Charles J. Antonelli

Hierarchical decomposition and simulation of manufacturing cells using Ada

A useful tool in the development of flexible automation is a system description language which can generate a complete func tional description of a manufacturing cell of arbitrary complexity. We propose a description system based on the concept of hierar chical decomposition utilizing the Ada programming language in conjunction with established diagrammatical decomposition methods. The distinguishing aspect of our work is that it takes advantage of certain features of Ada (such as type checking) to create a description that can be automatically verified for con sistency Simulation is often an indispensable tool in the develop ment of manufacturing systems. We show how a simulation of the operation of the manufacturing cell can be embedded in its description. Finally, we apply the methodology to a specific instance of a manufacturing cell.

Evaluating delayed write in a multilevel caching file system

Delayed write in a multilevel file system cache hierarchy offers a way to improve performance in diverse client/server scenarios, such as integrating mass store into a distributed file system or providing distributed file system access over low-speed links. Using file system traces and cache simulations, we explore extensions and modifications to the traditional client-caching model employed in such file systems as AFS, Sprite, and DFS. High cache hit rates at an intermediate cache server-a machine logically interposed between clients and servers that provides cached file service to the clients-combined with high client cache hit rates lend practicality to an integrated mass storage file system. In such a system, magnetic tape or optical-based mass storage devices may be used as a first-class data repository, fronted by disk and RAM caches to offer acceptable access times to the large, but slow, mass storage system. Similarly, a high cache hit rate is necessary for users accessing file systems via low-speed links, where a delayed write intermediate caching server can mediate traffic to make better use of available bandwidth. In an example taken from mobile computing, an intermediate server might be used as a docking station at a users home. This arrangement would be convenient for users of mobile computers who upload large amounts of data generated while operating in disconnected mode. Simulations of delayed write caching strategies are applicable to both the mass storage and low-speed link scenarios.

Integrating mass storage and file systems

The authors describe current and anticipated work at the Center for Information Technology Integration at the University of Michigan in developing and integrating mass storage with distributed file systems, specifically with the Andrew File System (AFS). They present a specific approach to integrating AFS with mass storage: they consider the mass store itself to be the file system, not a bag on the side of a disk-based file system. Instead of developing a back-end server to manage the movement of data files between traditional disk-based storage systems (used, in the present case, by AFS) and magnetic-tape or optical-based mass storage systems (of which AFS has little or no knowledge), the authors envision the mass store as a first-class data repository. A traditional disk-based file system serves as a very large cache of the mass store system. On top of that is another, large, high-speed memory cache. All storage other than the mass store is used exclusively for caching. In this approach, cache management policies are of fundamental importance.<<ETX>>

Cryptographic wiretapping at 100 megabits

This paper describes the Advanced Packet Vault, a cryptographically secured archiver of network packet data that reliably captures all packets on a 100 Mbps Ethernet network, encrypts them, and writes them to long-term magnetic tape storage for later analysis and evidentiary purposes. Based on a previous prototype, the APV provides an enhanced cryptographic organization that allows site-specific selection of the encryption format and that permits selected traffic to be made available without compromising the security of other traffic. The APV operates reliably under a continuous 100 Mbps load. We conclude with a discussion of future work necessary to scale the APV beyond 100 Mbps.

An alternative to asynchronous transfer of control in Ada 9X

We have observed the introduction of the asynchronous transfer of control (herein named ATC) into Ada 9X with some concern. We analyze the fundamental problems of ATC in the real-time environment and propose an alternative solution which we believe has improved properties.