Application sharing is an important aspect of collaborative computing. One mechanism for sharing applications is a multiplexor, a tool that broadcasts input/output messages from a single client to multiple displays. We describe the design and implementation of a multiplexor for X windows that supports a heterogeneous environment and works effectively across both local and wide area networks. This X-multiplexor is composed of two distinct components, the Collaborative Computing Frameworks X-Multiplexor (CCFX) which acts as a pseudo server for the client, and the Collaborative Computing Session Manager (CCSM) which renders the client images and supplies the user interface to the system. In effect, CCSM acts as a distributed window manager, ensuring that all participants have the same view of the shared clients. This two headed approach offers significant gains in flexibility and resource usage. CCFX and CCSM can communicate using any reliable transport protocol. This can result in significant reduction of bandwidth, decreased latency, and avoids some TCP/IP communication required by the X protocol. In addition, this architecture facilitates the creation of a virtual X environment. Using this virtual environment, instead of an actual X server's environment, and then translating the virtual properties to a specific server's properties, allows for more robust support of heterogeneous systems.

CCF (Collaborative Computing Frameworks) is a suite of software systems, communications protocols, and tools that enable collaborative, computer-based cooperative work. CCF constructs a virtual work environment on multiple computer systems connected over the Internet, to form a Collaboratory. In this setting, participants interact with each other, simultaneously access and operate computer applications, refer to global data repositories or archives, collectively create and manipulate documents or other artifacts, perform computational transformations, and conduct a number of other activities via telepresence. Research issues addressed in this project include problem solving environments and methodologies for laboratory and instrument-based scientific disciplines, and computer science issues in heterogeneous distributed systems. New approaches are being investigated and developed for fast multiway communication, robust geographically distributed data management methodologies, high-performance computational transforms inlined within collaboration sessions, and related auxiliary issues such as active documents, security, archival storage, and experiment management and control. In this paper, we discuss the design philosophy and systems rationale behind CCF, describe the major subsystems of the collaborative computing environment, and discuss the salient features of the system.

In a closed, separable, queuing network model of a computer system, the number of customer classes is an input parameter. The number of classes and the class compositions are assumptions regarding the characteristics of the system's workload. Often, the number of customer classes and their associated device demands are unknown or are unmeasurable parameters of the system. However, when the system is viewed as having a single composite customer class, the aggregate single-class parameters are more easily obtainable. This paper addresses the error made when constructing a single-class model of a multi-class system. It is shown that the single-class model pessimistically bounds, the performance of the multi-class system. Thus, given a multi-class system, the corresponding single-class model can be constructed with the assurance that the actual system performance is better than that given by the single-class model. In the worst case, it is shown that the throughput given by the single-class model underestimates the actual multi-class throughput by, at most, 50%. Also, lower bounds are provided for the number of necessary customer classes, given observed device utilizations. This information is useful to clustering analysis techniques as well as to analysts who must obtain class-specific device demands.