Computer Networks

1. Computer Networks IGCSE ICT Section 4

2. Introduction Objective: To be acquainted with: • The definitions of networking • Network topology • Network peripherals, hardware and software

3. Network Definition • A network can be defined as two or more computers connected together in such a way that they can share resources. • The purpose of a network is to share resources. • A resource may be: • A file • A folder • A printer • A disk drive Or • just about anything else that exists on a computer. • A network is simply a collection of computers or other hardware devices that are connected together, either physically or logically, using special hardware and software, to allow them to exchange information and cooperate. • Networking is the term that describes the processes involved in designing, implementing, upgrading, managing and otherwise working with networks and network technologies.

4. Advantages of Networking • Computers can communicate and share data and files. • Storage facilities,and other hardware peripherals such as printers can be shared. • Data from all the computers can easily be backed up centrally. • Communication with other network users (e-mail, instant messaging, video-conferencing, etc.) is possible.

5. Disadvantages of networking • A computer on a network is vulnerable to hackers • If the network breaks, many tasks become very difficult • Your computer can more easily be attacked by a virus • Installing a network is expensive as it involves purchasing of networking hardware such as Hubs, cables, network cards, bridges, routers etc. • Large networks require specialist to maintain them, which again involves time and money.

6. Computers on a network • Computers connected together to create a network fall into two categories: • servers andclients (workstations). • Client computers are thenormal computersthat people sit at and get theirworkdone. • Servers are special, powerful computers that provide ‘services’ to the clientcomputers on the network.These services might include: • Providing a central, common file storage area • Sharing hardware such as printers • Controlling who can or can’t have access the network • Sharing Internet connections

7. Servers There are different types of servers, e.g. • File servers, which allow users to save and load data/files • Application servers, which allow distribution of application software to each computer • Printer server, which ensures that printing on a network is done in a queue. • Proxy server, which is used as a buffer between WAN and LAN.

8. Types of Networks Local Area Network • A local area network (LAN) is a computer network covering a small geographical area, like a home, office, or small group of buildings, such as a school, or an airport. • Often a LAN is a private network belonging to an organization or business.  • Because LANs are geographically small, they usually use cablesor low-power radio (wireless) for the connections.

9. Types of Network Wireless Local Area Network (WLAN) • A wireless LAN (WLAN) is a LAN that usesradio signals (WiFi) to connect computers instead of cables. • It is much more convenient to use wireless connections instead of running long wires all over a building. • However, WLANs are more difficult to make secure since other people can also try to connect to the wireless network. So, it is very important to have a good, hard-to-guess password for the WLAN connections.

10. Types of networks Wide Area Network (WAN) • A Wide Area Network is a network that extends over a large area.  • A WAN is often created by joining several LANs together, such as when a business that has offices in different countries links the office LANs together.  • The largest and most well known WAN is the Internet.

11. LAN, WAN and WLAN WLAN LAN WAN

12. Sample Questions • What is the difference between a LAN and a WAN ? • Give(a) one example of a LAN and (b) one example of a WAN. • Describe two advantages that a network has over a set of stand-alone computer systems. • What is the purpose of (a) a file server and (b) a printer server.

13. NETWORK HARDWARE • In order for a computer to operate on a network, there are a range of different hardware components that are required.

14. Network Hardware 1. Network Interface Card (NIC) • Any computer that is to be connected to a network, needs to have a network interface card (NIC). • Modern computers have inbuilt NICs. However you can also add an expansion NIC card. • Most laptops have two inbuilt NICs; one for the wireless network, and another for the wired network.

15. Network Hardware 2. Network cables • They are used to connect different devices together. • Cables are still used in most networks, rather than using only wireless, because • they can carry much more data per second, • They are more secure (less open to hacking).

16. Network Hardware 3. Hubs • A hub is a hardware device that connects multiple computers together in a network and allows them to share data packets. • Each computer will be connected to a single 'port' on the hub. • The typical use of a hub is at the centre of a star network • A hub is a ‘dumb’ device: if it receives a message, it sends it to every computer on the network. • This means that hub-based networks are not very secure - everyone can listen in to communications. These days hubs are not common. They have been replaced by switches.

17. Network Hardware 4. Switches • A switch is a more advanced form of a hub. • It is a more ‘intelligent’ device than a hub • If it receives a message, it checks which computer it is addressed to, and only sends it to that specific computer. • Because of this, networks that use switches are more secure than those that use hubs, • Switches are a little more expensive.

18. Network Hardware 5. Routers • A router is a network device that connects together two or more networks. • It transfers data from between the networks in an intelligent way; i.e. it forwards data packets to their destination using the most efficient route. A common use of a router is to join a LAN to the Internet (WAN).

19. Network Hardware 6. Bridges • A bridge is a network device that linkstogether two different parts of a LAN. • It allows a large network to be segmented into two LANs to reduce traffic being passed around the network. • The linked LANs act as if it is one LAN.

20. Network Hardware 7. Proxy servers • A proxy server is a computer setup to share a resource, usually an Internet connection. • It keeps a copy of often-used web pages in its local 'cache'. • If a client computer requests for a web site then the proxy server checks to see if it has a copy - if it does, then that local page is sent to the client and there is no need to use up bandwidth calling for it over the internet A proxy server can also act as a data filter, only allowing data packets that conform to company policy. E.g. blocking certain websites at certain times.

21. Network Hardwares 8. Firewalls • A firewall is a device, or a piece of software that is placed between a computer/LAN and the rest of the network to block unauthorizedconnections being made to your computer/LAN. • Normal data is allowed through the firewall (e.g. e-mails or web pages) but other suspicious data is blocked. • If you wish to protect your LAN from hackers out on the Internet, you would place a firewall between the LAN and the Internet connection.