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Unit4: Computer Networks

What is a Network?

• Local Area Network (LAN)
• Wide Area Network (WAN)

• Share hardware peripherals like expensive printers.
• Share files between users
• Share software through an application. e.g If i have a lot of computers , i bought photoshop, I want to put the software in every computer. I can installed it in each of the computer one by one. Or I can installed the software in the server called “Application server”, which when we are using we are using it from the server.
• Allows user to communicate better.
• Saves time when installing new software.
• Wireless “much more flexible and portable”

• Are expensive to installed “e.g to connect the computers with alot of cables and wires.”
• Are expensive to maintain (you need a network manager or a computer technician)
• Network manager: fixes everything involved with network only
• Computer technician: fixes everything involved with computer only
• Can be difficult to find where the problems occur in the computer networks when there are lots of computers.  
• Sometimes, the viruses can be easily spread from one computer to another computer because if one program on the network have a virus, that virus can go into the other computers on the network because those computers on the network are all connected together. 
• When you are doing a work as a group, there might be a connection problem with the network you've made change to a document because that document may not be changed in the computers of the other and they might ended up working in the old document instead. 
• Many people would be able to access the data on the network if they know the password, although the owners of the data didn't them want to.

1. The distance that data will need to travel: when the computers are far away from the wireless access point (WAP) the radio waves can get weak and the data can be transfer slower which mean that the radio waves are much more suitable for a short distance.   
2. The need for portability of devices: when you are using the devices such as laptops, to connect it to network you wouldn't to carry the cables around as you move the laptops with you. Thus, radio waves are the best for the portability because you can connect your laptops with the network wireless.          
3. Security: the cables is more secure than the radio waves because the radio waves can be hack but the cables cannot be hack. 
4. Future proofing: (A term used to describe any technology that will not become outdated any time in the near future)you also need to think about whether the cables or the radio waves will still be used or updated in the future or not. 
5. The speed of data transmission: the cables can transfer lots of data faster than the radio waves because the data are transfer directly through the wires which will be less disturb than the radio waves in which data are transfer through the air. 
6. The size of the computer network: if there are too many people used the network the radio waves can get weak and the data can be transfer slower. So cables would be better because they are connected directly to each computer.

• NIC- Network Interface Card

• LAN Port-Local Area Network

LAN ports

• Router  

Like a hub, a router is another type of device that connects the network and the internet together. It also can connect two or more networks together. Here is an example of a wired router:

A router functions a little differently than a hub. In fact, a router can be considered a little "intelligent" than the hub.

Like a hub, the computers and other devices are connected to a router using network cables. To make this possible, a router is equipped with holes, called ports, in the back. Here is an example:

• Cables-(Twisted pair & Fiber Optics)

Cable is used to connect computers. Although we may use wireless networking, you should always have cables with you. The most commonly used cable is referred to as Category 5 cable RJ-45. The ends of the cable appear as follows:

They can be in different colors:

You can purchase this cable from a general store, a computer store, or web store on the Internet. When purchasing it, get one with at least 6ft.

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• Server

A regular computer but it have lots of hard drive space (large storage space) and powerful because it have a fast processor. 

• WAP- Wireless Access Point

This allows the users to connect the devices e.g. computers, smartphones, etc with the networks wireless.

• Switch

It connects many computers together but the data will be send directly to the targeted computer or the receivers.  

Network switch

  

• Hub

A hub is rectangular box that is used as the central object on which computers and other devices are connected. To make this possible, a hub is equipped with small holes called ports. Here is an example of a hub:

Although this appears with 4 ports, depending on its type, a hub can be equipped with 4, 5, 12, or more ports.

• Bridge

It connects different networks together, Just like a real life bridge does.

Network bridge

• Bluetooth = use radio waves to transfer data from one device to another device such as mobile phones in a short distance (a few meters). 

Bluetooth

• Wi-Fi = use radio waves to transfer data from one device to another device such as computers in a long distance.  

Wi-Fi

What is an IP address?

An IP (stands for Internet Protocol) address. It is a set of numbers that is given to each computers by the NIC (network interface card). Hence the network are able to recognise each computers which make it easier for them to send data from one computer to another compute. However, IP addresses are unique, different computers will have different set of numbers. It will change evrytime you re-open your computer.

Some extras:

If you already have one or more computers that you plan to use as workstations, you can start by checking the hardware parts installed in the computer. As mentioned already, you can use use existing computers or purchase new ones.

The computers used in a network must meet some requirements. The system requirements depend on the (type of) operating system ). 

Building a network consists partly of connecting the computers:

Besides the computers, you will use other objects.

I have that you could connect one computer to another. This can be done using their serial ports:

This is possible because almost every computer has a serial port. If you have to connect many computers to produce a network, this serial connection would not be practical. The solution is to use a central object that the computers and other resources can connect to, and then this object becomes responsible to “distribute” or manage network traffic:

The most regularly used types of network distributors are the hub, the router, and the switch.

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Router vs Bridges

The routers and the bridges are similar to each other because they both connect different networks together but most people decided to use the routers rather than the bridges because of the main advantages that the routers have over the bridges. 

How does the Routers & the Bridges work?
Routers	Bridges
Deciding where the data should be going and the best way of going (the routes).	The data that pass through the bridges must be in the same format.
Deciding that the data should be transfer or not.	Cannot format the data.
Formatting the data so that it could be accessed by the receiving networks.
Can connect the networks that are very different together.

Facts about the Computer Networks:

1. The switch and hub can work together but it will work slower than the switch alone. 
2. The radio waves work well horizontal but not vertical. 

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Network Topologies (Structures)

Computer networks can be connected in a number of ways.The 3 most common structures, or topologies, are called star,ring and bus network. Here are the outline of their structure.

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.

Star Network

In this type of network every computer is connected to a central device. The device passes messages between computers.

At the centre of a star network you might use a hub (cheap, but slower) or a switch (more expensive, but faster).

A star network…

Is quite expensive to install (you have to buy lots of cable and the central device)

Is very fast since each computer has its own cable which it doesn’t need to share

Can cope with a broken cable (only one computer will be affected)

Will stop working if the central device breaks

Is the most common network topology.

Star Network

Ring Network

In this type of network each computer is connected to a loop of cable, the ‘ring’. (If you took a bus network and connected the ends of the bus cable together, you would have a ring network.)

A ring network… 

• Can cope with a break in the ring cable since all computers are still joined together (it is now a bus network)

Ring Network

Advantage:

• Can create very large networks because you can just keep on adding the devices into the network.    

Disadvantages:

• If one computer has broken down, the whole network will stop working because the data can only travel in one way.
• Difficult to add new computers into the network because if you'll have to put them between two computers and while you were putting them in the network will stop working until you finished and you'll also have to reconnected some of the computers with the new ones. 

Bus Network

In this type of network, a long, central cable, the ‘bus’ is used to connect all of the computers together. Each computer has a short cable linking it to the ‘bus’. At the end and the start of  the central line there is a terminator. It will stop data from bouncing around.

A bus network…

• Is cheap to install (just one long cable)
• Can be quite slow since all computers share the same cable when communicating
• Will stop working if there is a break in the central bus cable.

Bus network

Advantages:

• New devices can be easily added to the network because you can just connected them to the central line.
• Although, one or more computers on the network are broken down but this will not affect the whole network because all of the computers are connected to the central line not to the other computers.

Disadvantages:

• If the central line is broken, the whole network will collapsed because all of the devices are connected to the central line.
• The more devices you added to the network, the slower the network works.

Tree network

It is a combination of star network and bis network; some device are connected directly to the central line and some are connected together to form a star network. This kind of topology is quite popular because it contains the advantages of both star network and bus network.

Advantages:

• The advantages are the same as the star network and the bus network.
• Although, the central line is broken but the devices that are connected together in a star network can still communicate with the devices that are on the same network.   

Disadvantages:

• The disadvantages are the same as the star network and the bus network. 

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Local Area Network (LAN)

A Local Area Network is a network confined toone 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.

Wireless Local Area Network (WLAN)

A wireless LAN (WLAN) is a LAN that usesradio 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 forsending and receiving data to the computers. (Most laptops have a wireless antenna built into the case.)

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.

This is a list of hardware that are needed for this type of networks:

• Wireless Access Point (WAP): spread the network signals around certain areas.
• Network Interface Card (NIC): sent the network to the WAP.
• Boosters (amplifiers): makes the network signals spread further. 
• Wireless repeaters: increase the network areas because same access point that have the same IP can be uses in different locations.
• Routers: allows the computers to access the internet through the WAP.   

Advantages of WLAN

Here is a list of the main advantages of the WLAN:

1. More portable than the LAN because the users can work anywhere within the network areas without having to carry the cables around with them.
2. The WAP are mostly installed in the public areas (e.g. cafes, libraries, etc) which mean that the users can access the internet in many different areas. 

Disadvantages of WLAN

Here is a list of the main disadvantages of the WLAN:

1. Can sometimes become quite slow because there are many users and it can be disturb by the other signals or currents easier than the LAN and the WAN because the WLAN signals travel through the air not the wires. 
2. Not secure because many people can access to it. 
3. The computers and the servers can be connected together in a shorter distance than by using the cables because if they are far away the signals will get weak and sometimes when the distance is too far away they might be unable to connected together.

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 aglobal WAN .In fact it is the world’s largest WAN.

Computers on the International Space Station are linked to the Internet, so the you could say the the Internet is now the first off-planet WAN!

Advantages of WAN

1. The data can be share quickly across the countries. 
2. It is one of the fastest ways of transferring the data across the countries.

Disadvantages of WAN

1. If there is a connection problem within the WAN (e.g. if there the telecommunication links fail), it would became impossible to use the WAN. 
2. The viruses often spread through the WAN than the LAN because more people are using it.
3. The WAN are hacked more than the LAN because it is easier to access. 

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-powerradio transmitters and receivers. When devices are in range of other Bluetooth devices, they detect each other and can be 'paired' (connected)

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 or PDA
• Transferring data / music from a computer to an MP3 player
• Transferring photos from a phone / camera to another device
• Synchronising calendars on a PDA and a computer

Tip 1

While the networks are sending the data from one computer to another, the data can

 be disturb by the high-voltage electrical cables that are running along side the

 network cables or by different wireless signals.

Tip 2

When you are using the WLAN, the routers are unnecessary for you unless you want

 to connect the internet to the WLAN. 

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What is an ISP?

ISP stands for Internet Service Provider. It is a company that allows user to
 acces the internet by given us the router to connect the internet through their 
servers. However we have to pay the montly fees to those company.

A simple diagram to show us how we connect the internet through ISP

TOT is one of the ISP companies in Thailand

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What is a Data Packet?

Have you ever wondered that when we sending emails and data to other computer, if the files are too big(for instance the files contains lots of graphics) it will takes a long time to send? But nowadays it have improved. The network solve this problem by breaking the files into millions of small pieces before sending it. Those pieces are called Data packet. 

 The data packet have 4 main parts in each pieces.

• The header (Contains the IP address of the sender)
• The packet number (identify the part of the files is)
• The data
• The footer ( contains the IP address of the receiver)

Another reason that the network breaks the data into smaller pieces, is because if there is some interference, the file could get lose while the networks is sending them, therefore it will have to re-send the file again. So if they don't break the files up, the networks will have to resend the whole files again which is time-consuming. Thus, if they break the files into small pieces although there are some interference on the way, only parts of the files will get lost. Therefore, the networks'll only have to send the parts that are lost and not the whole files.

Why are the Header, the Footer & the Packet Number important?

The header which is the IP address of the sender and the footer which is the IP address of the receiver are important because if too much electricity disturb the files while they are sending the data part can get lost. So if the data get lost we can check that the receiver receive the data or not and what kinds of data that the sender wanted so the networks can resend the data again if they know who is the receiver and who is the sender by checking the IP addresses.


The packet number help the networks to arrange different parts of the files that were break down into the correct order. If those parts were arranged in the wrong orders, the files will be different from the original files or the receivers might not be able to open the files.   

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How does the Computer Network Connected?

The diagram below will show you how the computer network are connected:

How does the router works?

The router are used for sending data across the network. The reason that it is called a router" is because it will choose the best and quickest route to send the data. Thus, the data will be delivered as fast as possible.

Here is an example how the router works:

* In the diagram above there are just a few routers that are connected together but in the reality there are billions of routers connected together.

For example :This is what the routers do when A want to send the email to B:

1. Router A allow the laptop that A was using to connect to the internet.
2. After A click "send", the data (email) was send to router A because router A know the IP address of the laptop.
3. Router A know the IP address of router 1 so it send the data to router 1.
4. Router 2 and router 4 know the IP address of router B but the route to router 2 was better (faster way) so router 1 send the data to router 2.
5. Then router 2 send the data to router B.
6. Router B know the IP address of B's laptop (which is the receiver) so it send the data to B's laptop.  
7. Finally, B receive the email from A. 

However, this is what happen when router 2 was broken:

1. Router 2 will send the message to the routers that are connected to it that it was broken.
2. Instead of sending the data to router 2, router 1 will send the data to router 3.
3. Router 3 will send the data to router 4.
4. Router 4 will send the data to router B.
5. Router B will send the data to B's laptop.  
6. Finally, B receive the email from A.

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What is a Modem?

The term Modem comes from Modulate (Make digital) demodulate (to make analogue).The job of the modem is to convert a digital signal into an analogue signal and then convert the analogue signal back into  digital signal again.

A modem

The modems are uses when we want to the devices that understand digital signals (e.g. smartphones understand binary numbers) with the devices that understand analogue signals (e.g. land line telephone understand sound waves).

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. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ The Internet & Intranet What is Internet?    The internet is sometimes called simply net. It is basically a network of computers interconnected across the worldwide. It is accessible to everyone in the world. These interconnected computers share data through a standard protocal called TCP / IP. It’s connected with billions of computers across private, public,government,business and education sectors. To simplified it : The internet is an international network that is connected across or around the world which contain millions of websites and webpages. Also, the internet can be access by anyone.  Examples of the internet What Can We Use the Internet For? The Internet provides the network connections that links computers together. There are many ways that we can use these connections: View web pages on the WWW (World-Wide Web) Sending and receiving e-mail messages Sharing files Communicating using voice (VOIP) and video (video-conferencing) Playing multi-player games Listening to streamed music or watching streamed video Advantages of Internet This is a list of the advantages of the internet: Powerful search engine. Can get wide range of opinions or interests. Free email services. Wide range of resources.  Accessible for anyone.  Allow the users to receive the latest news or information. Disadvantages of Intrenet Here is a list of the disadvantages of the internet: There might be some wrong information on the internet because anyone can post the information.  Caused some people to addicted to the internet which can causes health and social problems for those people. Many people are being fraudulent by strangers on the internet. The children can access to the inappropriate stuffs on the internet. More chances of being hacked and insecure because anyone can access to the internet. Cyber bullies can occur more easily. What is a Intranet? An intranet is the name given to a private network that provides similar services to The Internet: e-mail, messaging, web pages, etc. However, these services are only for the users of the intranet – they are private, not public (unlike Internet services which are generally public). A diagram of the Intranet Businesses and other organisations often have intranets for use by their employees.  Typical uses of an intranet would be:  Viewing internal web pages (e.g. company calendars, etc.) Internal e-mail and instant-messaging between workers Sharing of internal documents Advantages of Intranet This is a list of the advantages of the intranet: The data can be examine by the organisation or the companies.  The data are directly and indirectly related to the users.  The data can be shared within the organisation easily.  Faster to communicate with each other and to search for something due to the smaller size of networks.  Less chances of being hacked and more secure than internet because there are just some people who can access to them.  Privacy because not anyone can access to the intranet.  Disadvantages of Intranet Here is a list of the disadvantages of the intranet: Limited resources. Limited range of views (can only view or see something but not all of it ). Limited accesses. Cost a lot for the hardware, setting up and maintenance because you'll need to pay for the technicians to set up and manage the intranet. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ What is a Webpage? A webpage is computer document that mostly contain texts, images, sounds, animations and videos. All of the webpages have their own address so that it is easier or the web browsers to find them. This type of address is called the URL (Uniform Resource Locator). A webpage This is how the URL are split into different part e.g. http://www.examples.gov.za/example/page1  Protocol Host Name Domain Name File Path Filename Second Level Domain (SLD) Sub Domain Top Level Domain (TLD) http:// www .examples .gov .za/ example page 1 What is a Website? A website is a collection of webpages (HTML) that are all linked together by the hyperlinks. The hyperlinks are the systems that will allow the users to get from one webpage to another webpage or from one website to another websites by clicking on the hyperlinks. The hyperlinks can be attach to both texts and images. Collection of webpages  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ What is a Proxy server? A proxy server is a powerful computer that acts as a gateway between the internet and our network. Its jobs are to improve security, store the copies of the websites that the users often uses and allows or blocks the users who are on the network from accessing the websites that are on the list inside the proxy server. 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. How the proxy server works        Advantages of Proxy Server This a list of the advantages of the proxy server: Speed up the web browsers because some websites that we want to access might be store in the server that far away from our network but if we have a copy of the website store in a proxy server, our network will just have to get the data from the proxy server without having to go all the way to the original server. Save times (the reason for this is just like what I've mentioned in the first point). Improve security e.g. the proxy server for a school can block the websites might be inappropriate for the students so the that students can’t access to those websites. Disadvantages of Proxy Server Here is a list of the disadvantages of the proxy server: Limited access to the internet because the proxy server can block some of the websites. Additional expense because we have to pay for the hardware and the maintenance required for the proxy server. What is a fire wall? A firewall is a software that protect the networks from being hacked which are often uses between the internal and external networks. The firewall works by preventing ineligible communications to or from the networks. It also works by identifying the IP addresses of the networks and blocking suspicious things away from the networks.  How the fire wall works