Cambridge Lower Secondary CheckpointStage 7

Networks and Digital Communication

Computing Stage 7 Chapter Notes

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Networks and Digital Communication
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In this chapter

  1. 1Network Addressing: IP, URL, and DNS
  2. 2Error Detection in Data Transmission
  3. 3Data Encryption and Ciphers
  4. 4Secure Websites and Digital Certificates

1. Network Addressing: IP, URL, and DNS

Every device connected to a network needs a unique identifier, just like every house needs a unique postal address. On the internet, this is the IP (Internet Protocol) address. It's a series of numbers (like 192.168.1.1 or 2001:0db8:85a3:0000:0000:8a2e:0370:7334) that allows data to be sent to the correct destination. However, numbers are hard for humans to remember. That's where URLs (Uniform Resource Locators) come in. A URL is a user-friendly web address like 'www.cambridge.org'. The Domain Name System (DNS) acts as the internet's phonebook. When you type a URL into your browser, a DNS server looks up that URL and finds its corresponding IP address, allowing your browser to connect to the correct web server and retrieve the website.

Key term

DNS (Domain Name System): A hierarchical and distributed naming system for computers, services, or other resources connected to the Internet or a private network, which translates human-readable domain names to machine-readable IP addresses.

Examiner insight

Examiners look for a clear, step-by-step description of the DNS lookup process. Mentioning the request from the browser, the role of the DNS server, and the final connection to the web server's IP address will score maximum marks.

Common pitfall

Confusing the roles of a URL and an IP address. Remember, the URL is the name you type, and the IP address is the numerical address the computer uses.

Fun fact

The original 'phonebook' for the internet was a single text file called HOSTS.TXT that was manually updated and shared between computers on the ARPANET.

Worked example 15 marks

A student types 'www.example.com' into their web browser and presses Enter. Describe the main steps, involving DNS, that allow the homepage to be displayed.

  1. 1

    Step 1: The student's web browser sends the URL 'www.example.com' to a DNS resolver, which is usually provided by their Internet Service Provider (ISP).

  2. 2

    Step 2: The DNS resolver queries a series of DNS servers to find the IP address associated with 'www.example.com'.

  3. 3

    Step 3: The DNS server responds with the correct IP address (e.g., 93.184.216.34) for the web server hosting the website.

  4. 4

    Step 4: The student's browser sends an HTTP request to this IP address, asking for the website's homepage.

  5. 5

    Step 5: The web server at that IP address receives the request and sends the website's data (HTML, CSS, images) back to the student's browser, which then renders and displays the page.

Recap

  • An IP address is a unique numerical label assigned to each device on a computer network.
  • A URL is a human-readable address for a resource on the internet.
  • DNS translates user-friendly URLs into machine-readable IP addresses.
  • A web browser uses the IP address provided by DNS to request data from a web server.
  • Without DNS, we would have to remember the IP address for every website we want to visit.

Quick check

  1. What is the primary function of a DNS server?1 mark
  2. State one difference between a URL and an IP address.1 mark

2. Error Detection in Data Transmission

When data is transmitted as binary code across a network, it can be corrupted by issues like electrical interference or faulty cables. This can cause bits to 'flip' from 0 to 1 or vice versa, introducing errors. To combat this, systems use error detection methods. One of the simplest is the parity bit. An extra bit (the parity bit) is added to each byte of data before transmission. The value of this bit (0 or 1) is set to make the total number of '1's in the byte (including the parity bit) either even (for even parity) or odd (for odd parity). The receiving computer checks if the parity is still correct. If not, it knows an error has occurred and can request the data to be sent again.

Key term

Parity Bit: An extra bit added to a string of binary code to ensure that the total number of 1-bits in the string is either even or odd, used for error detection.

Examiner insight

Marks are often awarded for showing the counting of '1's. Clearly state the original number of '1's, the parity rule being applied, and the resulting parity bit value.

Common pitfall

Forgetting whether to count the existing '1's to make the total even/odd, or just the new parity bit. Always count the total number of '1's in the final byte including the parity bit.

Worked example 14 marks

The 7-bit binary code 1011001 is to be transmitted using even parity.(a) What value should the parity bit have?(b) The receiver gets the 8-bit byte 10110011. Explain if an error would be detected.

  1. 1

    Part (a): First, count the number of '1's in the original 7-bit data: 1011001. There are four '1's.

  2. 2

    Part (a): Since we are using even parity, the total number of '1's (including the parity bit) must be an even number. As there are already four '1's (an even number), the parity bit must be 0 to keep the total even.

  3. 3

    Part (a): The 8-bit byte to be sent is 10110010 (assuming the parity bit is added at the end).

  4. 4

    Part (b): The receiver gets 10110011. The receiver counts the number of '1's in this byte. There are five '1's.

  5. 5

    Part (b): Since the system was using even parity, the receiver was expecting an even number of '1's. As it received five '1's (an odd number), it detects that a transmission error has occurred.

Recap

  • Data transmission errors can cause binary bits to flip.
  • Error detection methods are used to check if data has been corrupted during transmission.
  • A parity bit is an extra bit added to a byte for error checking purposes.
  • In even parity, the total number of 1s (including the parity bit) must be even.
  • In odd parity, the total number of 1s must be odd.
  • If the received parity does not match the agreed-upon type (even or odd), an error is detected.

Quick check

  1. A system uses odd parity. What parity bit should be added to the 7-bit data 0110101?1 mark

3. Data Encryption and Ciphers

When data is sent over a network, especially a public one like the internet, it can be intercepted by unauthorised parties. Encryption is the process of scrambling data into an unreadable format, called ciphertext, so that only authorised parties with the correct 'key' can unscramble (decrypt) it back into its original form (plaintext). A simple example of an encryption algorithm is a cipher. The Caesar cipher is a basic substitution cipher where each letter in the plaintext is shifted a certain number of places down or up the alphabet. The number of places to shift is the 'key'.

Key term

Encryption: The process of converting data or information into a code, especially to prevent unauthorized access.

Examiner insight

Show your working for each letter's transformation. For shifts that 'wrap around' the end of the alphabet (e.g., encrypting 'Y' with a key of +3), clearly show how Y -> Z -> A -> B.

Common pitfall

When decrypting, students sometimes apply the same shift instead of reversing it. If the encryption key is +4, the decryption key is -4.

Worked example 13 marks

Using a Caesar cipher with a key of +3 (a right shift of 3), encrypt the message 'HELLO'.

  1. 1

    Step 1: Identify the plaintext message: 'HELLO' and the key: +3.

  2. 2

    Step 2: Take the first letter, 'H'. Shift it 3 places to the right in the alphabet: I, J, K. So, H becomes K.

  3. 3

    Step 3: Take the second letter, 'E'. Shift it 3 places to the right: F, G, H. So, E becomes H.

  4. 4

    Step 4: Take the third letter, 'L'. Shift it 3 places to the right: M, N, O. So, L becomes O.

  5. 5

    Step 5: Take the fourth letter, 'L'. It also becomes O.

  6. 6

    Step 6: Take the fifth letter, 'O'. Shift it 3 places to the right: P, Q, R. So, O becomes R.

  7. 7

    Step 7: Combine the encrypted letters. The ciphertext is 'KHOOR'.

Worked example 23 marks

A message 'MJQQT' was encrypted with a Caesar cipher using a key of +5. Decrypt the message.

  1. 1

    Step 1: To decrypt, we must reverse the process. This means applying a key of -5 (a left shift of 5).

  2. 2

    Step 2: Take 'M'. Shift it 5 places to the left: L, K, J, I, H. So, M becomes H.

  3. 3

    Step 3: Take 'J'. Shift it 5 places to the left: I, H, G, F, E. So, J becomes E.

  4. 4

    Step 4: Take 'Q'. Shift it 5 places to the left: P, O, N, M, L. So, Q becomes L.

  5. 5

    Step 5: The next 'Q' also becomes 'L'.

  6. 6

    Step 6: Take 'T'. Shift it 5 places to the left: S, R, Q, P, O. So, T becomes O.

  7. 7

    Step 7: The decrypted plaintext is 'HELLO'.

Recap

  • Encryption protects data from being understood if intercepted.
  • Plaintext is the original message, and ciphertext is the encrypted message.
  • A cipher is an algorithm for performing encryption or decryption.
  • The Caesar cipher is a simple substitution cipher that shifts letters by a set number (the key).
  • To decrypt a Caesar cipher, you perform the shift in the opposite direction.

Quick check

  1. What is the difference between plaintext and ciphertext?2 marks

4. Secure Websites and Digital Certificates

You can tell if a website is secure by looking at its URL. A secure website's URL starts with 'https://' instead of 'http://'. The 'S' stands for 'Secure' and indicates that the connection between your browser and the web server is encrypted. This encryption is provided by protocols like SSL (Secure Sockets Layer) or its modern successor, TLS (Transport Layer Security). To prove its identity and enable this encryption, a secure website uses a digital certificate. This certificate is issued by a trusted Certificate Authority (CA) and acts like a digital passport. It confirms that the website is legitimate and not a fake. Your browser automatically checks this certificate to ensure you're connecting to the real site before encrypting any data you send, like passwords or credit card details.

Key term

HTTPS (Hypertext Transfer Protocol Secure): An extension of HTTP that is used for secure communication over a computer network, with the communication protocol encrypted using Transport Layer Security (TLS) or Secure Sockets Layer (SSL).

Common pitfall

Simply stating 'the website is secure' is not enough. Students must explain *what* makes it secure (encryption) and *why* that is important (prevents interception of data).

Fun fact

As of 2023, over 95% of web traffic loaded by the Firefox browser is over HTTPS, a massive increase from around 40% in 2015, thanks to a big push for a more secure web.

Worked example 14 marks

You are about to enter your personal details on an online shopping website. Identify two signs that indicate the website is secure and explain why each is important.

  1. 1

    Sign 1: The URL in the address bar begins with 'https://'. This is important because it shows that the data being transmitted between my browser and the website's server is encrypted. This prevents hackers from easily reading my personal details if they intercept the data.

  2. 2

    Sign 2: A padlock icon is displayed in the address bar. This is important because it is a visual confirmation that the site has a valid digital certificate from a trusted authority. Clicking on it allows me to verify the website's identity and check that my connection is secure.

Recap

  • HTTPS indicates a secure, encrypted connection to a website.
  • The 'S' in HTTPS stands for 'Secure'.
  • SSL/TLS are the encryption protocols that provide the security for HTTPS.
  • A digital certificate is used to verify a website's identity.
  • A padlock icon in the browser's address bar is a visual indicator of a secure connection.
  • Never enter sensitive information on a website that does not use HTTPS.

Quick check

  1. What does the 'S' in HTTPS stand for?1 mark
  2. What is the purpose of a website's digital certificate?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Explain the relationship between a URL, a DNS server, and an IP address in the context of accessing a website.4 marks
  2. A 7-bit data byte 1101010 is to be transmitted. A parity bit will be added at the end of the byte. What will the final 8-bit transmission be if the system uses (a) even parity, and (b) odd parity?2 marks
  3. A computer receives the 8-bit byte 01101101. The system uses odd parity and the parity bit is the last bit. Has an error occurred? Justify your answer.3 marks
  4. Encrypt the word 'SECURE' using a Caesar cipher with a key of +4. Show your working.3 marks
  5. The ciphertext 'HSXF' was created using a Caesar cipher with a key of +2. What is the original plaintext message?2 marks
  6. Describe two differences between a website with a URL starting with 'http://' and one starting with 'https://'.4 marks
  7. Explain the role of a digital certificate in establishing a secure connection to a website.3 marks
  8. Parity checking can detect an error, but it has a significant limitation. Describe a scenario where a parity check would fail to detect an error in transmission.2 marks
  9. A user wants to log in to their online bank. Explain why it is crucial that the bank's website uses HTTPS, referencing both privacy and authentication.5 marks
  10. Describe two ways that Artificial Intelligence (AI) can be used to improve the experience of using the internet.4 marks

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