Communication and Networks

What you'll learn

This revision guide covers all aspects of communication and networks required for WJEC GCSE Computer Science. You'll learn about network types, topologies, hardware components, protocols, and how data is transmitted across networks. These topics are essential for both Paper 1 (Understanding Computer Science) and practical understanding of how modern systems communicate.

Key terms and definitions

Protocol — A set of rules that governs how data is transmitted across a network, ensuring devices can communicate effectively.

Topology — The physical or logical arrangement of devices (nodes) and connections in a network.

Packet — A small unit of data transmitted across a network, containing the payload (actual data), sender address, receiver address, and error-checking information.

IP address — A unique numerical identifier assigned to each device on a network, typically written in dotted decimal notation (e.g., 192.168.1.1).

Bandwidth — The maximum amount of data that can be transmitted across a network connection in a given time period, typically measured in bits per second (bps).

MAC address — A unique hardware identifier permanently assigned to a network interface card (NIC) by the manufacturer.

Encryption — The process of encoding data so that only authorized parties with the correct decryption key can read it.

Latency — The delay between sending data from one point and it being received at another, measured in milliseconds.

Core concepts

Network types

Networks are classified by their geographical coverage and purpose:

Local Area Network (LAN)

• Covers a small geographical area such as a single building, school, or office
• Typically owned and managed by a single organization
• High data transfer speeds (typically 100 Mbps to 10 Gbps)
• Uses infrastructure like Ethernet cables or Wi-Fi
• Examples: School computer network, home Wi-Fi network

Wide Area Network (WAN)

• Covers a large geographical area, potentially worldwide
• Connects multiple LANs together
• Often uses third-party telecommunications infrastructure
• Lower data transfer speeds than LANs due to distance
• The Internet is the largest WAN
• Examples: A bank's network connecting branches across the UK, a multinational company's network

Personal Area Network (PAN)

• Very small network, typically within a range of a few meters
• Connects personal devices belonging to one person
• Often uses Bluetooth or USB connections
• Examples: Connecting a smartphone to wireless earbuds, linking a smartwatch to a mobile phone

Network topologies

The arrangement of devices in a network affects performance, reliability, and cost.

Star topology

• All devices connect to a central switch or hub
• Data passes through the central device to reach its destination
• Advantages:
  • If one cable fails, only that device is affected
  • Easy to add new devices
  • Better performance as each device has dedicated bandwidth
  • Easy to identify faults
• Disadvantages:
  • If the central switch fails, the entire network fails
  • More expensive due to additional cabling required
  • Requires more cable than bus topology

Bus topology

• All devices connect to a single backbone cable
• Terminators at each end prevent signal reflection
• Data is broadcast to all devices; each checks if the data is addressed to them
• Advantages:
  • Less cabling required, reducing cost
  • Easy to implement for small networks
• Disadvantages:
  • If the main cable fails, the entire network fails
  • Performance degrades as more devices are added
  • All data is transmitted to all devices, creating security concerns
  • Difficult to identify faults

Mesh topology

• Each device connects to multiple other devices
• Full mesh: every device connects to every other device
• Partial mesh: some devices connect to multiple others, but not all
• Advantages:
  • Highly reliable — multiple routes for data
  • If one connection fails, data can take alternative routes
  • No single point of failure
• Disadvantages:
  • Very expensive due to extensive cabling
  • Complex to install and maintain
  • Requires many network interface cards

Network hardware

Switch

• Connects devices within a LAN
• Operates at the data link layer (Layer 2)
• Uses MAC addresses to direct data only to the intended recipient
• Improves network performance by reducing unnecessary traffic
• Learns which devices are connected to which ports

Router

• Connects different networks together (e.g., LAN to WAN)
• Operates at the network layer (Layer 3)
• Uses IP addresses to route data between networks
• Determines the best path for data transmission
• Provides gateway between home/office network and the Internet
• Often includes firewall functionality

Wireless Access Point (WAP)

• Allows wireless devices to connect to a wired network
• Creates a Wi-Fi hotspot
• Transmits and receives radio signals
• Often integrated into home routers

Network Interface Card (NIC)

• Hardware component that enables a device to connect to a network
• Can be wired (Ethernet) or wireless (Wi-Fi)
• Has a unique MAC address
• Prepares data for transmission across the network
• Installed in or attached to computers, printers, and other networked devices

Modem

• Modulates and demodulates signals
• Converts digital data to analogue signals for transmission over telephone lines (and vice versa)
• Required for broadband Internet connections using phone lines or cable
• Stands for "modulator-demodulator"

Network protocols

Protocols ensure different devices and systems can communicate effectively.

TCP/IP (Transmission Control Protocol/Internet Protocol)

• The fundamental protocol suite for Internet communication
• TCP: Ensures reliable, ordered delivery of data packets
  • Establishes connections between devices
  • Breaks data into packets
  • Checks packets arrive correctly and requests retransmission if needed
  • Reassembles packets in correct order
• IP: Handles addressing and routing of packets
  • Assigns IP addresses to devices
  • Routes packets to their destination across networks

HTTP/HTTPS (HyperText Transfer Protocol / Secure)

• Used for transmitting web pages over the Internet
• HTTP operates on port 80
• HTTPS includes encryption (SSL/TLS) for secure transmission
• HTTPS operates on port 443
• Web browsers use these protocols to request and display web content

FTP (File Transfer Protocol)

• Used for transferring files between computers
• Operates on ports 20 and 21
• Allows uploading and downloading of files
• Commonly used for website maintenance
• Can require authentication (username and password)

SMTP (Simple Mail Transfer Protocol)

• Used for sending emails
• Operates on port 25 (or 587 for secure)
• Transfers email from client to server and between servers
• Works alongside POP3 or IMAP for receiving emails

POP3 and IMAP (Post Office Protocol / Internet Message Access Protocol)

• POP3: Downloads emails from server to device and typically deletes from server
• IMAP: Synchronizes emails across multiple devices, keeping them on the server
• IMAP is preferred for accessing email from multiple devices

Data transmission methods

Packet switching

• Data is broken into packets before transmission
• Each packet contains:
  • Packet number/sequence
  • Sender's IP address
  • Recipient's IP address
  • Payload (actual data)
  • Error-checking data
• Packets may take different routes across the network
• Reassembled in correct order at destination
• More efficient use of network resources
• Used for most Internet communication

Circuit switching

• Dedicated communication path established between sender and receiver
• Path remains open for entire communication session
• Traditional telephone networks use this method
• Guarantees bandwidth but wastes resources when no data is being sent

Network security

Encryption methods

• Protects data during transmission from unauthorized access
• Data scrambled using an algorithm and key
• Only recipients with correct decryption key can read data
• HTTPS uses encryption for secure web browsing
• WPA2/WPA3 encrypts Wi-Fi communications

Firewall

• Software or hardware that monitors network traffic
• Blocks unauthorized access while permitting legitimate communication
• Uses rules to filter incoming and outgoing traffic
• Can be network-based (hardware) or host-based (software on individual devices)
• Examines packet headers to make filtering decisions

MAC address filtering

• Only allows devices with specific MAC addresses to connect
• Creates a whitelist of permitted devices
• Provides basic security but can be circumvented
• Useful for small networks with known devices

Authentication methods

• Username and password combinations
• Two-factor authentication (2FA) adds extra security
• Biometric authentication (fingerprint, facial recognition)
• Verifies identity before granting network access

IP addressing and DNS

IPv4 addressing

• 32-bit addresses written in dotted decimal notation
• Format: xxx.xxx.xxx.xxx (e.g., 192.168.1.1)
• Each section ranges from 0 to 255
• Approximately 4.3 billion unique addresses
• Running out of available addresses

IPv6 addressing

• 128-bit addresses written in hexadecimal
• Format: xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx
• Provides vastly more addresses than IPv4
• Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
• Gradually replacing IPv4

DNS (Domain Name System)

• Translates human-readable domain names into IP addresses
• Acts as the "phone book" of the Internet
• When you type www.example.com, DNS finds the corresponding IP address
• Hierarchical system of DNS servers
• Allows users to access websites without memorizing IP addresses

Worked examples

Example 1: Network topology selection (4 marks)

Question: A small office with 12 computers needs a network installing. The office manager wants the network to continue working even if one cable fails, and wants to easily add more computers in the future.

Explain which topology would be most suitable and justify your choice.

Answer:

• Star topology would be most suitable (1 mark)
• If one cable fails, only that one computer is affected; the rest of the network continues working (1 mark)
• This meets the requirement for reliability (1 mark)
• New computers can be easily added by connecting them to the central switch without disrupting existing connections (1 mark)

Mark scheme notes: Accept alternative valid justifications. Reject bus topology as single cable failure affects entire network. Mesh topology would be unnecessarily expensive for 12 computers.

Example 2: Packet switching (5 marks)

Question: Explain how packet switching is used to transmit an email across the Internet, and state one advantage of this method.

Answer:

• The email is broken down into smaller units called packets (1 mark)
• Each packet contains the sender's address, recipient's address, and packet sequence number (1 mark)
• Packets may take different routes across the network to reach the destination (1 mark)
• Packets are reassembled in the correct order at the destination (1 mark)
• Advantage: More efficient use of network resources / if one route is busy, packets can take alternative routes / network can handle multiple transmissions simultaneously (1 mark for any valid advantage)

Example 3: Protocol identification (3 marks)

Question: Identify the most appropriate protocol for each of the following scenarios: a) Loading a secure online banking website b) Sending an email c) Downloading a file from a website server to update a web page

Answer: a) HTTPS (1 mark) b) SMTP (1 mark) c) FTP (1 mark)

Common mistakes and how to avoid them

• Confusing switches and routers: Remember switches connect devices within a LAN using MAC addresses, while routers connect different networks using IP addresses. A router connects your home network to the Internet; a switch connects your computer to your printer on the same network.

• Mixing up topology advantages and disadvantages: Create a comparison table for star, bus, and mesh topologies. Specifically remember that star topology's weakness is central point of failure, while bus topology's weakness is the backbone cable failure.

• Confusing HTTP and HTTPS: Always specify HTTPS when the question mentions security, online shopping, banking, or passwords. HTTP is unencrypted; HTTPS includes encryption.

• Incomplete packet switching explanations: Don't just say "data is broken into packets." Always mention that packets contain addresses, may take different routes, and are reassembled at the destination.

• Vague protocol descriptions: Learn the specific purpose of each protocol. "Used for the Internet" is too vague — be specific about whether it's web pages (HTTP/HTTPS), email (SMTP/POP3/IMAP), or file transfer (FTP).

• Not reading the mark allocation: A 1-mark question needs one distinct point; a 4-mark question needs four. Don't write extensively for 1 mark or give minimal detail for 4 marks.

Exam technique for "Communication and Networks"

• Command word awareness: "State" requires a brief answer (often 1 mark); "Explain" requires reasoning and justification (usually 2+ marks); "Compare" requires similarities AND differences; "Justify" requires a decision with supporting reasons.

• Use technical terminology precisely: The mark scheme rewards accurate use of terms like packet, protocol, topology, bandwidth, encryption. Avoid vague phrases like "sends stuff" or "makes it safe" — use "transmits packets" and "encrypts data."

• Structure extended answers: For questions worth 4+ marks, use separate sentences or bullet points for each distinct point. This helps ensure you don't accidentally combine two points into one sentence and lose marks.

• Relate answers to context: If a question describes a specific scenario (e.g., school network, online shop, home user), tailor your answer to that context. Generic answers may not score full marks.

Quick revision summary

Networks connect devices to share resources and communicate. LANs cover small areas; WANs connect LANs over distance. Star topology uses a central switch; bus uses one backbone cable; mesh provides multiple connections. Key hardware includes switches (connect LAN devices), routers (connect networks), and NICs (enable device connectivity). Protocols like TCP/IP, HTTP/HTTPS, and FTP govern communication. Data transmission uses packet switching, breaking data into packets with addressing information. Security measures include encryption, firewalls, and authentication. DNS translates domain names to IP addresses.