1. Introduction to Computers
Academic Year: 2014 – 2015
Fresh Year
Chapter Four: COMPUTER NETWORKS
Instructor: Mukhtar M Ali “Hakaale”
BCS

2. What is computer network
A computer network consists of two or more computers that are linked in order to share resources (such as information or data, printers, CD, scanners….etc), exchange files or allow electronic communications.
The three types of computer networks are:
1- LOCAL AREA NETWORKS. (LAN)
2- METROPOLITAN AREA NETWORK (MAN)
3- WIDE AREA NETWORK (WAN).

3. Local area networks (LAN)
 LOCAL AREA NETWORK IS A NETWORK CONSISTING OF TWO OR MORE COMPUTERS THAT ARE CONNECTED TOGETHER WITHIN SMALL GEOGRAPHICAL AREA SUCH AS A LAB, SCHOOL, BUILDING ….ETC. THE COMPUTERS ON A LOCAL AREA NETWORK CAN BE LINKED THROUGH CABLES, TELEPHONE LINES & RADIO WAVES.

4. Local area networks (LAN)

5. Metropolitan area network (MAN)
Metropolitan area network is a network consisting of two or more computers that are connected together within large geographical area such as a town, city, and metropolitan city. The computers on a local area network can be linked through cables, telephone lines.
A Metropolitan Area Network (MAN) is a network system that covers area of a single city. Usually, MAN connects more than one LANS in a city or town and covers a smaller geographical area than a WAN.

6. Metropolitan area network (MAN)

7. Metropolitan area network (MAN)

8. Wide area network (WAN)
Wide area network is a network consisting of two or more computers that are connected together within very large geographical area such as different cities of country or different countries of the world. The computers on a local area network can be linked through cables, telephone lines or satellites
 A Wide Area Network (WAN) is a network system that covers a very large (wide) geographical area such as different cities of country or different countries of the world. Most telephone companies are WANs.

9. Wide area network (WAN)

10. A gateway performs the translation between two different types of networks.

11. Data Communications over Standard Telephone Lines - Modems
Most telephone lines attached to home and businesses digital, not Because PCs transmit and receive data in digital format, a device called a modem is needed to convert digital data to analog format for transmission over phone lines. When receiving data from another computer, the analog format to digital format.

12. Data Communications over Standard Telephone Lines – Modems Cont …
This modem converts
digital to analog.
This modem converts
analog to digital.

13. Data Communications over Standard Telephone Lines – Modems Cont …
When choosing a modem, consider the following factors:
Transmission speed - the speed at which the modem sends data - which is measured in bits per second (bps) .
Data compression, the technology the modem uses to shrink data so it can be transmitted faster.
Error correction, the method the modem sent and received without errors. uses to ensure data is sent and received without errors.
Internal versus external, which describes whether or not the modem fits inside the PC case.

14. Data Communications over Standard Telephone Lines – Modems Cont …
Internal Modem
External Modem

15. Advantages of installing a network:
1- RESOURCE SHARING:
The computers on a network share different resources that are available on the network such as printers, scanners, information……ect.
2- COST CUTTING :
Having a computer network can help an organization to cut the unnecessary operating costs such unnecessary transportation, manual transferring of data……etc. 
3- SECURITY: 
Files & programmes on a network can be designated as copy protected or “copy inhibit” so that you do not have to worry about illegal coping of programmes & data.
4- SPEED : 
Networks must provide a very rapid method (very fast) for sharing & transferring files. Without a network files are shared by copying them to floppy disks, cd ……etc. 
5- FLEXIBLE ACCESS: 
The network users must be able to access their files & data from computers throughout the network where ever they where.
A) Simultaneous Access
B)Shared Peripheral Devices

16. Disadvantages of installing a network:
1- EXPENSIVE TO INSTALL:
Although a network will generally save money over time, the initial costs of installation can be expensive. Cables, network cards and software can be expensive
2- REQUIRES ADMINISTRATIVE TIME:
Proper maintenance of a network requires considerable time & expertise. For example many schools have installed a network, only to find that they did not budget for the necessary administrative support.
3- FILE SERVER MY FAIL:  
Although a file server is no more susceptible to failure than any other computer, when the server goes down (server fails) the entire network may come to a halt (stop). When this happens, the entire organization may lose access to necessary programs, files &data.  
4- CABLES MAY BREAK:
Some of the configurations are designed to minimize the inconvenience of broken cables. Some of the configurations are designed to minimize the inconvenience of broken cables. With other configurations one broken cable can stop the entire network because that depends on the topology type used.

17. The Uses of a Network - Simultaneous Access
In organizations, many people may need to use the same data or programs. A network solves this problem.
Shared data and programs can be stored on a central network server. A server that stores data files may be called a file server.
Managers may assign access rights to users. Some users may only be able to read data, others may be able to make changes to existing files.

18. The Uses of a Network - Simultaneous Access Cont …