, obsolete interests can be detected and deleted by a class-specific algorithm that runs periodically.

Whru a dilt~lhSC is replicated at, many sites2 maintaining mutual consistrnry among t,he sites iu the fac:e of updat,es is a signitirant problem.This paper descrikrs several randomized algorit,hms for dist,rihut.ingupdates and driving t,he replicas toward consist,c>nc,y.The algorit Inns are very simple and require few guarant,ees from the underlying conllllunicat.iollsystem, yc+ they rnsutc t.hat.the off(~c~t, of ('very update is evcnt,uwlly rf+irt-ted in a11 rq1ica.s.The cost, and parformancc of t,hr algorithms arc tuned I>? c%oosing appropriat,c dist,rilMions in t,hc randoinizat,ioii step.TIN> idgoritlmls ilr(' c*los~*ly analogoIls t,o epidemics, and t,he epi-dcWliolog)-litc\ratiirc, ilitlh iii Illld~~rsti4lldill~ tlicir bc*liavior.One of tlW i$,oritlims 11&S brc>n implrmcWrd in the Clraringhousr sprv(brs of thr Xerox C'orporat~c~ Iiitcrnc4, solviiig long-standing prol>lf~lns of high traffic and tlatirl>ilsr inconsistcllcp.

The Etherphone™ system was developed at Xerox PARC to explore methods of integrating voice into existing distributed personal computing environments. An important component of the Etherphone system, the voice manager, provides operations for recording, playing, editing, and otherwise manipulating digitized voice based on an abstraction that we call voice ropes. It was designed to allow:

This paper presents an overview of the Cedar programming environment, focusing on its overall structure—that is, the major components of Cedar and the way they are organized. Cedar supports the development of programs written in a single programming language, also called Cedar. Its primary purpose is to increase the productivity of programmers whose activities include experimental programming and the development of prototype software systems for a high-performance personal computer. The paper emphasizes the extent to which the Cedar language, with run-time support, has influenced the organization, flexibility, usefulness, and stability of the Cedar environment. It highlights the novel system features of Cedar, including automatic storage management of dynamically allocated typed values, a run-time type system that provides run-time access to Cedar data type definitions and allows interpretive manipulation of typed values, and a powerful device-independent imaging model that supports the user interface facilities. Using these discussions to set the context, the paper addresses the language and system features and the methodologies used to facilitate the integration of Cedar applications. A comparison of Cedar with other programming environments further identifies areas where Cedar excels and areas where work remains to be done.

WFS is a shared file server available to a large network community. WFS responds to a carefully limited repertoire of commands that client programs transmit over the network. The system does not utilize connections, but instead behaves like a remote disk and reacts to page-level requests. The design emphasizes reliance upon client programs to implement the traditional facilities (stream IO, a directory system, etc.) of a file system. The use of atomic commands and connectionless protocols nearly eliminates the need for WFS to maintain transitory state information from request to request. Various uses of the system are discussed and extensions are proposed to provide security and protection without violating the design principles.