Network Operating Systems

A network operating system (NOS) causes a collection of independent computers to act as one system. A network operating system is analogous to a desktop operating system like DOS or OS/2, except it operates over more than one computer. Like DOS, a network operating system works behind the scenes to provide services for users and application programs. But instead of controlling the pieces of a single computer, a network operating system controls the operation of the network system, including who uses it, when they can use it, what they have access to, and which network resources are available.

At a basic level, the NOS allows network users to share files and peripherals such as disks and printers. Most NOSs do much more. They provide data integrity and security by keeping people out of certain resources and files. They have administrative tools to add, change, and remove users, computers, and peripherals from the network. They have troubleshooting tools to tell network managers what is happening on the network. They have internetworking support to tie multiple networks together.

Redirection

At the heart of the NOS is redirection. Redirection is taking something headed in one direction and making it go in a different direction. With redirection, an operating program does not know or care where its output is going.

You are probably familiar with DOS redirection. For example, the DOS command DIR > FILENAME will redirect a directory listing to a file instead of to the screen. The ">" tells DOS to give the results of the command to the entity on the right.

Network operating systems depend heavily on redirection, only in this case data is being redirected from one computer to another over the network cable, not over the PC's bus to local files or printers. Nevertheless, the operation is similar. If you type "COPY C: FILEA F:", FILEA will be copied from your local drive C: to the network drive F:. The NOS makes it appear to the COPY command that drive F: is local, when it really resides on another computer that is attached to the same network. The COPY command doesn't know or care that drive F: is across the network. It sends the file to DOS and the NOS reroutes the file across the network to drive F:.

Redirection can be done with printers and other peripherals. Thus, LPT1: or COM1: can be a network printer instead of a local printer and the NOS redirects files to these devices. With a NOS, users don't need to know about redirection; they just type the drive designator or print from their word processors as always.

Server Software

The computer with drive F: must expect data, if the output from the user 's PC can be redirected successfully. To do this, it must make its drive available to network users. This is part of the NOS's function at the server.

A NOS is made of a redirector and a server. Not all machines need to run the server software, because not all computers need to share their resources. But all network workstations must run redirector software because every client has to be able to put data onto the network.

With some NOSs, the computer running the server software cannot be used as a workstation. This is called a dedicated server. Novell's NetWare uses this kind of setup almost exclusively (although the low-end NetWare Lite can use nondedicated servers). With some other NOSs, all workstations on the network can also be servers. This a nondedicated server setup. This approach is used by Sitka and Artisoft, among others.

The two server approaches have advantages and disadvantages. Nondedicated servers allow for more flexibility, since users can make resources available on their computers as necessary. However, a nondedicated server approach requires that the users are willing to take some administrative responsibility for their computers and it necessitates that they be somewhat network-literate. Backing up the shared data, setting up security, and setting up access rights become more complicated and often become the responsibility of the user, not the administrator. Another drawback is that nondedicated servers often suffer some performance degradation when being used simultaneously as a workstation and as a server.

Dedicated servers have the opposite advantages and disadvantages. They are easier to administer since all data is in one place. They are faster because they don't have a local user to serve. On the other hand, it is harder to make resources available on an ad hoc basis, since setting up a server is more difficult and time-consuming . If a dedicated server fails, all users are forced to stop working because all resources are centralized. Your choice of dedicated or non-dedicated operation will depend on the work your network is doing.

File Service

A file server's primary task is to make files available to users, although it also makes other resources available, including printers and plotters . File service allows users to share the files on a server. The server PC can make its whole disk, certain directories, or certain files available. The file server's hard disk becomes an extension of each user's PC.

The NOS can let the network administrator determine which users are allowed to use which files, for example, keeping the mail clerk out of the payroll file. Suppose a user wants to use a file residing on the file server's hard disk. Drive F: is set to correspond with the file server's hard disk. The actual process of setting up virtual drives has several names , including mapping, mounting, and publishing.

Now, suppose a user wants to run WordPerfect. At the F: prompt, the user types "WP" to load WordPerfect. WordPerfect is loaded from the server over the network, and into the user PC's memory. Meanwhile, other people can use WordPerfect from the file server ( assuming there is a license for multiple users). WordPerfect makes sure no other user can get the document file being used by "locking" the file. With many applications, file locking allows other users to read the document but not edit it.

File service is an extension of the local PC. Applications work just as they would on a local PC. Some programs, however, have been designed to take advantage of the network, rather than just run on one. For example, some databases allow two users to edit the same table but not the same record and each user can see the other's changes.

The NOS provides much more than just file service; it provides security, administration, printer sharing, backup, and fault tolerance.

Server Operation

The server software makes a single-user computer into a multiuser machine. Instead of just one user, a server has many users. But we must qualify what we mean by "many users." A NOS allows many users to share the server's peripherals, printers, disks, and plotters, but it does not allow multiple users to share its processor. For now let's see how the file server allows users to share its peripherals.

In many cases, the file server is running the PC's native operating system (such as DOS or the Macintosh OS) as well as the NOS. When users' requests come in, the NOS receives and interprets them, then hands them to the operating system for execution. So if a request comes in to open a file, DOS opens the file and gives it to the NOS, which gives it to the user. If many users make requests at the same time, the NOS queues them and hands them to DOS one at a time.

High-performance NOSs, including Novell NetWare, Banyan VINES, and Microsoft LAN Manager, do not run DOS in the file server. DOS is replaced with a multitasking operating system, thereby gaining a performance advantage; however, they lose some compatibility and require dedicated file servers. In NetWare's case, it is a proprietary OS. VINES runs Unix; LAN Manager currently runs OS/2 but eventually will use Windows NT.

This tutorial, number 6, written by Aaron Brenner, was originally published in the January 1989 issue of LAN Magazine/Network Magazine.