What you'll learn
This revision guide covers all the essential content on networks and communication required for your Edexcel GCSE Computer Science examination. You will understand different network types, topologies, hardware components, protocols and how data is transmitted across networks. This topic typically accounts for 10-15% of your final exam marks.
Key terms and definitions
LAN (Local Area Network) — a network confined to a small geographical area, such as a single building or school campus, where the infrastructure is owned by the organisation using it.
WAN (Wide Area Network) — a network that spans a large geographical area, connecting multiple LANs, typically using infrastructure leased from telecommunications companies.
Protocol — a set of rules that governs how data is transmitted across a network, ensuring devices can communicate effectively.
Packet — a small unit of data transmitted across a network, containing payload data, source and destination addresses, and error-checking information.
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 (Media Access Control address) — a unique hardware identifier assigned to a network interface card, consisting of 48 bits usually displayed as six pairs of hexadecimal digits.
IP address (Internet Protocol address) — a numerical label assigned to each device on a network, used for identification and routing data to the correct destination.
Encryption — the process of encoding data so that only authorized parties with the correct decryption key can read it, protecting information during transmission.
Core concepts
Network types and characteristics
Networks are classified based on their geographical spread and ownership.
LANs are used within single sites such as homes, schools, or office buildings. Key characteristics include:
- Connected via Ethernet cables or Wi-Fi
- High bandwidth (typically 100 Mbps to 10 Gbps)
- Low latency
- Infrastructure owned and maintained by the organisation
- No ongoing connection costs beyond initial setup and maintenance
WANs connect geographically dispersed locations. The Internet is the largest WAN. Key characteristics include:
- Connected via fibre optic cables, telephone lines, or satellite links
- Infrastructure leased from telecommunications providers
- Lower bandwidth than LANs (varies significantly by connection type)
- Higher latency due to greater distances
- Ongoing rental costs for connectivity
Personal Area Networks (PANs) connect devices within an individual's workspace, typically using Bluetooth technology, covering ranges of approximately 10 metres.
Network topologies
A network topology describes how devices are arranged and connected within a network. Each topology has distinct advantages and disadvantages.
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 data doesn't pass through unnecessary devices
- Disadvantages: if the central switch fails, the entire network fails; requires more cabling than bus topology; can be expensive due to switch costs
Bus topology:
- All devices connect to a single backbone cable
- Data is broadcast to all devices; only the intended recipient processes it
- Terminators at each end prevent signal reflection
- Advantages: cheap to install; uses less cabling than star
- Disadvantages: if the backbone cable fails, the entire network fails; performance degrades with more devices; difficult to isolate faults
Mesh topology:
- Devices have multiple connections to 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; if one connection fails, data can route via alternative paths; no single point of failure
- Disadvantages: expensive to install and maintain; complex to configure; requires significant cabling (in wired implementations)
Network hardware
Network Interface Card (NIC):
- Hardware component that enables a device to connect to a network
- Each NIC has a unique MAC address
- Can be wired (Ethernet) or wireless (Wi-Fi)
- Converts data into electrical signals for wired networks or radio waves for wireless
Switch:
- Connects multiple devices within a LAN
- Receives data packets and forwards them only to the intended recipient
- Uses MAC addresses to identify destination devices
- More efficient than hubs, which broadcast to all devices
Router:
- Connects different networks together
- Routes data packets between networks using IP addresses
- Connects LANs to WANs (e.g., home network to the Internet)
- Provides Network Address Translation (NAT) to allow multiple devices to share a single public IP address
Wireless Access Point (WAP):
- Allows wireless devices to connect to a wired network
- Broadcasts a wireless signal that devices can detect and connect to
- Often integrated into routers in home environments
Transmission media:
- Ethernet cables (copper): affordable, suitable for LANs, maximum effective distance approximately 100m, susceptible to electrical interference
- Fibre optic cables: very high bandwidth, used for backbone connections and WANs, transmit data as light pulses, not affected by electrical interference, expensive
- Wireless (Wi-Fi): convenient, allows mobility, limited range, performance affected by physical obstacles and interference
Protocols and layers
The TCP/IP protocol stack organizes network communication into four layers, each with specific functions:
Application layer:
- Provides network services directly to applications
- Protocols include HTTP (web browsing), HTTPS (secure web browsing), FTP (file transfer), SMTP (email sending), POP3/IMAP (email retrieval)
- Determines which protocol is appropriate for the data being transmitted
Transport layer:
- Manages end-to-end communication between devices
- TCP (Transmission Control Protocol): ensures reliable delivery by establishing connections, ordering packets, checking for errors, and requesting retransmission of lost packets
- UDP (User Datagram Protocol): faster but unreliable; sends packets without guaranteeing delivery; used for streaming and gaming where speed matters more than perfect accuracy
- Splits data into packets and adds port numbers to identify which application should receive the data
Internet layer:
- Routes packets across networks using IP addresses
- IP (Internet Protocol): adds source and destination IP addresses to packets
- Determines the best path for data to travel across interconnected networks
- Handles packet forwarding between routers
Network Access layer:
- Deals with the physical transmission of data
- Adds MAC addresses to identify specific hardware devices
- Manages how data is transmitted over the physical medium (cables or wireless)
Data transmission concepts
Packet switching is the method used to transmit data across modern networks:
- Data is divided into packets of fixed or variable size
- Each packet contains the destination address, source address, payload data, packet sequence number, and error-checking data
- Packets may take different routes across the network
- Packets are reassembled in the correct order at the destination
- Missing or corrupted packets are identified and retransmitted
Advantages of packet switching:
- Efficient use of network capacity as routes are shared
- Resilient; if one route fails, packets can take alternative paths
- Packets from multiple communications can be interleaved on the same connection
Circuit switching (used in traditional telephone networks) establishes a dedicated connection for the entire communication duration. While this guarantees bandwidth, it is inefficient as the connection cannot be used by others even during silence.
Error checking ensures data integrity:
- Checksums: a value calculated from the data and sent with the packet; the receiver recalculates it and compares with the transmitted value
- Parity bits: an additional bit added to ensure the number of 1s is even (even parity) or odd (odd parity); simple but can only detect single-bit errors
Network security
Encryption protects data during transmission:
- Plaintext is converted to ciphertext using an encryption algorithm and key
- Only recipients with the correct decryption key can convert ciphertext back to plaintext
- HTTPS encrypts web traffic to protect sensitive information like passwords and payment details
- Prevents unauthorized access if packets are intercepted
Authentication verifies user identity:
- Usernames and passwords are the most common method
- Two-factor authentication requires a second verification method (e.g., code sent to mobile phone)
- Prevents unauthorized network access
Firewalls monitor and control network traffic:
- Examine incoming and outgoing packets
- Block traffic based on predefined security rules
- Can block specific IP addresses, ports, or protocols
- Protect networks from unauthorized access and malicious traffic
MAC address filtering:
- Only allows devices with approved MAC addresses to connect
- Provides basic security for wireless networks
- Can be bypassed as MAC addresses can be spoofed
Worked examples
Example 1: A school uses a star topology for its network.
(a) State two advantages of using a star topology. [2 marks]
Answer:
- If one cable fails, only one device is affected / the rest of the network continues to function [1]
- Easy to add new devices / better performance as data doesn't pass through unnecessary devices [1]
(Mark scheme notes: Accept any two valid advantages. Do not accept vague statements like "more reliable" without explanation.)
Example 2: Describe how packet switching is used to send an email across the Internet. [6 marks]
Answer:
- The email is divided / split into multiple packets [1]
- Each packet contains the destination address and source address [1]
- Each packet also contains a sequence number / packet number [1]
- Packets may take different routes / paths across the network [1]
- Packets are reassembled in the correct order at the destination [1]
- Missing or corrupted packets are identified using checksums / error-checking data and retransmitted [1]
(Mark scheme notes: Award up to 6 marks. Look for clear explanation of the process with accurate technical terminology.)
Example 3: A company connects its offices in London and Manchester using a WAN.
(a) Explain why a WAN is required rather than a LAN. [2 marks]
Answer:
- The offices are in different geographical locations / far apart [1]
- A LAN only covers a small geographical area / single site, whereas a WAN covers a large geographical area [1]
(b) State one disadvantage of using a WAN compared to a LAN. [1 mark]
Answer:
- Lower bandwidth / slower data transmission speeds [1]
- OR: Higher cost due to leasing infrastructure from telecommunications providers [1]
- OR: Higher latency / longer delays [1]
(Mark scheme notes: Accept any valid disadvantage with correct technical explanation.)
Common mistakes and how to avoid them
Confusing LANs and WANs: Remember that geographical area and infrastructure ownership are the key differences. A LAN covers a small area with owned infrastructure; a WAN covers large areas using leased infrastructure.
Mixing up MAC and IP addresses: MAC addresses are permanent hardware identifiers burned into NICs. IP addresses are logical addresses assigned by networks and can change. Switches use MAC addresses; routers use IP addresses.
Stating topology advantages too vaguely: Instead of writing "star topology is reliable," specify "if one cable fails in a star topology, only that device is affected and other devices can still communicate."
Not explaining packet switching fully: Exam questions often ask you to "describe" or "explain" packet switching. Ensure you mention that data is split into packets, packets contain addresses and sequence numbers, packets may take different routes, and they are reassembled at the destination.
Confusing protocols and their purposes: HTTP is for web pages, HTTPS for secure web browsing, FTP for file transfer, SMTP for sending email, POP3/IMAP for receiving email. Learn which protocol does what.
Forgetting the layers in the TCP/IP stack: The four layers in order are Application, Transport, Internet, and Network Access. Questions may ask you to state which layer performs a specific function.
Exam technique for "Networks and Communication"
Command words matter: "State" requires a brief answer (usually one mark per point). "Describe" requires more detail about how something works. "Explain" requires reasons or justifications, often linked with "because" or "therefore."
Two-mark questions typically require two distinct points: If asked for two advantages, ensure your points are genuinely different. "Faster" and "high bandwidth" might be considered the same point.
Use technical terminology accurately: Examiners look for precise vocabulary. Use terms like "packet," "protocol," "bandwidth," "latency," and "topology" correctly to demonstrate understanding.
Check mark allocations: A 6-mark question on packet switching requires six distinct points. Plan your answer to ensure you cover enough detail without repetition.
Quick revision summary
Networks are classified as LANs (small area, owned infrastructure) or WANs (large area, leased infrastructure). Common topologies include star (devices connect to central switch), bus (single backbone cable), and mesh (multiple connections). Essential hardware includes NICs, switches, routers, and WAPs. The TCP/IP stack has four layers: Application, Transport, Internet, and Network Access. Data is transmitted via packet switching, where information is divided into packets containing addresses and sequence numbers. Security measures include encryption, authentication, and firewalls. Understanding these concepts and using precise terminology is essential for exam success.