Chapter 9 How Do Users Share Computer Files?

What is a File Server A (central) computer which stores files which can be accessed by network users.

File Servers (Continued) Example: The CS dept has a file server in its network where each user can store files that can be accessed from other computers. It is easy to connect to from my department’s network and possible (but complicated) to access from elsewhere. My files are available to me and to no one else. If I wish to share my files I have to send copies to other individuals via or put them on a web site where others can download them.

How are files accessed on a network? Download the file from the server and make changes to that file. This leaves the original on the server unchanged unless the user uploads the change (This is what I do to update my website). The user modifies the file on the server without downloading it. (I sometimes do this to fix something quick.)

FTP (File Transfer Protocol) What you do to copy files to your Web site. It can also be used to copy files from your Web site. It can also be used independent of the Web to copy files from one computer to another as long as you have permission to use both resources.

Remote File Access This technique is used when a program on your computer is acting on the file on the server and not a copy of it. Updating the (only) copy of the file  Write-Through – immediate update, each change is automatically reflected in the document  Write-on-close – the changes are reflected in the document when the user is finished.  Write-delayed – batch updates, for example the file could be updated every two minutes.

How are files shared? FTP  We’ve covered this a bunch of times Anonymous FTP  It’s just like regular FTP except that you don’t need a username and password to access the account. There is a standard username usually anonymous or possibly guest. The password is your address.  So if you were to make something anonymously available to me I would use anonymous as my username and as my

How are files shared (continued) Remote File Access The problem occurs if several people have the right to access the file simultaneously, i.e. if you and I were editing the same paper at the same time or if you were reading it while I was editing it. Cache cohesion – the copy you’re reading is not the copy I’m editing so you’re out of sync Client-initiated update – the server version updates when I finish editing but you aren’t told Server-inititated update – when I finish your copy is automatically updated.

Why can some computers access some files but not others? Necessities for file access  Your computer must be able to physically connect to the server.  You must have permission to access the server and the directory and the file in the way that you need to access the file.  Your computer (software) must be able to interpret the data in the file. Encryption Common file format

When someone wants a file from a server, how does the server know what method to use? TCP/IP (Transmission Control Protocol/Internet Protocol  Data is sent in packets.  Each packet consists of the data being sent plus a header for that packet.  One of the items in the header is a 16 bit port address (which is essentially a number from 0 to 65535).  The ports are associated with particular applications.  For example ports 20 and 21 are associated with FTP, port 80 with HTTP.

What makes TCP/IP work? Logical levels are used to subdivide the work to be done. The ISO (international standards organization) has specified OSI (open systems interconnection reference model) with seven levels:

What Makes TCP/IP work (cont)? OSI Layers  Layer 1: Physical Layer – listens to the cables to see if stuff is being sent and sends bits when the line is clear  Layer 2: Data Link Layer – moves groups of bits called frames from machine to machine  Layer 3: Network Layer – routes packets from source to destination  Layer 4: Transport Layer – divides messages into packets  Layer 5: Session Layer – higher level capabilities  Layer 6: Presentation Layer – higher level capabilities  Layer 7: Application Layer – higher level capabilities

Summary Files can be shared in three different ways:  Each member of a group can keep their own personal copy of the data and they can copies to each other and be personally responsible for keeping the copies in sync.  The members could agree on a common file format and keep copies of the file on a server and use FTP to transfer between their machines and the server. They would work on the files on their own machines

Summary (Continued) Files can be shared in three different ways  The members could work on the file on the server itself using remote file access.  The application that the group is using must insure that either only one person can work on a particular file at one time or that if several people work on a file simultaneously, that the correct copy is maintained.

Summary (Continued) If different software (o/s) is used by different members of a team to view data, it may look different on different platforms. TCP/IP is probably the most common method of transmitting data through a network. Port numbers tell the receiving computer which application to use. These port numbers are standardized by ICANN.

Terminology Anonymous FTP Cache Coherence Client Client-initiated update Concurrent update problem File Server File Transfer Protocol (FTP) Frame Location Independence Location Transparency Lock Port Address Packet Remote File Access Server Server-initiated Update Transmission Control Protocol/Internet Protocol (TCP/IP)