Microsoft MS-DOS CD-ROM Extensions
Networking CD-ROMS
29 March 1989

Although it is possible to share CD-ROM drives on a local area network or LAN, it is not as easy as it should be. While MS-DOS provides a single, stable platform to develop a file system driver like the Microsoft CD-ROM Extensions, there are a wide variety of LANs and LAN server implementations that do not provide a stable platform for which a file system driver like MSCDEX could be provided. Because each LAN implementation takes a different approach for server support, the approach for CD-ROM support on a server depends on what LAN implementation is being used.

This document should help clarify the present situation and help get you started.

At present, there are several CD-ROM products that allow sharing of CD-ROM drives on a LAN. These are:

	Microsoft	MSCDEX - The Microsoft CD-ROM Extensions
	Meridian Data	CD-NET
	Online		Opti-Net

Choosing which product depends on your LAN and your needs. 

There are some LANs, such as Lantastic by Artisoft, that can share CD-ROM drives using any version of MSCDEX on a Lantastic server. This is possible because their servers run as an MS-DOS application and do I/O with standard MS-DOS INT 21 services. LAN servers like this, that do not make assumptions about the underlying media or try to bypass MS-DOS and do use standard MS-DOS INT 21 services to access the drive letter, will likely work with all versions of MSCDEX.

There are several LAN products based on MS-NET or a similar LAN server model such as Ungermann-Bass or 3COM. Unfortunately, these products do not access files on the server using standard INT 21 calls and for several reasons due to assumptions inside MS-DOS about non-standard calls from the server, you cannot share CD-ROM drives on MS-NET based servers. Although the server seems to allow sharing of the CD-ROM drive letter, requests to the server from workstations do not work correctly.

Fortunately, MSCDEX Version 2.10 has a command line switch (/S) that instructs MSCDEX to patch the in-memory image of MS-DOS during its initialization to fix these problems. By including this parameter on the MSCDEX command line, MSCDEX can be loaded before the network server software is started, and the CD-ROM drive letters can then be shared by MS-NET based server software, and workstations will see the correct behavior. This solution requires only that the server use MSCDEX Version 2.10 and no software or hardware changes to the workstation. Only the server runs MSCDEX or loads any CD-ROM related device drivers. To the workstation, the CD-ROM server drives are indistinguishable from other server drives.

For LAN products that are not MS-NET based yet have NETBIOS support such as Novell or IBM PC-NET, both Online and Meridian Data have adapted the MSCDEX and CD-ROM Device Driver model to provide LAN CD-ROM support. Each workstation runs MSCDEX and a special CD-ROM device driver that accepts normal CD-ROM driver requests from MSCDEX and uses the NETBIOS to transmit the command to a network driver on a server. The network driver submits the request to a true CD-ROM device driver on the server and transmits the results back to the workstation pseudo CD-ROM driver. The psuedo driver in turn responds to MSCDEX. So long as the workstation CD-ROM device driver responds appropriately, MSCDEX is unaware that the command has passed through the network to a server. Contact Meridian Data and Online for information for these networks as they can both describe their products and features best.

Online offers one potential configuration for computer systems that do not wish to dedicate a machine to be a server. The workstation operates as above, but instead of communicating the workstations driver request to a dedicated server process, another user's workstation running a special TSR version of their system can field the driver request, submit it to the CD-ROM driver, and respond to the requesting workstation. This allows a network of workstations to share the CD-ROM drives that each computer has connected to it at the same time all workstations are available to the users. This option may work for many users although it does slow performance of the workstation when outside requests come in and uses up memory for the TSR system code.

At present, there is no available version of the CD-ROM Extensions for OS/2 although there is a way to access CD-ROM data in OS/2 on a network. Since from the outside, workstations cannot tell MS-DOS server drives that are shared CD-ROM drives using version 2.10 of MSCDEX from traditional block drives, even OS/2 machines can access the CD-ROM drive on the server. Although this does mean including an MS-DOS server on an OS/2 LAN, it does provide at least an interim way to access CD-ROM data under OS/2 at this time.

For more information on CD-ROM LAN products for LANs other than MS-NET, contact:

Online Computer Systems
Mr. Mike Romanies
20251 Century Blvd
Germantown, MD 20874
301.428.3700

Meridian Data Inc.
Mr. Greg Smith
4450 Capitola Road Suite 101
Capitola CA 95010
408.476.5858
408.476.8908 (Fax)
MSCDEX - Microsoft MS-DOS CD-ROM Extensions Version 2.20

Networking CD-ROM's - Copyright (C) Microsoft Corp. 1989. All rights reserved - page {page|1}

Networking CD-ROM's - Copyright (C) Microsoft Corp. 1989. All rights reserved - page {page|1}