Cambridge Lower Secondary CheckpointStage 9

Networks and Digital Communication

Computing Stage 9 Chapter Notes

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Networks and Digital Communication
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In this chapter

  1. 1Network Topologies: Bus, Ring, and Star
  2. 2Understanding Network Protocols: TCP/IP
  3. 3Error Detection with Parity Bits
  4. 4Parity Blocks for Error Correction
  5. 5Designing for Growth: Network Scalability
  6. 6Balancing Network Security

1. Network Topologies: Bus, Ring, and Star

Network topology refers to the physical or logical arrangement of devices (called nodes) in a network. The topology determines how nodes communicate with each other. The three fundamental topologies you need to know are Bus, Ring, and Star. Each has distinct advantages and disadvantages that make it suitable for different situations.

Key term

Network Topology: The physical or logical arrangement of nodes and connections in a network, which dictates how data is transferred.

Examiner insight

Examiners frequently ask for comparisons between topologies, so be prepared to discuss the advantages and disadvantages of each in a given scenario, focusing on cost, performance, and reliability.

Common pitfall

Confusing a hub and a switch in a star topology. A hub broadcasts data to all ports, whereas a more intelligent switch sends data only to the intended recipient's port.

Worked example 14 marks

A small startup with 6 employees is setting up its first office. They need a cheap and simple network. Which topology would you recommend and why? Discuss one major drawback of your chosen topology.

  1. 1

    Recommendation: A Bus topology would be the most suitable choice.

  2. 2

    Justification 1 (Cost): It is the cheapest to install as it requires the least amount of cable. All devices connect to a single central cable (the bus).

  3. 3

    Justification 2 (Simplicity): It is simple to set up, as you just need to run one main cable and tap each device into it using connectors.

  4. 4

    Drawback: The main drawback is that the central cable is a single point of failure. If the main bus cable breaks anywhere, the entire network will fail. Also, performance degrades as more devices are added and try to communicate at the same time, leading to data collisions.

Recap

  • A Bus topology connects all devices to a single central cable, which is cheap but has a single point of failure.
  • A Ring topology connects each device to two others, forming a circle, with data often passed using a 'token'.
  • A Star topology connects every device to a central hub or switch, which is robust but requires more cable.
  • In a Bus topology, data is broadcast to all nodes, but only the intended recipient accepts it.
  • In a Star topology, if one cable fails, only that one device is disconnected from the network.
  • The choice of topology involves a trade-off between cost, reliability, and performance.

Quick check

  1. In which topology does the failure of a central device cause the entire network to fail?1 mark
  2. What is the main cable in a bus topology called?1 mark

2. Understanding Network Protocols: TCP/IP

Imagine trying to have a conversation where everyone speaks a different language. It wouldn't work. Protocols are the 'agreed languages' of computer networks. They are a set of rules that govern how data is formatted, transmitted, and received. The most important protocol suite is TCP/IP (Transmission Control Protocol/Internet Protocol), which powers the internet. TCP and IP work together but have different jobs. TCP manages the data, and IP manages the delivery.

Key term

Protocol: An agreed set of rules that computers follow to communicate with each other over a network.

Examiner insight

Marks are awarded for clearly separating the roles of TCP (packet splitting, numbering, reassembly, error control) and IP (addressing and routing).

Common pitfall

Stating that 'TCP/IP sends data' without explaining the distinct roles of TCP (packet management and reliability) and IP (addressing and routing).

Fun fact

The first message sent over the ARPANET (the internet's predecessor) was 'LO'. The user was trying to type 'LOGIN' but the system crashed after the first two letters.

Worked example 16 marks

Explain the roles of TCP and IP when you send an email with an attachment from your computer.

  1. 1

    Step 1 (TCP's role - Sender): The Transmission Control Protocol (TCP) on your computer takes the email and attachment data.

  2. 2

    Step 2 (TCP's role - Sender): It breaks the data down into smaller, numbered chunks called packets.

  3. 3

    Step 3 (IP's role - Sender): The Internet Protocol (IP) takes each packet and adds a header containing the source IP address (your computer) and the destination IP address (the email server).

  4. 4

    Step 4 (Transmission & Routing): The packets are sent over the network/internet. Routers along the way read the destination IP address to forward the packets towards the email server.

  5. 5

    Step 5 (TCP's role - Receiver): At the email server, TCP receives the packets. It uses the numbers to check if all packets have arrived and puts them back in the correct order to reconstruct the original email and attachment.

  6. 6

    Step 6 (TCP's role - Receiver): If any packets are missing or corrupted, TCP requests for them to be re-sent.

Recap

  • A protocol is a set of rules for network communication.
  • TCP/IP is the fundamental protocol suite for the internet.
  • TCP is responsible for splitting data into packets, numbering them, and reassembling them at the destination.
  • TCP also handles error checking and requests re-transmission of lost packets.
  • IP is responsible for addressing packets with source and destination IP addresses and routing them across the network.

Quick check

  1. Which protocol, TCP or IP, is responsible for numbering data packets?1 mark
  2. What piece of information does IP add to a packet to ensure it gets to the right computer?1 mark

3. Error Detection with Parity Bits

When data is transmitted over a network, interference can cause bits to 'flip' (a 0 becomes a 1, or a 1 becomes a 0). This corrupts the data. Parity checking is a simple method for detecting such errors. A parity bit is an extra bit added to a byte (a group of 8 bits). The value of this bit (0 or 1) is set to make the total number of 1s in the byte either even (even parity) or odd (odd parity), depending on the system's rules.

Key term

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

Examiner insight

Candidates must be able to calculate the correct parity bit for a given byte and also check a received byte for errors, clearly stating the steps of counting the 1s.

Common pitfall

A parity check can detect a single bit error, but it will fail to detect an error if an even number of bits (e.g., 2 or 4) are flipped, as the parity will still appear correct.

Worked example 13 marks

A network uses an odd parity system. The 7-bit data 1101010 is to be transmitted. What should the 8th bit (the parity bit) be, and what will be the final transmitted byte?

  1. 1

    Step 1: Identify the parity system. The system uses odd parity, meaning the total number of 1s must be an odd number.

  2. 2

    Step 2: Count the number of 1s in the original 7-bit data: 1101010.

  3. 3

    Step 3: The number of 1s is 4 (an even number).

  4. 4

    Step 4: To make the total number of 1s odd, the parity bit must be 1. (4 + 1 = 5, which is odd).

  5. 5

    Step 5: The final transmitted byte will be the original data with the parity bit appended. Assuming the parity bit is added at the end, the byte is 11010101.

Worked example 22 marks

A computer using even parity receives the byte 01101101. Has an error occurred? Explain your answer.

  1. 1

    Step 1: The system uses even parity, so the total count of 1s in a correct byte should be even.

  2. 2

    Step 2: Count the number of 1s in the received byte: 01101101.

  3. 3

    Step 3: The number of 1s is 5.

  4. 4

    Step 4: 5 is an odd number. Since the system expects an even number of 1s, an error has been detected.

Recap

  • Data can be corrupted by interference during transmission.
  • A parity bit is an extra bit added to a byte for error detection.
  • In an even parity system, the total number of 1s (including the parity bit) must be even.
  • In an odd parity system, the total number of 1s must be odd.
  • If the received byte's parity does not match the system's rule, an error is detected and the data is usually re-sent.

Quick check

  1. For the 7-bit data 1010101, what would the parity bit be in an even parity system?1 mark

4. Parity Blocks for Error Correction

A simple parity bit can tell you *that* an error has occurred, but not *where*. This means the entire byte of data must be re-transmitted. A more advanced method is the parity block, also known as 2D parity. Data is arranged in a grid, and a parity bit is calculated for each row and each column. If a single bit flips, both its row and column parity checks will fail. The intersection of the failed row and failed column points directly to the incorrect bit, allowing the receiving computer to correct it by flipping it back.

Key term

Parity Block: A method of error checking that organises data into a grid and uses both horizontal and vertical parity bits to detect and locate a single-bit error.

Examiner insight

Questions on parity blocks require methodical working. Show your checks for each row and column clearly to gain partial marks even if your final answer is incorrect.

Common pitfall

Misidentifying the intersection of the erroneous row and column, or forgetting to flip the bit to correct it after finding it.

Worked example 15 marks

The following data block was received. It was sent using even parity for both rows and columns. One bit has been flipped during transmission. Identify and correct the erroneous bit. Data: 1101 0011 1010 Column Parity: 0100

  1. 1

    Step 1: Check the parity of each row using the even parity rule.

  2. 2

    Row 1 (1101): Contains three 1s (odd). This row is incorrect.

  3. 3

    Row 2 (0011): Contains two 1s (even). This row is correct.

  4. 4

    Row 3 (1010): Contains two 1s (even). This row is correct.

  5. 5

    Step 2: Check the parity of each column using the received column parity bits.

  6. 6

    Column 1 (1,0,1): Contains two 1s (even). The column parity bit should be 0. Received is 0. Correct.

  7. 7

    Column 2 (1,0,0): Contains one 1 (odd). The column parity bit should be 1. Received is 1. Correct.

  8. 8

    Column 3 (0,1,1): Contains two 1s (even). The column parity bit should be 0. Received is 0. Correct.

  9. 9

    Column 4 (1,1,0): Contains two 1s (even). The column parity bit should be 0. Received is 0. Correct.

  10. 10

    Step 3: Re-calculate the column parity from the received data to find the incorrect column.

  11. 11

    Column 1 (1,0,1): two 1s -> even. Correct.

  12. 12

    Column 2 (1,0,0): one 1 -> odd. Incorrect.

  13. 13

    Column 3 (0,1,1): two 1s -> even. Correct.

  14. 14

    Column 4 (1,1,0): two 1s -> even. Correct.

  15. 15

    Step 4: Identify the error location. The error is in Row 1 and Column 2.

  16. 16

    Step 5: The bit at the intersection of Row 1, Column 2 is currently 1. To correct it, it must be flipped to 0.

  17. 17

    Step 6: The corrected data block is: 1001 0011 1010

Recap

  • A single parity bit can detect an error but cannot locate or correct it.
  • A parity block arranges data in a grid with parity bits for each row and column.
  • The intersection of the row and column with incorrect parity identifies the exact location of a single-bit error.
  • Once located, a single-bit error can be corrected by flipping the bit (0 to 1, or 1 to 0).
  • Parity blocks provide a mechanism for error correction, not just detection.

Quick check

  1. What is the main advantage of a parity block over a single parity bit?1 mark

5. Designing for Growth: Network Scalability

Scalability is the ability of a network to grow and handle increased demand without a significant drop in performance. A network that works well for 10 users might grind to a halt with 100 users if it wasn't designed to be scalable. When designing a network, you must think about the future. Key scalability factors include the number of users, the amount of data traffic, and the physical area the network needs to cover. Choosing the right topology and hardware is critical for building a network that can scale up effectively.

Key term

Scalability: The capability of a system or network to handle a growing amount of work, or its potential to be enlarged to accommodate that growth.

Examiner insight

High-scoring answers link specific design choices (e.g., using a star topology with a switch) directly to improved scalability, explaining the technical reason why it helps.

Common pitfall

Simply stating a network should be 'scalable' without explaining *how* to achieve it (e.g., using switches, star topology) or *what factors* it needs to scale for (e.g., more users, more video streaming).

Worked example 14 marks

A school is planning a new network. They expect to have 500 users initially, but this could double in five years. Explain two design choices that would ensure the network is scalable.

  1. 1

    Choice 1: Use a Star Topology. In a star topology, each device has its own dedicated cable to a central switch. This means adding a new user is as simple as running a new cable and does not affect the performance of other users. This is much more scalable than a bus topology, where more users would increase data collisions and slow the entire network down.

  2. 2

    Choice 2: Use Switches instead of Hubs. A switch is an intelligent device that sends data packets only to the specific port of the intended recipient. A hub, by contrast, broadcasts packets to all ports. As the number of users and traffic grows, a switch-based network will perform much better because it minimizes unnecessary traffic and collisions, making it highly scalable.

  3. 3

    Bonus Choice: Use a structured cabling plan and modular hardware. Planning cable runs to allow for easy additions and using network devices (like switches) that can be stacked or have empty ports for expansion makes the physical process of scaling up much simpler and cheaper in the future.

Recap

  • Scalability is a network's ability to handle future growth.
  • Key factors affecting scalability are user numbers, traffic volume, and geographical area.
  • A Star topology is more scalable than Bus or Ring topologies.
  • Using switches instead of hubs is crucial for a scalable network as they reduce unnecessary traffic.
  • Planning for scalability from the start is cheaper and more effective than trying to fix an overloaded network later.

Quick check

  1. Why is a star topology more scalable than a bus topology?2 marks

6. Balancing Network Security

Securing a network isn't about creating an impenetrable fortress; it's about finding the right balance. Implementing network security always involves a trade-off between three key factors: cost, accessibility, and the security requirements of the data itself. The goal is to provide 'appropriate' security—enough to protect the assets without crippling the budget or preventing legitimate users from doing their jobs.

Key term

Authentication: The process of verifying the identity of a user or device to allow access to a computer system or network.

Examiner insight

Examiners look for a balanced discussion that considers multiple factors. Good answers justify security choices by linking them to the specific scenario and the sensitivity of the data involved.

Common pitfall

Assuming that 'more security is always better'. Recommending Fort Knox-level security for a school's lunch menu website is not a good answer as it ignores the cost and accessibility trade-offs.

Worked example 16 marks

A local library wants to provide free Wi-Fi for its visitors but also needs to protect its own administrative network which contains staff details and financial records. Discuss the security choices they must make, considering cost, accessibility, and requirements.

  1. 1

    Step 1 (Analyze Requirements): There are two distinct datasets with very different security needs. The public Wi-Fi data is low-risk, while the administrative data is high-risk.

  2. 2

    Step 2 (Public Wi-Fi - Accessibility vs Cost): For the public Wi-Fi, accessibility is the top priority. Security should be minimal. A simple 'splash page' where users agree to terms is sufficient. This is very low cost to implement and makes access easy for visitors.

  3. 3

    Step 3 (Admin Network - Security vs Accessibility): For the administrative network, security is the top priority. This network should be physically and logically separate from the public Wi-Fi (e.g., using VLANs or a separate network switch). Access should require strong authentication, such as a username and a complex password. This increases security but makes it slightly less accessible for staff.

  4. 4

    Step 4 (Admin Network - Cost): Protecting the admin network will have higher costs. It will require a good firewall to block unauthorized access from the internet and from the public Wi-Fi network. Data on the server should be encrypted. This is a necessary cost to protect sensitive data and comply with data protection laws.

  5. 5

    Conclusion: The library should implement a two-tiered security approach: an open, low-cost system for the public and a robust, secure, and more costly system for its internal administrative data. This balances all three factors appropriately for each use case.

Recap

  • Network security is a balance between cost, accessibility, and the value of the data.
  • Higher security often means higher cost and lower accessibility for users.
  • The security measures chosen should be appropriate for the data being protected.
  • Not all data requires the same level of security; a public website needs different protection than a bank's customer database.
  • Authentication (verifying who you are) and encryption (scrambling data) are key security techniques.

Quick check

  1. List the three main factors that must be balanced when choosing network security measures.3 marks

End-of-chapter exercise

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

  1. Compare and contrast a star topology with a bus topology. You should mention the hardware used, reliability, and cost.6 marks
  2. A system uses odd parity. A byte is received as 10110110. Has an error occurred? Explain your reasoning.2 marks
  3. Explain the difference between the role of TCP and the role of IP in the TCP/IP protocol suite.4 marks
  4. A small business is growing rapidly. Explain why choosing switches over hubs for their network is a good decision in terms of scalability.3 marks
  5. The 7-bit ASCII code for the character 'C' is 1000011. If a system is using even parity, what 8-bit byte would be transmitted for 'C'?2 marks
  6. Describe what a 'protocol' is in the context of computer networks and give an example of a protocol.2 marks
  7. A bank is designing a network for its new branch. Explain the trade-offs it must consider between security and accessibility for its staff.4 marks
  8. What is the key limitation of using a single parity bit for error detection?2 marks
  9. A youth centre wants to set up a network for gaming. They have a limited budget but want a reliable network that won't go down if one person's computer cable is faulty. Recommend and justify a suitable network topology.3 marks
  10. A 3x3 data block is sent using even parity for rows and columns. The received data is shown below, but one bit has been flipped. Find the incorrect row and column, state the location of the error (e.g., Row 2, Column 3), and state the corrected 3x3 data block. Data: 101 111 0105 marks

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