Exam Board:
OCR A-Level
3.3 - Networks
Specification:
Computer Science H446
Watch on YouTube:
Purpose of networks (LAN & WAN)
Protocols & standards
Protocol layering & TCP/IP stack
Domain Name System (DNS)
Packet & circuit switching
Network security threats
Network security protection
Network hardware
Client-server & peer-to-peer
This topic looks at how data is transferred between computer systems on networks, including the required devices, protocols, network types and potential security threats.
Purpose of Networks
A network is a group of connected computers that can share data, resources and communicate with each other.
The main purpose of a network is to allow users to share files, hardware (like printers), internet connections and other services efficiently.
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A Local Area Network (LAN) covers a small geographical area, such as a single building or school, and is usually owned and managed by one organisation.
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A Wide Area Network (WAN) covers a large geographical area, connecting multiple LANs through public or leased communication lines such as the internet.
Protocols & Standards
Protocols are sets of rules that define how data is transmitted and received over a network, ensuring that devices can communicate reliably.
Standards are agreed specifications that ensure different hardware and software systems are compatible and can work together. They are needed so that networks remain interoperable, secure and efficient, regardless of the devices or manufacturers involved.
Common network protocols include:
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HTTP/HTTPS is used for transferring web pages over the internet.
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FTP aids the transfer of files across a network.
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SMTP is used to send emails and IMAP/POP receive emails.
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TCP/IP is the core suite of protocols that controls how data is packaged, addressed, transmitted and received across networks.
Protocol Layering & TCP/IP Stack
Protocol layering is used to divide complex networking tasks into manageable sections, making systems easier to design, understand and troubleshoot. It also allows different technologies or protocols to work together, as each layer only interacts with the ones directly above and below it.
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The four layers are:
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Application layer: Provides network services to end users, such as web browsing (HTTP) or email (SMTP).
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Transport layer: Manages data transmission between devices, ensuring it arrives reliably and in the correct order (e.g. TCP, UDP).
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Internet layer: Handles addressing and routing of data packets between networks using IP (Internet Protocol).
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Link layer: Manages the physical connection between devices and controls how data is transmitted over the network hardware.
Domain Name System (DNS)
The Domain Name System (DNS) translates human-readable domain names (like www.csnewbs.com) into IP addresses that computers use to identify each other on a network.
When a user enters a web address, the request is sent to a DNS server to find the matching IP address. If the server doesn’t have it stored locally, it queries other DNS servers higher in the hierarchy until it finds the correct address.
The IP address is then returned to the user’s device, allowing it to connect to the correct web server to access the requested web page.
Packet & Circuit Switching
Packet switching and circuit switching are methods of data transmission, describing how data is sent across a network from one device to another.
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With packet switching, data is split into small packets, each sent independently across the network and reordered at the destination. This makes efficient use of network resources and allows many users to share the same connections. However, packets can arrive out of order or be delayed, causing variable performance.​
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With circuit switching, a dedicated communication path is established between two devices for the duration of a session, as in traditional phone networks. It provides a reliable and consistent connection with guaranteed bandwidth. The drawback is that it wastes resources, as the dedicated line cannot be used by others.
Network Security Threats
There is a range of potential threats associated with network use to be aware of, including the following:
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Hackers can attempt to gain unauthorised access to computer systems or networks, often to steal, alter or destroy data.
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Viruses are malicious programs that attach themselves to other files and spread, potentially damaging or deleting data.
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Denial of Service (DoS) attacks overload a network or website with traffic, making it unavailable to legitimate users.
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Spyware secretly monitors user activity and collects information such as passwords or browsing habits.
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An SQL injection involves inserting malicious SQL code into a database query to access or alter sensitive data.
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Phishing uses fraudulent emails or messages to trick users into revealing personal information.
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Pharming redirects users from legitimate websites to fake ones designed to steal login details or financial information.
Network Security Protection
Minimising or preventing network threats is vital and can be achieved with the following measures:
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Firewalls monitor and control incoming and outgoing network traffic, blocking unauthorised access while allowing safe communication.
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Secure passwords help protect user accounts by making it difficult for attackers to guess or crack them, especially when they are long and complex.
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Anti-virus software scans and removes malicious programs, such as viruses and worms, before they can damage files or systems.
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Anti-spyware software detects and removes spyware, preventing it from secretly collecting personal or sensitive information from a user’s device.
Network Hardware
A range of network hardware is required for devices to transfer data to another location, including the following:
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A modem converts digital data into analogue signals and back, allowing internet access over phone or cable lines.
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A router directs data between networks and assigns IP addresses to connected devices.
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Cables provide the physical connections between devices.
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A Network Interface Card (NIC) enables a computer to connect to a network.
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A Wireless Access Point (WAP) allows wireless devices to join a wired network via WiFi.
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On a local area network (LAN), hubs broadcast data to all devices, whereas switches send data only to the intended destination, improving network efficiency.
Client-Server & Peer-to-Peer
A client–server network has a central server that provides resources and services to client computers. It allows for centralised management, making it easier to back up data and enforce security policies. However, it relies heavily on the server - if it fails, users may lose access to resources.
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A peer-to-peer (P2P) network has no central server; instead, each computer can act as both a client and a server, sharing resources directly. It is cheap and easy to set up, making it suitable for small networks. The drawback is that it can be less secure and harder to manage, as data and security depend on individual users.
Questo's Key Terms
Purpose of networks: local area network (LAN), wide area network (WAN)
Protocols: protocol, standard, protocol layers, TCP/IP stack, application layer, transport layer, internet layer, link layer
DNS: Domain Name System
Switching: packet switching, circuit switching
Network security: hackers, viruses, unauthorised access, denial of service, spyware, SQL injection, phishing, pharming, firewalls, secure passwords, anti-virus, anti-spyware
Network hardware: modem, router, cable, NIC, Wireless Access Points, hub, switch
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Client-server & peer-to-peer
Did You Know?
The first computer worm is considered to be Creeper (in 1971), which spread across ARPANET computers and displayed the message: “I’m the creeper, catch me if you can!”. A second program called Reaper was then created to delete Creeper, making it arguably the first antivirus.
