Welcome to Networks!

Hello and welcome to your study notes on Networks for CCEA A2 Digital Technology (Unit A2 1: Information Systems). Whether you are already tech-savvy or feel a bit overwhelmed by acronyms like TCP/IP, DNS, and OSI, do not worry! We will break down every concept step-by-step using clear language, everyday analogies, and helpful memory tricks to ensure you are fully prepared for your exam.

Why study networks? In modern information systems, standalone computers are rare. Businesses, hospitals, and schools rely on interconnected computers to share files, process database transactions, communicate globally, and maintain secure services. Understanding how data travels from device A to device B is central to mastering digital technology.

1. Network Types and Architectures

A computer network is simply two or more computer systems linked together to communicate, share data, and share resources (like printers or internet connections).

Geographical Classifications

Networks are often categorised by their physical size and range:

• LAN (Local Area Network): Covers a small geographical area, such as a single room, school building, or office. The hardware and cabling are usually owned entirely by the organisation using it.
Analogy: Think of a LAN like the internal intercom system inside your own house.

• WAN (Wide Area Network): Connects computers over a large geographical distance, such as across cities, countries, or continents. WANs use telecommunication links and external service providers (e.g., telephone lines, undersea cables, satellite links). The ultimate example of a WAN is the Internet.
Analogy: Think of a WAN like the international postal service connecting different countries.

• WLAN (Wireless Local Area Network): A LAN that uses high-frequency radio signals (such as Wi-Fi) rather than physical cables to connect devices within a limited area.

• PAN (Personal Area Network): A very short-range network centred around an individual person (typically within \(10\) metres), often connecting devices like smartphones, wireless headphones, and smartwatches via Bluetooth.

• SAN (Storage Area Network): A dedicated, high-speed network that provides block-level network access to storage devices, allowing servers to access disks as if they were locally attached.

• VPN (Virtual Private Network): An encrypted tunnel created over a public network (like the Internet) that allows remote users to securely access a private organisation's network as if they were directly plugged into the office LAN.

Network Models: Client-Server vs Peer-to-Peer (P2P)

Networks can also be organised according to how control and data storage are managed:

1. Client-Server Model
In this setup, central computers called servers provide services, manage resources, and store data. Other computers, called clients (the workstations used by users), request services from the server.

• Advantages: Centralised security, centralised backups (no need to back up every PC individually), centralised management of user access and software updates, and high performance.
• Disadvantages: Expensive to set up and maintain (requires specialist hardware and a network manager); if the central server fails, dependent services become unavailable (single point of failure).

2. Peer-to-Peer (P2P) Model
In a P2P network, there is no central server. Every computer (called a peer) has equal status and responsibility. Each computer can act as both a client and a server, sharing its own files and resources directly with other peers.

• Advantages: Very cheap and easy to set up; does not require expensive dedicated servers or a dedicated network administrator; if one peer fails, the rest of the network continues to operate.
• Disadvantages: Security is decentralised (passwords and permissions must be set on each machine); backups must be managed on individual computers; network performance can slow down when other users access files stored on your machine.

Key Takeaway for Section 1:
LANs cover small areas (owned infrastructure); WANs cover large areas (third-party infrastructure). Client-Server offers centralised control and security but is costly, while Peer-to-Peer is cheaper and decentralised but harder to secure and manage.

2. Network Topologies

A network topology describes the physical layout or logical arrangement of cables, computers, and peripheral devices on a network.

Star Topology

In a star topology, every host or workstation is connected directly to a central node, typically a switch or hub.

• How it works: When Device A wants to send data to Device B, it sends the data to the central switch, which directs it to Device B.
• Advantages: Highly reliable; if a single cable or computer breaks, no other device is affected. Easy to add new devices without disrupting the network. Data collisions are minimised when using a switch.
• Disadvantages: Requires a lot of cabling (which increases cost); if the central switch fails, the entire network goes down.

Bus Topology

All devices are connected to a single central cable called the bus or backbone. At each end of the backbone, a component called a terminator absorbs electrical signals to prevent them from bouncing back and corrupting data (known as signal reflection).

• Advantages: Inexpensive to install because it uses minimal cabling; easy to set up for temporary or very small networks.
• Disadvantages: Heavy traffic causes data collisions and slows down the network; if the main backbone cable breaks, the entire network fails; difficult to isolate and troubleshoot faults.

Ring Topology

Computers are connected in a continuous circular loop. Data packets travel in one direction around the ring from device to device until they reach their destination.

• Advantages: Orderly data transmission; every device gets an equal opportunity to transmit, avoiding collisions.
• Disadvantages: A break in the ring cable or the failure of a single computer can bring down the entire network; adding or removing devices requires briefly shutting down the network.

Mesh Topology

In a full mesh topology, every device has a dedicated point-to-point connection to every other device. In a partial mesh, only some devices are interconnected with multiple redundant links.

• Advantages: Extremely resilient and robust; if one link fails, traffic is automatically rerouted along an alternative path; excellent data throughput and security.
• Disadvantages: Very expensive and complex to install due to the massive amount of cabling and large number of network ports required.

Quick Review:
• Star = Central switch (most common in schools and offices).
• Bus = One backbone cable with terminators.
• Ring = Circular loop in one direction.
• Mesh = Multiple redundant links (used in the Internet backbone and mission-critical systems).

3. Transmission Media and Data Transmission

Wired vs Wireless Transmission Media

To transfer binary bits (\(0\)s and \(1\)s) between devices, physical or wireless channels are used:

1. Twisted Pair Copper Cable (e.g., Cat 5e / Cat 6 Ethernet)
Pairs of insulated copper wires twisted together to cancel out electromagnetic interference (EMI) and crosstalk.
• Pros: Inexpensive, flexible, easy to install.
• Cons: Vulnerable to electrical interference; signals weaken over long distances (known as attenuation), generally limited to runs of \(100\text{ m}\).

2. Coaxial Cable
A single central copper core surrounded by insulation and a braided metallic shield.
• Pros: Better shielding against interference than unshielded twisted pair.
• Cons: Bulkier, less flexible, and largely superseded by twisted pair and fibre optics.

3. Fibre Optic Cable
Transmits data as pulses of light through flexible strands of pure glass or plastic using total internal reflection.
• Pros: Huge bandwidth (very high data transmission rates), signals travel tens of kilometres without significant attenuation, completely immune to electromagnetic interference, and extremely hard to tap (secure).
• Cons: Expensive to manufacture, requires specialist tools to splice and install.

4. Wireless Media (Wi-Fi, Microwaves, Satellites)
Transmits data using electromagnetic radio waves through the air.
• Pros: High mobility (users can move around freely), no physical cabling needed to connect new devices.
• Cons: Susceptible to physical obstructions (thick walls) and radio interference; lower security (signals can be intercepted, requiring strong encryption like WPA3); shared bandwidth means speeds drop as more users connect.

Modes of Transmission

• Simplex: Data travels in one direction only. Example: A television broadcast or a computer sending data to a standard monitor.
• Half-Duplex: Data travels in both directions, but only one direction at a time. Example: A walkie-talkie (you press to speak, then release to listen).
• Full-Duplex: Data travels in both directions simultaneously. Example: A standard telephone call or modern full-duplex Ethernet switch connections.

Packet Switching vs Circuit Switching

Packet Switching (Used by the Internet and LANs):
Large files are split into small chunks called packets. Each packet contains the payload (data), a sequence number, the sender's IP address, and the destination IP address. Packets travel independently across the network along the fastest available path determined by routers. Because different packets may take different routes, they may arrive out of order; the receiving computer uses the sequence numbers to assemble them correctly. If a packet is lost or corrupted, only that individual packet needs to be retransmitted.

Circuit Switching (Used by traditional telephone lines):
A dedicated physical or virtual circuit (channel) is established between the sender and receiver for the entire duration of the communication session. All data travels down this exact same fixed path in order. The path remains open and reserved even if no data is currently being spoken or sent, which can be inefficient.

Key Takeaway for Section 3:
Fibre optic offers the highest speed and range without interference. Packet switching breaks files into independently routed packets, maximising network efficiency compared to dedicated circuit switching.

4. Network Hardware and Interconnecting Devices

To connect computers and route data properly, specific hardware components are required:

• NIC (Network Interface Card): An internal hardware component (built-in or expansion card) that connects a computer to a network. It converts computer data into electrical, optical, or radio signals. Every NIC has a unique, hardcoded physical identifier known as a MAC (Media Access Control) address.

• Hub: A simple, basic hardware device that connects multiple computers. When it receives a data packet on one port, it blindly broadcasts it to all other connected ports, creating unnecessary network traffic and security risks.

• Switch: An intelligent device used in LANs. It inspects the destination MAC address of incoming data frames and directs the data only to the specific port connected to the intended recipient. This reduces collisions and improves network speed and security.

• Router: A device that connects two or more different networks together (e.g., connecting a home LAN to the WAN of the Internet). Routers inspect IP addresses and use internal routing tables to choose the best and fastest path for packets to reach their destination.

• Wireless Access Point (WAP): A device containing a radio transmitter/receiver that connects to a wired network, allowing wireless devices (laptops, phones) to join the LAN.

• Bridge: Connects two separate LAN segments that use the same communication protocols, filtering traffic so local traffic stays within its own segment.

• Gateway: Connects two completely different networks that use different protocols by translating data formats between them.

Common Mistake to Avoid:
Do not confuse a Switch and a Router on your exam! A switch connects devices within the same local network using MAC addresses. A router connects different networks together using IP addresses.

5. Protocols and Communication Models

A protocol is an agreed set of rules and standards that determines how data is formatted, transmitted, and received across a network so that different hardware and software can communicate successfully.

The OSI 7-Layer Model

The Open Systems Interconnection (OSI) model divides network communication into seven logical layers. Dividing network tasks into layers allows developers to update or change one layer without breaking the others.

Mnemonic to remember the layers (from Top Layer 7 to Bottom Layer 1):
"All People Seem To Need Data Processing"

• Layer 7: Application Layer – The layer closest to the end user. Provides networking services directly to software applications (e.g., web browsers, email clients).
• Layer 6: Presentation Layer – Translates, formats, encrypts, and compresses data so the receiving system can interpret it (e.g., SSL/TLS encryption, converting ASCII to Unicode).
• Layer 5: Session Layer – Establishes, manages, maintains, and terminates communication sessions between applications.
• Layer 4: Transport Layer – Responsible for end-to-end communication, segmenting data, controlling flow rate, and ensuring error checking/recovery (e.g., TCP and UDP).
• Layer 3: Network Layer – Handles logical addressing (IP addresses) and routes packets across different networks.
• Layer 2: Data Link Layer – Organises data into frames, handles physical addressing (MAC addresses), and manages error detection on the local physical link.
• Layer 1: Physical Layer – Transmits raw binary bits (\(0\)s and \(1\)s) over physical media as electrical voltages, light pulses, or radio waves.

Common Internet Protocols

You must be familiar with the key protocols operating within modern TCP/IP networks:

• HTTP (HyperText Transfer Protocol): Used by web browsers to request and display unencrypted web pages from a web server (typically port \(80\)).
• HTTPS (HTTP Secure): Encrypts web traffic using SSL/TLS to protect sensitive transactions (like online banking and logins) from eavesdropping (typically port \(443\)).
• FTP (File Transfer Protocol): Used for uploading and downloading files between a client and a remote server.
• SMTP (Simple Mail Transfer Protocol): Used to send email messages from a client to an email server, or between email servers.
• POP3 (Post Office Protocol 3): Downloads emails from a mail server to a local device and deletes them from the server.
• IMAP (Internet Message Access Protocol): Allows users to view and manage emails directly on the server, keeping mail synchronized across multiple devices.
• TCP (Transmission Control Protocol): A connection-oriented protocol at the transport layer that ensures reliable, error-checked delivery of packets.
• UDP (User Datagram Protocol): A lightweight, connectionless protocol that sends packets quickly without checking for errors or retransmitting lost packets (ideal for live video streaming and gaming).
• IP (Internet Protocol): Responsible for addressing packets and directing them to their destination.
• DNS (Domain Name System): The "phonebook of the Internet" – translates human-friendly domain names (e.g., www.ccea.org.uk) into numerical IP addresses (e.g., \(192.0.2.1\)).
• DHCP (Dynamic Host Configuration Protocol): Automatically assigns IP addresses, subnet masks, and default gateways to devices joining a network.

IP Addressing: IPv4 vs IPv6

• IPv4: Uses \(32\text{ bits}\) displayed as four decimal numbers separated by dots (e.g., \(192.168.1.1\)). Provides approximately \(2^{32} \approx 4.3 \text{ billion}\) unique addresses. Due to the explosion of internet-connected devices, IPv4 addresses have run out.
• IPv6: Introduced to solve IPv4 address exhaustion. Uses \(128\text{ bits}\) written as eight groups of hexadecimal digits separated by colons (e.g., \(2001:\text{0db8}:85a3::8a2e:0370:7334\)), providing \(2^{128}\) unique addresses – virtually unlimited.

Key Takeaway for Section 5:
Protocols are universal rules allowing diverse systems to interact. The OSI model standardises this into 7 distinct layers, from the physical wiring (Layer 1) up to user applications (Layer 7).

6. Network Security Measures

Connecting systems creates security vulnerabilities. Robust information systems employ layered network security:

• Firewall: A hardware or software barrier that monitors incoming and outgoing network traffic based on predefined security rules, blocking unauthorised access while permitting safe communication.
• Encryption: Scrambles plain text data into unreadable ciphertext using an algorithm and encryption key. Only authorised parties with the matching decryption key can decode the data.
• MAC Address Filtering: A switch or wireless router can be configured with an allowlist of specific MAC addresses, preventing unapproved devices from connecting to the network even if they know the Wi-Fi password.
• User Authentication & Access Levels: Requiring strong passwords, multi-factor authentication (MFA), and setting role-based permissions (read, write, execute) to ensure users only access the data necessary for their role.

Chapter Summary & Revision Checklist

Before sitting your exam on Networks, ensure you can confidently:

• Compare LAN, WAN, WLAN, PAN, and VPN with real-world examples.
• Contrast Client-Server and Peer-to-Peer models with their distinct advantages and disadvantages.
• Draw and describe Star, Bus, Ring, and Mesh topologies, noting single points of failure.
• Explain the characteristics of Twisted Pair, Fibre Optic, and Wireless transmission media.
• Differentiate Simplex, Half-Duplex, and Full-Duplex data transmission.
• Describe how Packet Switching works from source to destination.
• Identify the roles of Network Hardware (NIC, Switch, Router, Hub, WAP, Bridge, Gateway).
• List all 7 layers of the OSI model in order and describe their key roles.
• State the purpose of core protocols: HTTP, HTTPS, FTP, SMTP, POP3, IMAP, TCP, IP, UDP, DNS, and DHCP.
• Compare IPv4 (\(32\text{-bit}\)) and IPv6 (\(128\text{-bit}\)) addressing.