Chapter Four




4-1 What is  a computer network?

A computer network is a collection of computers and peripheral devices connected by communication links that allow the network components to work together.

4-2 What is the important of computer network ?

Hardware sharing .(e.g. high speed laser printer, plotter )
Information sharing.
It allows for the electronic transfer of text.
It allows for communication between organizations.
High reliability by having alternative sources of  supply. For example, all files could be replicated on two or three machines, so if one of them is unavailable (due to a hardware failure) the other copies could be used.

4-3 Examples of computers networks :

Banking networks in which one bank card can be used at many automatic teller machines over a widespread area.

Message networks that allow people to use computers to send and receive mail electronically.

Airline, car rental, and hotel computer networks that enable travelers to make reservations from any wherein the country.

corporate computer networks that transfer the results of data processing operations between corporate locations.

4-4 Types of computer networks

4-4-1 Local Area Networks (LAN):

It can range in scope from a few city blocks to almost global

Is a system of interlinked personal computers, sharing common resources like disks, printers, etc...                       

It is usually thought of as being restricted to a single location.

Local Area Networks (LANs) can be useful for those frequent situations in which computer users wish to share information, secondary storage, peripheral devices, or a data base. For example a user may wish to send another user information in the form of a document, a graph, or a spreadsheet analysis.

A part of the concept of sharing information in a LAN is electronic mail. using electronic mail on a network involves sending documents, pictures, and messages to one or more persons. The user simply turns on the terminal or PC, enter the proper commands, types in the document, gives the names of those persons who are to receive it, and press a key. Instantaneously, a message that " mail is waiting " appears at the proper terminals or PCs. If the recipient is working at the terminal at that time, he or she can read the mail immediately, otherwise, the message appears on the screen the next time the terminal or PC is turned on.
By sharing a single hard disk drive, all users have access to certain information, while other information may remain specific to a given computer within the network. this hard disk is referred to as a file server, because it contains files that all users may access. These files may be data files or software packages.

In terms of sharing peripherals, having one high-speed letter-quality or laser printer or a flatbed plotter in the network that can be accessed by all PCs saves equipment costs. Instead of each PC having its own printer, print jobs can simply be sent to the print queue to be printed in their turn .

Sharing data base gives all users access the data base but cannot change it, thereby ensuring the security of the information.

Classification of LANs:

LANs can be classified according to the configuration of the network into :

start network:
In a star network a host computer has multiple computers connected to it. Since the host controls all data communications, any communications between any two computers must be routed through the host computer. if the host fails, then the entire system goes down. Multiple PCs hooked to a file server form a star LAN.

 Bus network:
A bus network has computers that tie into a main cable or bus, with no one central computer, the failure of any one computer doesn't affect the overall performance of the network. To send a message to another computer requires only that the software be able to signal the correct computer to receive the message.

Ring network:
In a ring network, all computers are treated the same and any communications between two computers must be processed by any intervening computers. To avoid having the system go down when a single PC fails, ring LANs have ways to drop the single PC without interfering with the operation of the LAN.

4-4-2 Wide Area Network (WAN):

 It is a network covering large areas, usually connected via telephone links rather than dedicated lines. (usually Modems are used in WAN)

there are basically three types of WAN in use today:

  1. Academic and research networks.
  2. private networks designed to provide communications between a company's host computer and the employee's and customer's terminals.
  3. value-added networks.

Press to in large
Figure 4-1 Networks