CD-ROM was conceived as a single-user technology, but it can be expensive to dedicate a PC, drive, and CD-ROM disk to each user. Therefore, CD-ROM LAN configurations were developed. A CD-ROM drive cannot be attached to a network without special hardware and software because most networks use a centralized file server and require their own file structure--one different from the file structure used by CD-ROM. Another problem is the fact that some networks dont support devices with more than 250 MB of storage. Finally, there is the MS-DOS limitation of 32 Mbytes (overcome by Microsoft CD-ROM Extensions).
The early CD-ROM LAN products were nothing more than switching devices, allowing the connection of any one of several workstations to any one of several disk drives. Only recently has "caching" been introduced to make it possible for multiple users to access the same disk in rapid succession. The cache stores information which has been frequently accessed so that the next request will come out of RAM, rather than off a CD-ROM disk. The composition of the cache is constantly changing. Caching makes it possible for several users to access a CD-ROM disk without degrading performance.
Today's CD-LAN products act as self-contained nodes connecting directly to the network like a workstation. After connecting a CD-LAN node to the network, the library decides which workstations will have access to the CD-LAN node and adds a device driver and a Microsoft CD-ROM extension to the CONFIG.SYS file of the selected workstations. The number of workstations depends on the capacity of the LAN used. At that point, the operating system automatically assigns drive letters to each CD-ROM drive. In the end, each CD-ROM drive appears as a logical device on the workstation. If a CD-ROM drive is assigned the letter "E," the user only types "E" to access that drive.
Unfortunately, there are few benchmarks available for the popular CD-ROM LANs to determine the impact of having several workstations seek access to the same disk at the same time. The vendors claim that it is very unlikely that there will be contention. One company representative told the editors that if 20 users on a network access the same drive an average of 20 times a day for a total of 440 "hits," there would be only a 1.38 percent chance of a conflict. However, that assumes an absolute even distribution of demand throughout the hours the library is open. A more realistic scenario is a group of students coming to the library after class to pursue an assignment and several of them seeking to access the same data base. In response to that situation, a second, and possibly a third, CD-ROM drive may have to be devoted to the same data base.
Anecdotal evidence-collected by calling six libraries using CD-ROM LANs--suggests that up to five persons can be working with the same data base at one time without an unacceptable deterioration of response time (more than 15 seconds). A possible reason for this is that most users are inexpert and relatively slow. Therefore, the network doesn't really encounter concurrence for much of the time the five users are active.
The most widely used CD-ROM LANs in libraries are the products of Meridian Data (CD NET), Online Computer Systems (OPTINET), Information Access Corporation, SilverPlatter (MultiPlatter) and H.W. Wilson--the last three all vendors of CD-ROM disks. The first two generally support more disk drives and more workstations and offer greater file caching than the other products. The last three are proprietary products which generally are sold as part of complete product packages. This article will focus on the first two because most libraries seek to obtain CD-ROM publications from several sources and want to avoid the limitations of a proprietary product.
CD NET is the most popular CD-ROM network product in both North America and Europe. In the last year, CD NET has been installed in over 100 locations in the United States and Europe, including university, health sciences, and corporate libraries.
CD NET from Meridian Data is a CD-ROM read-only network server which provides the hardware and software to share data bases over a local network of up to 75 workstations. CD NET runs under Novell network software on a Local Area Network using Arcnet, Ethernet, or Token Ring network interface cards. Recently 3Com, Ungermann-Bass and IBM PC-LAN options have been added. CD NET also can be used on TCP/IP networks and on Digital VAX equipment through gateway servers. CD NET is compatible with all High Sierra and ISO 9660 formatted CD-ROM disks (the accepted industry standards).
There are two basic 14 drive devices: the 286 CPU-based CD NET Model 214 at $20,430 and the 386 CPU-based CD NET Model 314 at $22,430. A system with fewer drives uses the same cabinet. The simple formula for calculating cost is to subtract $1,000 per drive not installed--down to a minimum of $7,995 for a single drive 214, and $9,995 for a single drive 314. The price includes Ethernet or ARCNET LAN support. Token Ring is $400 (4 Mbits) to $600 (16 Mbits) more. The prices for Models 214 and 314 also include CD NET Version 3.0 software. The configuration includes up to 7 MB of data caching, allowing data retrieval at full network bandwidth. This cached data is then retrieved at up to 1.5 times local CD-ROM transfer speeds. The Microsoft CD-ROM extensions are available through Meridian on a site license basis at a cost of $159 for up to ten users, $299 for 22 to 25, and $699 for 26 to 100.
CD NET accepts multiple copies of the same disk for increased performance and is designed with caching software. This means that information can be temporarily transferred from the CD-ROM--where access time is relatively slow--to the computer's RAM (Random Access Memory) cache for faster retrieval.
CD NET comes complete with CD-ROM players, a network interface card, cables and connectors, server and workstation software. It is not possible to use CD-ROM drives other than those provided by the vendor on CD NET, but they could be attached to the LAN if the vendor of them provides a CD-LAN network product. The recommended workstation configuration is a 286-8MHz class CPU, 640K RAM memory, a 20 MB hard disk and a floppy disk drive.
OPTINET from Online Computer Systems operates on IBM PC and Ethernet networks, using 3COM or Novell software. The IBM PC LAN supports up to 32 simultaneous users; the Ethernet LAN up to 128. OPTINET produces single-drive units that may be linked to multiple stations on the network, as well as four-drive and eight-drive units that can run different disks simultaneously. Either CD-ROM or WORM drives can be used on OPTINET. Unfortunately, the interface to the CD-ROM drives is proprietary. OPTINET includes a feature that makes it easy to find software on a multiple drive configuration: the system manager names each CD-ROM application so that users don't need to know which title is on which drive.
OPTINET is less widely used than CD NET, in part because it is less aggressively marketed; however, for small networks of fewer than ten drives it appears to be less costly. For example, a four drive system is under $6,000--approximately half the cost of CD NET. Expanding CD NET, once the basic drive cabinet is installed, is less expensive, however, and the maximum capacity substantially greater.