1. Chapter 4: Computer Networks
Revision

2. Computer Networks What is a Network? Firewall Bridge
What is a Computer Network?
A network is two or more computers, or other electronic devices, connected together so that they can exchange data.  For example a network allows:
Computers to share files,
Users to message each other,
Share Resources
Network connections between computers are typically created using cables (wires).
However, connections can be created using radio signals(wireless / wi-fi), telephone lines (and modems) or even, for very long distances, via satellite links.
ISP
Router
Firewall
Server
wireless access point
Bridge

3. Computer Networks Overview of Network Devices Setting up a Network:
Switch to connect Network Devices together using cables
A separate wireless access point - to allow wireless devices (e.g. laptops or smart-phones) to join the network
A router to connect your LAN to the Internet (WAN)
A firewall to protect your network from hackers
A Bridge to connects two parts of a Local Area Network together
Server(s) to manage network functions such as network security, network file storage (central), shared resources (such as printers).
Set up an account with an Internet Service Provider (ISP) to receive an internet connection to your location

4. Computer Networks Common Network Devices Router
Connects network/computers to the internet
Connects LANs/networks together
Transfers data between networks (Receives and Sends Data Packets)
Router can connect to devices using cables or wireless signals.
It stores information about which computer is connected to which network
Hubs
A hub is a device that connects a number of computers together to make a LAN.
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.
Normally used in small home based networks.
Switches
A switch, like a hub, is a device that connects a number of computers together to make a LAN.
A switch is a more ‘intelligent’ device than a hub: if it receives a message, it checks who 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, but also a little more expensive
Normally used in larger networks found in schools, offices etc.
Bridges
A bridge is a network device that typically links together two different parts of a LAN.

5. Computer Networks
Using a computer connected to a network allows us to…
Advantages of using Networks
Easily share files and data
Share resources such as printers and Internet connections
Communicate with other network users ( , instant messaging, video-conferencing, etc.)
Store data centrally (using a file server) for ease of access and back-up
Keep all of our settings centrally so we can use any workstation
Disadvantages of using Networks
Greater risk of hackers
Greater risk of viruses (spreading and disabling network)
The significant cost of extra equipment
When the network is down, cannot use network computers without network connection.
Print queues can be long

6. Computer Networks Use of Router, Wi-Fi and Bluetooth
WiFi is a technology which makes it possible to connect a device to a network or to a single computer through wireless communications, for example:
Laptops
Mobile Phones, PDAs and other handheld devices
Large computers which are WiFi enabled.
Bluetooth
Bluetooth is a wireless networking technology designed for very short-range connections (typically just a few metres).
Typical uses of Bluetooth:
Connecting wireless devices such as mouse, keyboard, headset to a computer.
Transferring files between devices.
Printing Wirelessly from a PDA or Mobile Phone.
Advantages of Wireless Technology:
Reduced cost of cabling
Safer – won’t trip over wires
Easier to connect other devices to the network
Makes the computer portable as long as it’s within range of the wireless access point
Disadvantage of Wireless Technology:
Limited area of network
Strength of signal is weaker
Possible slow data transfer speeds
Easier to hack into/less secure
Physical obstacles can interfere with signal/can cause disconnection

7. Computer Networks LAN Types of Networks (LAN & WAN)
LAN is a Local Area Network
LAN covers a small area/one building
Often a LAN is a private network belonging to an organisation or business. 
Because LANs are geographically small, they usually use cables or low-power radio (wireless) for the connections.
WAN
A WAN is 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. 
Because WANs are often geographically spread over large areas and links between computers are over long distances, they often use optical fibre (glass) cables, satellite radio links, microwave radio links, etc.

8. Computer Networks WLAN WLAN – Wireless Local Area Network
A wireless LAN (WLAN) is a LAN that uses radio signals (WiFi) to connect computers instead of cables.
At the centre of the WLAN is a wireless switch or router
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.

9. Computer Networks Internet Intranet
Common Networks – The Internet & Intranets
Internet
Internet is Public (available to all users)
Internet is network of networks
Internet is global
Internet has more information than an intranet
Typical uses of an internet would be: 
Viewing web pages 
Sending and receiving   messages
Sharing files
Communicating using voice (VOIP) and video (video-conferencing)
Playing multi-player games
Streaming Video/audio Content
Online Shopping/Banking
Intranet
Intranet is within one organisation (Private)
Intranets tend to be policed/managed
Intranet has an extra layer of security
Data found in an intranet is likely to be more reliable/relevant than that found on the Internet
  Typical uses of an intranet would be: 
Viewing internal web pages (e.g. company schools, university's etc.)
Internal  and instant-messaging between workers
Sharing of internal documents

10. Can be used to monitor Internet usage
Computer Networks
Network and Data Security: Proxy Server
Proxy Server
Can be used to monitor Internet usage
Can block certain sites
Server passes on requests to the Internet
Passes the requested web pages to individual computers
Can cache/store the webpages
Can act as a buffer (between Internet and LAN)
Can act as a web server

11. Computer Networks Network and Data Security: Firewall
Prevents computer accessing undesirable/unauthorised sites
Monitors and controls incoming and outgoing data traffic
Prevents unauthorised computers using the internet accessing the computer
Helps to prevent malware (viruses) getting into computer/from the internet
Keeps a list of undesirable sites/IP addresses
Keeps a list of acceptable sites/IP addresses
Warns you regarding threats/allows you to accept/reject downloaded programs

12. Computer Networks Network and Data Security: Data Encryption
Encryption is the process of converting information into a form that is meaningless to anyone except holders of a ‘key’. 
1. Data is scrambled up (Data is Encrypted).
2. If the data is intercepted it then it prevents people from understanding and reading the data.
3. The person with the key is able to Decrypt and understand the data.
Disadvantages
Data can still be deleted from system
Criminals can use encryption to keep incriminating material secure

13. Computer Networks Network and Data Security
Methods of Authentication (Network Security)
User Name & Password
User ID: When logging onto any network system, a user will be asked to type in a user ID. This will be unique to the user. A user ID could be made up of letters and numbers
Passwords: When the user has entered their user ID into the login they will then be asked to type in their password. This will appear at asterisks (******).
If the password does not match up with the user ID then then login will be rejected.
Biometrics
Finger prints, retina, face, voice can be used to gain entry to a system.
Main advantage is the data will be unique and hard to replicate.
Magnetic Swipe Cards
Will only allow people with the swipe card to gain access to the system.
Swipe cards are quite easy to update.
Disadvantages is the cards can be easily lost or replicated.

14. Computer Networks Network and Data Security
If your computer is connected to a network, other people can connect to your computer.  A person who gains unauthorised access to a computer system is often called a hacker.
How can hackers effect your network and data?
Introduce of viruses/malware/spyware to the network:
… which could result in data being deleted
Log key presses to get passwords to access the system
Threat to Data:
Data could be deleted
Data could be replicated
Data could be corrupted
Data could be edited
Data could be sold or pass on to other people.
Threat to Network:
Network may have to be shut down to quarantine virus.
This could stop production and cost an organisation a lot of money.
Methods to keep Network & Data Secure other than authentication:
Install Anti Virus
Spyware
Use of Firewall