Chapter 6

Networks and eMail

A computer network is a collection of computers and other devices 
that communicate to share data, hardware, and software. If computers 
are in a limited area, called LAN or local area network.

A major advantage of being on a network is the ability to share 
resources - software, storage space, peripherals - beyond those available 
on your own computer.

Each device on a network is called a node. The network server is a 
special computer that "serves" or delivers resources to the other 
computers. In order to use the network, you must log on by specifying 
your user id and password. 

To access the server's hard drive, you must know the drive mapping 
or letter assigned to it. Normally, the server hard drive will be denoted 
as F:, your hard drive is denoted as C: . There are two kinds of files 
stored on drive F:, data files and program files.

When you start an application program that is stored on drive F:, the 
program is loaded into your computer's RAM memory and executes on 
your computer, not the server's computer. It acts just as if you had 
started it from your own computer's disk. Many users can launch the 
same program from the server at the same time. A copy of the program 
is stored in each computer's RAM.

What's the advantage? There is no need to maintain duplicate 
copies of the same software on the hard drives for all the computers in 
the network. You only need to store it on the server's hard drive. If 
there is a modification or upgrade, you only have to upgrade one 
copy, not several. You are certain that everyone in the company is 
using the same version of the software. May not be the case if they 
have their own copies. Although it costs more for a site license for 
software to be used on a network, it will still be less expensive than 
purchasing multiple copies.

You can also access data files on the server. This is a little more 
complicated because it is not OK for several people to be accessing 
the same file at the same time. If they were just reading it, that would 
be fine. But if they are editing it, one user's changes may contradict the 
other's. Let's suppose two people were editing at the same time. They 
would both get the original file, say version A. When the first person 
finished, she would save her work, version B. But the other person is still 
working with the original copy, so he has no knowledge of the changes 
that were made in version B. When he saves the edited file, version C, it 
will supercede versions B and A. 

To prevent this problem, a file will be locked once it has been 
accesssed. This means that it cannot be opened by another person on 
the network until it has been closed by the first person who opened 
it. This is actually the case on a single computer that allows multiple 
tasks to be performed simultaneously. A file that has been opened by 
one application cannot be opened by another until the first applicatin 
closes it.

Network Printing

If you are on a network, stuff you send to the printer will print on a 
local printer if you have one. If not, it will be diverted to the network 
printer. Files waiting to be printed are saved in a print queue. 

Network Hardware

TO be part of a network, your computer must have a newwork 
interface card plugged into an expansion slot. Cabling that connects 
your computer to the network plugs into the card's ports. Different 
network cards are needed for each type of network: ethernet, token 
ring, etc. 

Server types:

Dedicated file srver: used for communicatin only. Delivers programs and 
files to workstations. Does not run programs. Usually an upgraded 
microcomputer. However, sometimes it is a minicomputer or mainframe.

Non-dedicated file_sever: One computer in the network can be used 
as a Personal computer. But other computers in the newtowk can access 
data and programs on its hard disk. Usually ha increased disk capacity.

Print server A computer that manages the print queue. Accetpint and 
storing jobs to be printed and sending print jobs to the printer. It can 
be the same computer as the file server or a different computer.

Application server: A compuer that runs application software and 
forwards the results to a workstation (computer). Makes it possible to 
use the processing power of both the workstation (the client) and the 
server. Called client-server archtitecture. The server is usually a 
more powerful computer than the client.

Example: A very large data base and a database program could be stored 
on a minicomputer. You send a request from your computer to locate 
records wth particular features. This request is processed by the server 
which runs the database search program on the database file. More 
efficient than downloading the search program and database to your 
machine and having your machine do the task. The results of the search 
(a list of records) is sent to the client.

Host computers: These are computers, minicomputers or mainframes, 
that have several terminals connected to them. The terminals do not 
have processing power. All processing takes place on the host. The 
terminals are for input (keyboard) and output (monitor). This is called 
timesharing because the different users all share the same processor. 
You can use microcomputer equipped  with terminal emulation 
software as smart terminals. The microcomputer can act like a 
terminal and leave the processing to the host or it can also run its own 
applications. But it can't do both at the same time.







Elliot Koffman, Professor
CIS Department
Temple University