Computer Science - Topic 5 - Communication and the Internet

Advantages of Networks

A network is a linked collection of computer systems that allows resources and computer power to be shared. There are advantages and disadvantages to using networks:


  • allows users to share resources
  • saves space and money - not all individual devices need their own printers
  • good for sharing files and collaboative group projects
  • software can be installed on a server and shared across the network - quicker and cheaper
  • data can be backed up centrally
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Disadvantages of Networks


  • initial installation may be expensive
  • if the main server fails, then the whole network will fail and staff will be unable to work
  • viruses and malware can be easily spread across the network
  • a network mananger is usually needed for all but the smallest of networks 
  • wireless networks can dropm out due to interference and may be slow at transferring data 
  • wireless networks are mor susceptible to being hacked and "piggybacking" (when someone accesses a network without authorisation) can happen 
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LANs and WANs

There are LANs (Local Area Networks) and WANs (Wide Area Networks). Local Area Networks are used when all of the devices on the network are in one building (e.g. a school or an office) and do not require internet access for the devices to be able to coorperate with each other. Wide Area Networks are created when many different LANs are connected together and span over a large area. They require media such as broadband cables and can connect with organisations based in different geographical locations. The most common example of a WAN is the internet. 

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Wired and Wireless Networks

Networks can either be wired (where all components are physically connected together through an ethernet cable) or wireless (where no devices are connected physically and rather communicate through a hib). Wired networks are usually used in a LAN and wireless networks are usually used in a WAN. Wired networks need a Network Interface Card (NIC) and requires a port on each device in the network for a network cable to be connected. Wireless networks also have a similar standard NIC however they also have a transmitter/receiver for sending/recieving signals via radio waves and have no network cable attached to each device making them more flexible to working conditions - people are able to work on their laptops outside of the office.

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Ways of connecting a network

There are various different methods of connecting devices in a network together. A device that requests information is called a client and a device that provides information is called a server. There are two network connection models that are used most often - peer-to-peer and client-server. The client-server model involves two devices where one (the client) makes requests to the other (the server) for information. In this model the client is dependent on the server for managing and providing information. In a peer-to-peer model there is no one designated client and peer, all of the devices connected in the netwoek sgare equal responsibilty for storing and sharing data. In this model, many users can store part of one large file and then share the data they each store with the others so that any device can access the whole file.

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Network Topologies - part 1

A network topology is the way in which the devices on the network are organised and connected together. There are four main network topologies: bus, ring, star and mesh. A bus topology is where all the devices are connected to a central line and that central line has terminators on either end. They are easy and cheap to install and it is easy to add new devices to the network however if the main cable gets damaged then the whole network fails, it can be difficult to identify where along the cable the fault is, as more devices are connected the performance becomes slower and every device on the network can see all the data from all the devices posing a security risk. A ring topology is where all the devices are connected in a ring with each device connected to two others. Ring topologies are quick at transferring data as they find the quickest route to the device the information is being requested from/sent to and any extra devices added do not impact the performance of the network. However, in ring topologies if the main cable or any device becomes faulty the whole network fails, in order to add or remove any devivces to the network the network must be temporarily shut down, it can be difficult to identify where a fault is and it is the most expensive topology to install as it requires a lot of cabling.

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Network Topologies - part 2

 A star topology involves a hub or switch that all the devices are individually connected to and the hub/switch sends the data being transferred to the right place. Star topologies are very reliable as even if one cable or device fails, the others will continue to work, any faults are very easy to locates, a new device can be added easily without having to shut down the whole network and the performance is always high as there will be no data collisions. However star topoloies are expensive to install  as they use the most cable, they require extra hardware (hub or switch) and if the hub/switch fails all the devicves connected to it will have no network connection. A mesh topology where all the devices are connected to each other either wirelessly or using cables. Every device in this network sends its own signal but also rlays data from other devices. Mesh topologies have a very high performance because each device is connected to multiples devices, adding more devices does not slow the transmission of data and the network can handle high volumes of data traffic. However, they are difficult and expensive to install, it can be difficult to manage them due to the large amount of connections within the network and they can require specialist supervision. 

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Protocols - what they include

A protocol is a set of rules that determine how communication between devices are formatted and how these communications will be sent/received. There are three catagories of protocols: email protocols, network protocols and internet protocols. Protocols have to dictate:

  • how each computer is identified (its address)
  • what route the data will take to get to its destination (routing information)
  • how errors will be detected and dealt with (error checking)
  • whether each part of a message should send a signal/be acknowledged as received correctly
  • what to do if the data isn't recieved correctly
  • how the data is to be formatted
  • how the data is to be sequenced (does is need to be sent in a specific order?)
  • how the speed of the sender and receiver can be synchronised
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Protocols - email protocols

There are three main email protocols:

  • SMTP - (Simple Mail Transfer Protocol) - used when sending an email over the internet
  • POP3 - (Post Office Protcol version 3) - used when retrieving and downloading emails from the server 
  • IMAP - (Internet Message Access Protocol) - used when synchronising different devices so that the emails can be accessed on different devices (e.g. phones, tablets and laptops)
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Protocols - network protocols

There are many different network topologies such as:

  • Ethernet - not exactly a protocol but the physical cables that connect wired networks together 
  • Wi-Fi - used in wireless networks to allow digital communication between devices 
  • HTTP - (HyperText Transfer Protocol) - pre-fix to website URLs that allow the search engine (google chrome, internet explorer, firefox etc) to communicate with the web server and decides how the data should be formatted 
  • HTTPS (HyperText Transfer Protocol Secure) - secure version of HTTP used with websites where a login is required to prevent a third party from accessing passwords
  • FTP - (File Transfer Protocol) - used when publishing any files to the internet (uploading photos, creating a website etc) 
  • TCP - (Transmission Control Protocol) - provides a reliable connection between devices on a network 
  • TCP/IP - (Transmission Control Protocol/Internet Protocol) - a protocol stack that allows different protocols to work together with a reliable connection 
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Protocols - TCP/IP stack

The TCP/IP is a protocol stack that has four layers and allows a collection of protocols to work together with a reliable connection. Each layer deals with a particular function of the protocol and passes the information from that function to the other layers as the request is processed. The irst layer is the application, this is the layer that the user interacts with and provides the user with access to services and data sent/recieved over a network. The nect layer is called the transport and this layer transports the data from the server to the user and vice-versa and also splits the data into packets. Data is split into any number of packets depending on how big the document is; each of these packets contains a small section of the data that makes up the document and a packet header. The packet header contains information specific to that packet such as the addresses of the sender and the recipient, how many packets the data has been split into and the number of this particular packet. The next layer in the protocol stack is the internet layer. This layer adds the information to the packet header and prepares to send the packets to the recipient over the network. Finally, the last layer of the protocol stack is the link layer which decides how the packets will be sent over the network - if they will be sent using Wi-Fi or an ethernet cable. 

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Protocols - internet protocols

There is only one internet protocol:

  • IP - (Internet Protocol) - deals with an adreesing system that allows every device to be identified and adds the details to the packet header of data packets. Each device linked to the internet has its own unique IP address so that the devices can be identified as the sender or the recipient of data and so the data knows which device to go to 
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Network Security - what is it?

Network security is making a network safe against third parties do that only users with authorised access can use the network and the resources shared on it as well as ensuring the users on the network can only access data that is relevant to them. This prevents the missusage of the network as it means that other people's files cannot be deleted, edited or copied without their permission and new softwara cannot be installed as well as preventing damage to hardware components. Network security is important because it helps businesses become successful (if the company's vital data on a network becomes corrupted it could cause disorganisation and possible failure of a business. Also, sensitive data on the network can be protected and stored safely if the network is secure and therefore there will not be a violation of the data protection act. Finally the data on a network might be financially valuable (e.g. details of a new product) that could cause people to lose money if they were accessed by unauthorised users and so they must be protected.

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Network Security - cyber attacks

A cyber attack is any kind of electronic attac on a computer system, server, network or other IT service. These attacks can be designed to do a number of things such as gain access to data on the system, delete or modify information on the system, make the system unavailable for use or physically damage a device connected to the network. There are two types of cyber attacks: social engineering and technical weaknesses. Social engineering attacks are attacks that are human behaviour related due to the fact that an attacker engineers the situation. In comparison, attcks against technical weaknesses are where vulnerabilities in a system are attacked.

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Network Security - social engineering cyber attack

There are two main types of social engineering attacks:

  • Phishing - attempt to get sensitive, confidential information from the user of a computer network (tends to be passwords or bank details). This information tends to be gained through emails, fake websites, phone calls or instant messages 
  • Shoulder surfing - gaining access to sensitive information by directly observing the user (could be literally looking over their shoulder). Used to get a person's pin number or username/password 
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Network Security - technical weakness cyber attack

There are four main ways that a technical weakness in a system can be exploited and used for a cyber attack:

  • Unpatched software - when software is used, security issues arise and as a result the company in charge of the software releases patches to fix these issues. However these patches have to be applied manually by the user and therefore some people forget leaving software vulneravle to an attack
  • USB devices - portable storage devices such as USBs can be used to install a virus on the network or copy confidential information from the network 
  • Eavesdropping - this is where a user's digital communications are accessed by a third party who reads the information (usually usernames, passwords, PIN numbers or bank details) but does not copy it 
  • Smart devices - electrical appliances in the home that are controlled by an app are unprotected by any security measures and as a result are vulnerable to hackers. Normally hackers will stop the device from doing what it needs to do
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Network Security - ways to secure a network (softw

People can secure a network using either software or physical measures against attackers. 

Software measures:

  • Authentification and validation - checking the identity of a user that logs onto a computer network. This is done by validating the username and password against data stored on the system. If a user's login is incorrect they will not be allowed access onto the network 
  • Access control - determines how much the user is allowed access to once they have logged onto a network. This is set up by the organisation that contols the network according to management requirements 
  • Firewall - monitors and controls the data that is moving from one network to another. Designed to stop certain protocols from being used, block data coming from or going to certain network addresses or websites and stop attempts at hacking the network 
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Network Security - ways to secure a network (physi

Physical measures:

  • Lock doors - prevent people from accessing the room where valuable devices are stored
  • Biometric system recognition - used to only allow one person access to a device (e.g. phone)
  • CCTV - monitor the interior/exterior of the building where devices are connected 
  • RFID chips - Radio Frequency Identification chips that are installed so that if the device is stolen an alarm is sounded 
  • Chains/locks - extreme method used to secure devices  
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The Internet

The internet is the largest WAN in the world. The internet is a global system of interconnected computer networks that serve worldwide and provides many services such as email. The internet requires three basic components to work: ISPs (Internet Service Providers), TCP/IP protocol stack and routers. The computer that uses the interent has to be connected to it using an ISP or else there will b no internet connection, the TCP/IP protocol stack is used to send the data packets to the right address and routers are used to forward packets between networks. 

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The World Wide Web

The World Wide Web (WWW) is a service that is run on the internet to provide the user with access to web pages that contain text, graphics and multimedia. To access the WWW, the user needs to use a web browser such as Google Chrome or Firefox. There are many different web browsers but they all use the sam protocols and the internet to transfer information. The job of a web browser is to convert data received from a web server to a format that can be easilty read by humans. The WWW works by transferring data from one computer system to another - the computer accessing the WWW connects to the internet using an ISP and will need to be running a web browser. A step-by-step account of how a user uses the WWW is:

  • the user enters the web address (URL - Uniform Resource Locator) into a web browser to find the information they want to look at
  • the computer finds the IP address of the required web server using a system called Domain Name Service (DNS)
  • the web browser then connects to the web server using the IP address and requests the webpage relevant to the user
  • the web page is then transferred from the web server to the computer that requested it using HTTP or HTTPS 
  • data sent from the web server to the web browser is in HTML (HyperText Markup Language) an so the web browser displays the web page to the user as described in the HTML
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