Networking AT-class computers for image distribution

The Lister Hill National Center for Biomedical Communications is investigating a local area network approach to a system for the distribution of digital document images among a set of image retrieval and display workstations, each based on an AT-class computer. The goal of the study is to investigate rapid, online, multi-user access to a database of document images stored on optical disks. The display workstations have access to the document image server node via Ethernet physical and data link layers and inhouse-developed upper layer protocols, The demonstrated reliability of the Ethernet, combined with the restricted functionality of the application, allowed the development of upper layers involving a minimum of control and processing overhead. In addition, certain protocol tasks are performed by an 80186 coprocessor located on the Ethernet controller board, thereby improving the performance of the server ' node to a level beyond the constraints imposed by the AT-class architecture and the single tasking DOS. This strategy results in acceptable retrieval speeds perceived by a user at a display node, even with other active users in the system. This paper explains the protocol used in the networked system, describes the system's performance under various simulated user loads, and discusses the optimum network configuration in an operational environment. Introduction The Lister Hill National Center for Biomedical Communications (LHNCBC) is the research and development arm of the National Library of Medicine (NLM). It is currently developing a distributed Electronic Document Storage and Retrieval (EDSR) system based on AT-class computers. This prototype system is a testbed for investigating the applicability of recent advances in electronic imaging, optical disk storage, and high-speed data networks to document preservation, document image access, and library information processing issues[l]. The system consists of several workstations, each based on an AT-class computer, with peripherals and software to support the specific task of the workstation. There are, for example, scanning workstations, archiving workstations, and display workstations. These workstations can be loosely coupled, where images and other files are transported among workstations via removable magnetic or optical media, or tightly coupled, where the workstations share images and files over a local area network (LAN)[2]. Backaround An important aspect of the prototype distributed EDSR system and of any operational system is rapid access to and display of stored images. This paper describes a LAN approach for document image distribution in which a database of document images are stored on optical disks mounted on one or more drives located at an image server workstation. Several display workstations request and receive images from the single server via the LAN and display the images locally. Such a configuration allows many users simultaneous access to a large database of images. However, the throughput of the server and the LAN must be sufficient to support a full compliment of users without causing noticeable delay in service time at any display workstation. Anolication Confiouration Figure 1 illustrates an anticipated configuration of a document image distribution network. In this application, the images have already been scanned, compressed and stored on optical disks. One or more image servers have optical disks mounted on one or more drives. The image display workstations retrieve compressed image files from the servers via a LAN, expand the image and display it on the local high resolution image display monitor. The bulk of the network traffic is image data from the servers to the display workstations. Each server responds to individual requests for page images. These requests come from several display workstations, each supporting a user that is reading various documents from various subject areas. The probability that sequential requests at the server are for the same page is so small that caching page images at the server node would result in negligible improvement in