Upgrading and Repairing Networks (5th Edition)

Understanding how a particular protocol functions will better enable you to troubleshoot network problems that prevent users from timely access to file resources. For example, using a LAN analyzer to review network traffic during a troubleshooting session will be of little use unless you know what types of frames you are looking for and understand their function in the file-sharing process. You also can use the knowledge gained from this chapter to assist in making decisions about future additions to the network.

There are many ways you can share files. You can copy them to a floppy disk or tape cartridge (or more likely today, a CD you just burned) and pass them around the office (a la sneaker-net ). This is not a very efficient method when your volume of data grows and you find yourself trying to keep track of multiple versions of a file.

When TCP/IP was developed, several handy utilities were created to work with the protocol to provide clients some useful file- and system-sharing functionality. One of these, the File Transfer Protocol ( FTP ) , enables a user to copy a file to or from a remote computer to his own so that manipulation of the data can be done locally, and then the file can be transferred back onto the original server.

FTP is an ideal mechanism when you want to distribute multiple copies of a file to multiple people. When used in an environment where the goal is to allow multiple users to modify a file, however, FTP doesn't really improve much on the floppy method, except that the network can often handle a larger number of files over a greater distance more quickly. However, because users end up with more than one copy of the file, there is always the potential of creating mismatched versions when trying to coordinate multiple access to a file by making many copies of it. For example, if the user forgets to copy the file back to its original location after making changes, the next user who makes a copy of the file will find herself working on a file that does not contain these changes. Another problem with the copying method is that the network bandwidth (or lack of it) can become a problem for very large files, though this is less of a problem in today's faster networks. It may be a hindrance, however, in slow dial-up connections to the network.

FTP, telnet, and a host of other useful network utilities are covered in Chapter 26, "Basic TCP/IP Services and Applications."

Other TCP/IP utilities can be used to access files remotely. For example, you can establish a telnet session to a remote computer and then issue commands locally to manipulate data.

Using telnet, the user's PC or workstation acts as nothing more than a terminal emulator, and a rather expensive one at that. All applications that are needed to manipulate the data must be installed on the remote computer. This is probably a better method to use than copying from an FTP site or floppy disk when trying to share a single file among many users, because it maintains only one copy of the file. However, it is still not a very convenient method for several reasons. If you want to access files on more than one remote system, you need a separate telnet session for each one. This means it is not possible for an application, such as a word processor or database, to access files or remote systems at the same time (because the word processor or other application is running on the target system and not the client workstation). When using a telnet client, the user must have a user account set up on each remote system so that the logon can be validated . This can be overcome by using a single sign-on technology such as the Active Directory. But using telnet is not as transparent a process as simply running an application on the user's workstation and accessing files in the local file system.

The Network File System ( NFS ) protocol was developed by Sun Microsystems to make remote file access as simple a process for the user as local file access. NFS enables a user to access a remote file system while making it appear to the user to be a local file system. There is no need to copy files back and forth from servers. Using NFS, a file system (or a portion of it) residing on a remote system can be made to appear to the client as though it were simply part of the local file system. Early on, NFS was found only on Unix boxes. However, its popularity spread, and you can now find NFS server applications and client applications for most major operating systems. A minor disadvantage to using NFS is that network problems can interfere with file access. However, this is true for any kind of network file-sharing protocol.

Microsoft operating systems have long used the Server Message Block ( SMB ) protocol to provide file and printer access to networked clients. This protocol has developed over the years and has been adopted into Windows NT and Windows 2000. Whereas NFS is built on top of several other complex protocols and is used to provide only file-sharing capabilities, SMB is a more basic protocol that can be used across a network to provide network access to files and print sharing to interprocess communication, and other resources such as named pipes and mailboxes. The latest incarnation of SMB is called the Common Internet File System (CIFS) protocol.

In this chapter, you will briefly look at these protocols that enable you to share files on a network.