1. UNIVERSITY OF LUSAKA BIT 101 – LECTURE NOTES 7
INTRODUCTION TO BUSINESS INFORMATION SYSTEMS
UNIVERSITY OF LUSAKA
N. MOOLA

2. COMPUTER NETWORK

3. Computer Network Technology

4. What is a 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, a whole room of computers to share a single printer, etc.
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.
A computer that is not connected to a network is referred to as a stand-alone computer.

6. Why do we need a Network?
Using a computer connected to a network allows us to…
Easily share files and data
Get access to a huge range of information for research
- 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
- Make use of on-line services such as shopping (e-commerce) or banking
- Access different forms of entertainment (games, video, etc.)
- Join on-line communities (e.g. MySpace, Facebook, etc.)

7. The negative aspects of a Network?
Using a computer connected to a network means that…
The computer is vulnerable to hackers
- Your computer can more easily be attacked by a virus
- If the network breaks, many tasks become very difficult
In particular, if we use a computer connected to The Internet…
- We have to be careful about revealing personal information
- We have to be careful to avoid suspect websites that might contain malware
- We have to be aware that information found on The Internet is not always accurate or reliable

9. Computers in a Network
Computers connected together to create a network fall into two categories: servers and clients (workstations).
Clients
Client computers, or workstations, are the normal computers that people sit at to get their work done.
When you use your Web browser, you are in fact using a Web client. When you type in the URL (Uniform Resource Locator) of a web page, you are actually providing the address of a Web server.
e.g. is the address of the BBC’s web server.
Your Web browser/client asks this server for the web page you want, and the server ‘serves’ the page back to the browser/client for you to see.

10. Servers
Servers are special, powerful computers that provide ‘services’ to the client computers on the network.
These services might include:
- Providing a central, common file storage area
Sharing hardware such as printers
Software Development Server
- Controlling who can or can’t have access the network
- Sharing Internet connections
Servers are built to be very reliable. This means that they are much more expensive than normal computers. In a small network one server might provide all of these services. In a larger network there might be many servers sharing the work.

11. Types of Network
Local Area Network (LAN)
A Local Area Network is a network confined to one building or site. 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.

12. Types of Network
Wireless Local Area Network (WLAN)
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 - a small box with one or two antennas sticking out the back - used for sending and receiving data to the computers. It is much more convenient to use wireless connections instead of running long wires all over a building.

13. Types of Network
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.
Typically, the range of a wireless connection is about 50m, but it depends how many walls, etc. are in the way.

14. Types of Network
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.
Because WANs are often geographically spread over large areas and links between computers are over long distances, they often use quite exotic connections technologies: optical fibre (glass) cables, satellite radio links, microwave radio links, etc.
The Internet is an example of a global WAN. In fact it is the world’s largest WAN.

15. Types of Network
Wide Area Network (WAN)

16. Types of Network
Bluetooth (Personal Area Network)
Bluetooth is a wireless networking technology designed for very short-range connections (typically just a few metres).
The idea of Bluetooth is to get rid of the need for all of those cables (e.g. USB cables) that connect our computer to peripheral devices such as printers, mice, keyboards, etc.
Bluetooth devices contain small, low-power radio transmitters and receivers. When devices are in range of other Bluetooth devices, they detect each other and can be 'paired' (connected)

17. Types of Network
Bluetooth (Personal Area Network)

18. Bluetooth (Personal Area Network)
Typical uses of Bluetooth:
- Connecting a wireless keyboard to a computer
- Connecting a wireless mouse to a computer
- Using a wireless headset with a mobile phone
- Printing wirelessly from a computer
- Transferring data, music or photos between devices
- Synchronising calendars between two devices
Because Bluetooth networking only works over very short distances, and with devices belonging to one user, this type of network is sometimes called a 'Personal Area Network'

19. Bluetooth and WiFi are different standards for wireless communication.
Bluetooth technology is useful when transferring information between two or more devices that are near each other when speed is not an issue, such as telephones, printers, modems and headsets. It is best suited to low-bandwidth applications like transferring sound data with telephones (i.e. with a Bluetooth headset) or byte data with hand-held computers (transferring files) or keyboard and mice.
Wi-Fi is better suited for operating full-scale networks because it enables a faster connection, better range from the base station, and better security (if configured properly) than Bluetooth.

20. Types of Network
Wi-Fi is better suited for operating full-scale networks because it enables a faster connection, better range from the base station, and better security (if configured properly) than Bluetooth. “Wi-Fi" is a type of wireless networking protocol that allows computers (devices) on a Local Area Network to communicate without cords or cables. It provides an alternative to the use of cables when setting up a Local Area Network.
Wi-Fi is technically an industry term that represents a type of wireless local area network (LAN) protocol based on the IEEE network standard. It's the most popular means of communicating data wirelessly, within a fixed location, today.

21. Bluetooth Vs Wi-Fi Bluetooth Wifi Frequency 2.4 GHz 2.4, 3.6, 5 GHz
Cost
Low
High
Bandwidth
Low ( 800 Kbps )
High (11 Mbps )
Security
It is less secure
More Secure
Year of development
1994
1991
Primary Devices
Mobile phones, mouse, keyboards, office and industrial automation devices
Notebook computers, desktop computers, servers, TV, Latest mobiles.
Hardware requirement
Bluetooth adaptor on all the devices connecting with each other
Wireless adaptors on all the devices of the network, a wireless router and/or wireless access points
Range
5-30 meters
typical range is 32 meters indoors and 95 meters (300 ft) outdoors. Antennas can also increase the range.
Power Consumption
Ease of Use
Fairly simple to use. Can be used to connect up to seven devices at a time. It is easy to switch between devices or find and connect to any device.
It is more complex and requires configuration of hardware and software.

22. LAN Network Topologies

23. LAN Network Topologies
The word topology means ‘arrangement’, so when we talk about the topology of a network, we mean how the different parts are arranged and connected together. In computer networking, topology refers to the layout of connected devices.
The following are some of the most common categories of Network topologies:
- Bus
- Star
- Ring
- mesh
More complex networks can be built as hybrids of two or more of the above basic topologies.

24. THE BUS TOPOLOGY
Bus topology is a fairly old topology and you probably won't be seeing much of this type in any modern office or home.
With the Bus topology, all workstations are connected directly to the main backbone that carries the data.
Traffic generated by any computer will travel across the backbone and be received by all workstations.
This works well in a small network of 2-5 computers, but as the number of computers increases so will the network traffic and this can greatly decrease the performance and available bandwidth of your network.

25. As you can see in the above example, all computers are attached to a continuous cable which connects them in a straight line. The arrows clearly indicate that the packet generated by Node 1 is transmitted to all computers on the network, regardless the destination of this packet.

26. THE BUS TOPOLOGY
Also, because of the way the electrical signals are transmitted over this cable, its ends must be terminated by special terminators that work as "shock absorbers", absorbing the signal so it won't reflect back to where it came from.
The value of 50 Ohms has been selected after carefully taking in consideration all the electrical characteristics of the cable used, the voltage that the signal which runs through the cables, the maximum and minimum length of the bus and a few more.

27. THE BUS TOPOLOGY
If the bus (the long yellow cable) is damaged anywhere in its path, then it will most certainly cause the network to stop working or, at the very least, cause big communication problems between the workstations.
Bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result.

28. THE STAR TOPOLOGY / THE HUB TOPOLOGY

29. THE STAR TOPOLOGY
The Star or Hub topology is one of the most common network topologies found in most offices and home networks.
It has become very popular in contrast to the bus type (which we just spoke about), because of the cost and the ease of troubleshooting.
A star network features a central connection point called a "hub node" that may be a network hub, switch or router.
Compared to the bus topology, a star network generally requires more cable.

30. THE STAR TOPOLOGY
The advantage of the star topology is that a failure in any star network cable (ie one computer of the star topology fails) will only take down one computer's network access (ie only the failed computer will NOT be able to send and receive data). The rest of the network will continue functioning normally.
The disadvantage of using this topology is that because each computer is connected to a central hub or switch, if this device fails, the entire network fails!

31. THE RING TOPOLOGY
In the ring topology, computers are connected on a single circle of cable. Unlike the bus topology, there are no terminated ends. The signals travel around the loop in one direction and pass through each computer, which acts as a repeater to boost the signal and send it to the next computer.
In a ring network, every device has exactly two neighbours for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.

32. THE RING TOPOLOGY

33. THE RING TOPOLOGY
The method by which the data is transmitted around the ring is called token passing. IBM's token ring uses this method. A token is a special series of bits that contains control information. Possession of the token allows a network device to transmit data to the network. Each network has only one token.
Ring topologies are found in some office buildings or school campuses. On a larger scale, multiple LANs can be connected to each other in a ring topology.

34. THE MESH TOPOLOGY

35. THE MESH TOPOLOGY
In a mesh topology, each computer is connected to every other computer by a separate cable. This configuration provides redundant paths through the new work, so if one computer blows up, you don't lose the network.
On a large scale, you can connect multiple LANs using mesh topology.
Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination.
(Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) Some WANs, most notably the Internet, employ mesh routing.

36. THE MESH TOPOLOGY
A mesh network in which every device connects to every other is called a Full Mesh.
Partial Mesh networks also exist in which some devices connect only indirectly to others.
Again, the big advantage of this topology is its backup capabilities by providing multiple paths through the network.

37. THE HYBRID TOPOLOGY
With the hybrid topology, two or more topologies are combined to form a complete network.
For example, a hybrid topology could be the combination of a star and bus topology.
These are also the most common in use.

38. The Hybrid – Star Bus Topology

39. The Hybrid – Star Bus Topology
This star/bus hybrid approach supports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.
In this topology, if a computer fails, it will not affect the rest of the network. However, if the central component, or hub, that attaches all computers in a star, fails, then you have big problems since no computer will be able to communicate.

40. The Hybrid – Star Ring Topology

41. The Hybrid – Star Ring Topology
In the Star-Ring topology, the computers are connected to a central component as in a star network. These components, however, are wired to form a ring network.
Like the star-bus topology, if a single computer fails, it will not affect the rest of the network. By using token passing, each computer in a star-ring topology has an equal chance of communicating. This allows for greater network traffic between segments than in a star-bus topology.

42. Networking Hardware

43. Network Interface Card (NIC)
Any computer that is to be connected to a network, needs to have a network interface card (NIC).
Most modern computers have these devices built into the motherboard, but in some computers you have to add an extra expansion card (small circuit board)

44. Network Cables
To connect together different devices to make up a network, you need cables.
Cables are still used in most networks, rather than using only wireless, because they can carry much more data per second, and are more secure (less open to hacking).

45. Network Cables
The most common type of network cable in use today looks like the one shown above, with plastic plugs on the ends that snap into sockets on the network devices.
Inside the cable are several copper wires (some used for sending data in one direction, and some for the other direction).

46. A HUB

47. A HUB
A hub is a device that connects a number of computers together to make a LAN.
The typical use of a hub is at the centre of a star network (or as part of a hybrid network) - the hub has cables plugged into it from each computer.
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 into communications.
Hubs are pretty much obsolete now (you can't buy them any more), having been superseded by cheap switches.

48. Switch
A switch, like a hub, is a device that connects a number of computers together to make a LAN.
The typical use of a switch is at the centre of a star network (or as part of a hybrid network) - the switch has cables plugged into it from each computer.
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.

49. Switch

50. Router
A router is a network device that connects together two or more networks.
A common use of a router is to join a home or business network (LAN) to the Internet (WAN).
The router will typically have the Internet cable plugged into it, as well as a cable, or cables to computers on the LAN.

51. Router

52. Router
Alternatively, the LAN connection might be wireless (WiFi), making the device a wireless router. (A wireless router is actually a router and wireless switch combined)
Routers are the devices that join together the various different networks that together make up the Internet.
These routers are much more complex than the one you might have in your home.

53. Router

54. Proxy Server
A proxy server is a computer setup to share a resource, usually an Internet connection.
Other computers can request a web page via the proxy server. The proxy server will then get the page using its Internet connection, and pass it back to the computer who asked for it.
Proxy servers are often used instead of router since additional software can be easily installed on the computer such as anti-virus, web filtering etc.

55. Proxy Server

56. Bridge
A bridge is a network device that typically links together two different parts of a LAN.

57. Bridge
Whereas a router is usually used to link a LAN to a WAN (such as the Internet), a bridge links independent parts of a LAN so that they act as a single LAN.

58. Firewall
A firewall is a device, or a piece of software that is placed between your computer and the rest of the network (where the hackers are!)
If you wish to protect your whole LAN from hackers out on the Internet, you would place a firewall between the LAN and the Internet connection.

59. Firewall

60. Firewall
A firewall blocks unauthorised connections being made to your computer or LAN. Normal data is allowed through the firewall (e.g. s or web pages) but all other data is blocked.

61. Firewall
In addition to physical devices, firewalls can also be software.
In fact most computer operating systems have a software firewall built in (e.g. Windows, Linux and Mac OS)

62. Modem
Before the days of broadband Internet connections, most computers connected to the Internet via telephone lines (dial-up connections).
The problem with using telephone lines is that they are designed to carry voices, which are analogue signals. They are not designed for digital data.

63. Modem
The solution was to use a special device to join the digital computer to the analogue telephone line. This device is known as a modem.
A modem contains a DAC and an ADC.
The DAC in the modem is required so that the digital computer can send data down the analogue telephone line (it converts digital data into noises which is exactly what the telephone line is designed to carry.)
The ADC in the modem is required so that the analogue signals (noises) that arrive via the telephone line can be converted back into digital data.

64. Modem
The reason telephone lines were used is that almost every building in the world is already joined to every other via the telephone system.
Using the telephone system for connecting computers meant that people didn’t have to install new wires to their houses and offices just for computer use.
In the last few years however, this is exactly what people have done. Special cables have been installed just for Internet access.
These special cables are designed to carry digital data, so no modem is required.
The word modem is an abbreviation of MOdulator DEModulator.
A modulator acts as a DAC, and a demodulator acts as an ADC.

65. Modem
So, simply put, a modem is required because computers are digital devices and the telephone system is analogue. The modem converts from digital to analogue and from analogue to digital.
If you have ever used a dial-up connection, you have probably heard the noises sent by the modem down the telephone line.

66. Setting Up a Small Network
If you were asked to build a small, Internet-connected network from scratch, what would you need to do?
You would need to buy some hardware:
- One or more switches / hubs - to link devices together
- Network cables to connect devices to the switch, etc.
- A separate wireless access point (or this could be part of the switch) - 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
- Possibly a bridge if you already have a section of network and you want your new network to connect to it
- Server(s) to manage network functions such as network security, network file storage, shared resources (such as printers)

67. Setting Up a Small Network
You would need to organise some other things:
- Set up an account with an Internet Service Provider (ISP)
- Get an Internet connection installed from the ISP to your location
- Configure various bits of hardware and software so that everything worked with the network
For any network that is more complex than a small home network, there is a lot to do.
It's not just a case of buying the parts and connecting them together...
- Routers and switches have to be configured (settings changed)
- Network devices need to be given network addresses
- Software needs to be configured to use the network
- Etc...

68. Networks are pretty complex thing to set-up
Networks are pretty complex thing to set-up. The people who do this are called Network Engineers. It's a very interesting technical job, if you like that sort of thing!

69. Network & Data Security

70. Network & Data Security
As soon as your computer is connected to a network, you have to start thinking about security – security of your files, information, etc.
A network allows a person who does not have physical access to your computer (they are not sitting in front of it) to gain access all the same. 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.

71. Preventing Unauthorised Access
There are a number of security
measures that you can take to
prevent hackers accessing your
computer and all of the data
stored on it:
Physical Security
The first thing to make sure of is that no unauthorised people can physically access (sit down in front of) any of the computers on your network.
For example, by keeping office doors locked.

72. Use a Username and Have a Good Password
The most common way to protect your computer’s data is to setup user accounts with usernames and passwords. Anyone not having a username, or not knowing the correct password will be denied access.
For this to be effective passwords must be chosen that are not easy to guess. Passwords should be a random combination of lowercase letters, uppercase letters and numbers (and symbols if this is allowed):
‘Weak’ passwords: password, , david, 27dec1992
‘Strong’ passwords: s63gRdd1, G66ew$dQ, gdr298783X
Some computer systems replace the typing of usernames and passwords with other forms of user identification such as ID cards, fingerprint readers, voice-print recognition, etc.

73. Strong passwords are often hard to remember
Strong passwords are often hard to remember. Here is a good method for creating a password that is very strong, but also easy to remember:
Think of a phrase that you will never forget…
- “My favourite food is chocolate ice cream”
- Take the first letter of each word…
- mfficic
- Change some letters to similar numbers: I to 1, o to 0, s to 5, etc. and make some letters (e.g. the first and last) uppercase…
- Mff1c1C
A random-looking mixture of letters and numbers. As long as you like chocolate ice cream you will never forget your password!

74. Always Install and Use a Firewall
A firewall is a device, or a piece of software that is placed between your computer / LAN and the rest of the network / WAN (where the hackers are!)
Securing Your Data
Often we have data that is private or confidential. This data needs to be protected from being viewed by unauthorised people. This is especially true if the data is to be sent via a public network such as The Internet.
The best way to protect data is to encrypt it...

75. Data Encryption
Encryption is the process of converting information into a form that is meaningless to anyone except holders of a ‘key’.
For example, if Alice wants to send important, personal messages to Bob, she must go through the following steps...
- First Alice needs to generate a secret ‘key’.
The key is usually a very long, random number.

76. Data Encryption
- The encryption scheme shown here is called Symmetric Key, or Single Key encryption.
There are many better schemes, such as Public Key Encryption, but the one shown here is the easiest to understand!
All that is required is to have the concept of Data Encryption.

77. Data Encryption
- Alice must then give a copy of this key to Bob. She must make sure that nobody else can get to the key
(So maybe Alice will visit Bob and give him a copy of the key on a memory stick or floppy disc).

78. Data Encryption
- Now that Bob has a copy of the key, each time Alice needs to send him a message she starts by encrypting it using special encryption software and the secret key.
- The encrypted message now looks like a jumble of random letters and numbers.

79. Data Encryption
- Alice then sends the encrypted message to Bob.
- She can use a public network like the Internet, since, even if it gets stolen, the encrypted message cannot be read or understood without the key.

80. Data Encryption
- When Bob receives the message, he uses special decryption software and his copy of the secret key to decrypt the message.
- Bob can now read the original message from Alice.

81. END
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