Computer Systems: Network Systems

What you'll learn

This revision guide covers all testable content on network systems from the Pearson Edexcel International IGCSE Computer Science specification. You will understand network types, topologies, transmission methods, hardware components and network protocols. These concepts underpin how devices communicate in homes, schools and businesses worldwide.

Key terms and definitions

Local Area Network (LAN) — a network that connects devices within a limited geographical area such as a school, office building or home

Wide Area Network (WAN) — a network that connects devices across large geographical areas, often using third-party telecommunication links

Protocol — a set of rules that governs the transmission of data between devices on a network

IP address — a unique numerical identifier assigned to each device on a network, allowing it to be located and communicated with

Packet switching — a method of data transmission where data is broken into packets that travel independently across a network

MAC address — a unique hardware identifier permanently assigned to a network interface card by the manufacturer

Bandwidth — the maximum amount of data that can be transmitted over a network connection in a given time period

Latency — the time delay between sending a data request and receiving a response

Core concepts

Network types and their characteristics

Networks are classified by their geographical coverage and ownership structure.

LANs are owned, controlled and maintained by a single organisation. A school computer network connecting all classrooms and administrative offices is a typical LAN. LANs typically use Ethernet cables or Wi-Fi, offering high bandwidth and low latency. All infrastructure (cables, switches, routers) belongs to the organisation.

WANs connect geographically dispersed locations. The Internet is the largest WAN. A UK-based company with offices in London, Birmingham and Manchester would use a WAN to connect these sites. WANs often use leased telecommunication lines from third-party providers, resulting in higher costs and typically lower bandwidth than LANs. A bank connecting branches across the Caribbean would implement a WAN infrastructure.

Virtual Private Networks (VPNs) create secure connections over public networks. Employees working from home use VPNs to securely access their company's LAN via the Internet. The VPN encrypts data, protecting it from interception during transmission.

Network topologies

A network topology describes how devices are physically or logically arranged and connected.

Bus topology connects all devices to a single backbone cable. Data travels in both directions along the cable. Each end requires a terminator to prevent signal reflection. If the backbone cable fails, the entire network fails. Bus topologies are rarely used in modern networks due to poor performance with many devices and difficulty troubleshooting faults.

Star topology connects all devices to a central switch or hub. Each device has a dedicated cable to the central point. If one cable fails, only that device loses connectivity; other devices continue functioning. The central switch represents a single point of failure—if it fails, the entire network fails. Star topologies are the most common in modern LANs because they are easy to troubleshoot and expand.

Mesh topology connects devices with multiple redundant paths between them. Full mesh connects every device to every other device; partial mesh connects some devices with multiple paths. Mesh topologies offer high reliability—if one connection fails, data reroutes through alternative paths. The Internet uses a partial mesh topology. The cost and complexity increase significantly with the number of devices due to multiple connections required.

Network hardware components

Network Interface Card (NIC) — a hardware component that enables a device to connect to a network. Each NIC has a unique MAC address. Modern computers have NICs built into the motherboard; older systems used expansion cards.

Switch — a device that connects multiple devices on a LAN and uses MAC addresses to forward data only to the intended recipient device. Switches operate at Layer 2 of the OSI model and create dedicated communication paths between devices, improving network performance compared to hubs.

Router — a device that forwards data between different networks, typically connecting a LAN to a WAN (like the Internet). Routers use IP addresses to determine the best path for data packets. A home router connects devices on your home network to your Internet Service Provider's network.

Wireless Access Point (WAP) — a device that allows wireless devices to connect to a wired network using Wi-Fi. WAPs are often integrated into routers in home and small office environments.

Hub — an older networking device that broadcasts data to all connected devices rather than just the intended recipient. Hubs create unnecessary network traffic and are largely obsolete, replaced by switches.

Data transmission methods

Packet switching breaks data into small units called packets. Each packet contains the destination address, source address, packet number, error-checking data and a portion of the actual data. Packets may travel different routes across the network and arrive out of order. The receiving device reassembles packets in the correct sequence.

Benefits of packet switching:

• Efficient use of network capacity—multiple communications share the same infrastructure
• Resilient—if one route fails, packets automatically reroute
• Different packets from the same message can take different paths

Circuit switching creates a dedicated communication path between two devices for the duration of the communication. Traditional telephone networks used circuit switching. The entire bandwidth is reserved for that connection, even during silence or inactive periods, making it less efficient than packet switching for data networks.

Network protocols

Protocols ensure different devices and networks can communicate effectively by following agreed standards.

TCP/IP (Transmission Control Protocol/Internet Protocol) is the fundamental protocol suite for Internet communication. IP handles addressing and routing packets to their destination. TCP ensures reliable, ordered delivery of packets. TCP checks for errors and requests retransmission of corrupted or missing packets.

HTTP (Hypertext Transfer Protocol) governs communication between web browsers and web servers. When you type a URL, your browser sends an HTTP request to the server, which responds with the requested web page. HTTPS (HTTP Secure) encrypts the communication, protecting sensitive data like passwords and credit card numbers.

FTP (File Transfer Protocol) enables file uploads and downloads between devices. Web developers use FTP to upload website files to web servers.

SMTP (Simple Mail Transfer Protocol) handles sending email messages between mail servers. When you send an email, SMTP transfers it from your mail server to the recipient's mail server.

IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol version 3) handle retrieving emails from a mail server. IMAP synchronises emails across multiple devices, keeping messages on the server. POP3 typically downloads messages to one device and removes them from the server.

IP addressing and DNS

Every device on a network requires a unique identifier. IP addresses serve this function.

IPv4 addresses consist of four numbers (0-255) separated by dots, for example 192.168.1.1. Each number is an octet (8 bits), making IPv4 addresses 32 bits total. IPv4 provides approximately 4.3 billion unique addresses, which is insufficient for the growing number of Internet-connected devices.

IPv6 addresses use 128 bits, represented as eight groups of hexadecimal numbers, for example 2001:0db8:85a3:0000:0000:8a2e:0370:7334. IPv6 provides vastly more unique addresses (approximately 340 undecillion).

Static IP addresses are manually assigned and remain constant. Servers typically use static IPs so they can be reliably located. Dynamic IP addresses are automatically assigned by a DHCP (Dynamic Host Configuration Protocol) server and may change when a device reconnects to the network.

Domain Name System (DNS) translates human-readable domain names (like www.kramizo.com) into IP addresses. When you enter a URL, your device queries a DNS server, which returns the corresponding IP address. This allows users to access websites without memorising numerical IP addresses.

Network security considerations

Networks face numerous security threats requiring protective measures.

Firewalls monitor incoming and outgoing network traffic and block suspicious or unauthorised communications based on predetermined security rules. Hardware firewalls protect entire networks; software firewalls protect individual devices.

Encryption scrambles data so unauthorised parties cannot read it during transmission. WPA2 and WPA3 encrypt Wi-Fi communications. HTTPS encrypts web traffic.

MAC address filtering allows only devices with approved MAC addresses to connect to a network. This provides basic security but can be bypassed as MAC addresses can be spoofed.

Authentication verifies user identity before granting network access. Methods include passwords, biometrics and two-factor authentication.

Worked examples

Example 1: Network topology comparison (6 marks)

Question: A company is designing a network for a new office building with 50 employees. Compare star and bus topologies for this scenario. Recommend which topology the company should use, justifying your answer.

Mark scheme answer:

Star topology advantages:

• If one cable fails, only one device is affected; other devices continue working (1 mark)
• Easy to add new devices—connect them to the central switch (1 mark)
• Easy to identify and isolate faults to specific cables or devices (1 mark)

Bus topology disadvantages:

• If the backbone cable fails, the entire network fails (1 mark)
• Performance degrades significantly with 50 devices on one cable (1 mark)

Recommendation: The company should use star topology because reliability is essential for business operations and 50 devices would severely impact bus topology performance (1 mark)

Example 2: Packet switching (4 marks)

Question: Explain how packet switching transmits data across a network.

Mark scheme answer:

• Data is divided into small units called packets (1 mark)
• Each packet contains destination address, source address, packet number and data portion (1 mark)
• Packets may travel different routes across the network (1 mark)
• The receiving device reassembles packets in the correct order using packet numbers (1 mark)

Example 3: Protocols (5 marks)

Question: A student uses a laptop to access a school's website and send an email to a teacher.

(a) Identify the protocol used when accessing the website. (1 mark) (b) Identify the protocol used when sending the email. (1 mark) (c) Explain the purpose of DNS in this scenario. (3 marks)

Mark scheme answer:

(a) HTTP or HTTPS (1 mark)

(b) SMTP (1 mark)

(c)

• DNS translates domain names into IP addresses (1 mark)
• The student enters the website address as a domain name (1 mark)
• DNS server returns the IP address of the web server, allowing the browser to connect to it (1 mark)

Common mistakes and how to avoid them

• Confusing LAN and WAN ownership: Remember LANs are owned by one organisation; WANs typically use third-party infrastructure. Don't say "a WAN is just a bigger LAN"—ownership and infrastructure differ fundamentally.

• Mixing up topology failure modes: Star topology—one cable fails, one device affected; central switch fails, all devices affected. Bus topology—backbone cable fails, all devices affected. Learn the specific failure points for each topology.

• Muddling MAC and IP addresses: MAC addresses are hardware identifiers assigned by manufacturers and permanent; IP addresses are software-assigned and can change. Use the correct address type when explaining switches (MAC) versus routers (IP).

• Vague protocol explanations: Don't just list protocols—explain their specific purposes. "HTTP is for websites" is insufficient; "HTTP governs communication between web browsers and web servers" is precise.

• Confusing packet switching terminology: Packets contain multiple pieces of information; don't forget to mention addressing, packet numbers and error-checking data, not just "data is divided."

• Overlooking command words: "Describe" requires characteristics or features; "Explain" requires reasons or how something works; "Compare" requires similarities AND differences. Match your answer structure to the command word.

Exam technique for "Computer Systems: Network Systems"

• Command word awareness: "State" or "Identify" typically award 1 mark for a correct term with no explanation needed. "Describe" requires characteristics (2-3 marks). "Explain" requires reasoning with connectives like "because," "therefore," "this allows" (2-4 marks). "Compare" requires both similarities and differences, not advantages/disadvantages.

• Use precise technical terminology: Write "switch" not "network device," "packet" not "piece of data," "MAC address" not "hardware address." Examiners award marks for accurate terminology that demonstrates understanding.

• Context-specific answers: If a question provides a scenario (school network, business, home), relate your answer to that context. Generic answers often miss available marks for application.

• Mark allocation guides detail: A 4-mark question requires four distinct, creditworthy points. If you've only written one sentence for 4 marks, you haven't provided sufficient detail. Aim for one developed point per mark.

Quick revision summary

Networks connect devices for communication and resource sharing. LANs cover limited areas with single-organisation ownership; WANs span large distances using third-party infrastructure. Star topology is most common in modern LANs due to reliability and easy troubleshooting. Switches use MAC addresses to forward data on LANs; routers use IP addresses to forward data between networks. Packet switching divides data into packets that travel independently and reassemble at the destination. Protocols like TCP/IP, HTTP and DNS enable standardised communication across different networks and devices.