Chapter 2: Communication - 9618 AS Level Notes

2.1 Networks Including the Internet

Purpose and Benefits of Networking Devices

Why Create Networks?

Purpose of networking:

Share resources (printers, storage, internet connections)
Enable communication between users (email, messaging, video calls)
Centralise data management and backups
Provide access to shared applications
Enable collaborative working
Improve security through centralised control
Reduce costs by sharing expensive peripherals

Benefits of networking:

Resource sharing: Multiple users can access the same hardware/software
Communication: Easy and fast communication between users
Centralised management: IT staff can manage all devices from one location
Data backup: Centralised backup of all important data
Security: Consistent security policies across all devices
Cost-effective: Fewer peripherals needed (e.g., one printer for many users)
Scalability: Easy to add new users and devices

LAN vs WAN

Local Area Network (LAN)

Characteristics:

Covers a small geographical area (single building, office, school campus)
Owned and managed by a single organisation
High-speed connections (typically 1 Gbps to 100 Gbps)
Low cost to set up and maintain
Low error rates
Uses technologies like Ethernet and Wi-Fi

Examples:

School computer lab network
Office network in a company
Home network

Wide Area Network (WAN)

Characteristics:

Covers large geographical area (cities, countries, continents)
Connections are leased from telecommunications companies
Slower speeds compared to LANs (due to distance)
Higher cost to set up and maintain
Higher error rates
Uses technologies like fibre optics, satellites, leased lines

Examples:

The Internet (largest WAN)
Bank connecting branches across countries
Multinational company connecting global offices

Feature	LAN	WAN
Geographical area	Small (building/campus)	Large (city/country/global)
Ownership	Single organisation	Multiple organisations/leased
Speed	Very high (1-100 Gbps)	Lower (Mbps to Gbps)
Cost	Relatively low	High
Error rate	Low	Higher
Technology	Ethernet, Wi-Fi	Leased lines, satellite, fibre

Network Models

Client-Server Model

How it works:

Central server provides services and resources
Clients request services from the server
Server manages security, data storage, and user access

Roles of computers:

Server: Powerful computer running server software; manages network resources, security, and data
Client: User’s computer; makes requests to server; has limited local storage/processing

Benefits:

Centralised management and security
Easy to backup all data (stored on server)
Users can access files from any client device
Scalable (add more clients easily)
Better security through central control
Easier software updates and maintenance

Drawbacks:

Server is a single point of failure (network stops if server fails)
Can be expensive to set up and maintain
Requires specialist IT staff
Network congestion if many clients request simultaneously
Server performance limits overall network performance

When to use:

Schools and universities
Large businesses and organisations
Banking systems
Email and web services
Database applications

Peer-to-Peer Model

How it works:

All computers are equal (peers)
Each computer can act as both client and server
No central server; resources shared directly between peers
Each user manages their own sharing and security

Roles of computers:

Every computer shares its own resources and accesses others’ resources
No dedicated server; any computer can provide services

Benefits:

No expensive server hardware needed
Easy and cheap to set up
No single point of failure (network continues if one computer fails)
Users control their own resources
Good for small networks (home, small office)

Drawbacks:

Poor security (users must manage their own)
No central backup; data distributed across devices
Performance depends on individual computers
Difficult to manage as network grows
Files may be duplicated across multiple computers
Users must remember where files are stored

When to use:

Small offices with few computers (under 10)
Home networks
File sharing (BitTorrent)
Collaborative applications where central server isn’t needed

Comparison Summary

Aspect	Client-Server	Peer-to-Peer
Management	Centralised	Distributed
Security	Central control	User responsibility
Cost	Higher (server hardware)	Lower
Scalability	Good	Poor
Single point of failure	Yes (server)	No
Backup	Centralised	Distributed
Performance	Server-dependent	Peer-dependent
Best for	Large organisations	Small networks

Thin Client vs Thick Client

Thin Client

Definition: A computer that relies heavily on a server for processing and storage; essentially a terminal that displays information and sends input.

Characteristics:

Minimal local processing power
Limited or no local storage
Runs applications on the server
Data stored on server
Cheaper hardware
Longer lifespan (less to upgrade)
Lower power consumption

Examples:

Network terminals in libraries
Virtual Desktop Infrastructure (VDI) workstations
Older computers repurposed as thin clients
Chromebooks (in some configurations)

Thick Client (Fat Client)

Definition: A computer that performs most processing locally and uses the network primarily for data storage and communication.

Characteristics:

Powerful local processing
Local storage for applications and data
Runs applications locally
Can work offline
More expensive hardware
Requires regular upgrades
Higher power consumption

Examples:

Typical desktop PC in an office
Gaming computers
Development workstations
Most laptops

Differences Summary

Feature	Thin Client	Thick Client
Processing	On server	Locally
Storage	On server	Local
Hardware cost	Low	High
Offline working	Not possible	Possible
Maintenance	Centralised	Per device
Network dependency	High	Low
Upgrade frequency	Low (server only)	Regular
Security	Centralised	Distributed

Network Topologies

Bus Topology

Structure:

All devices connected to a single central cable (the backbone)
Terminators at both ends to prevent signal reflection
Devices connect via taps or connectors

How packets are transmitted:

Packet sent onto the bus by source device
Signal travels in both directions along the cable
All devices receive the packet but only destination accepts it
Terminators absorb signals to prevent bouncing

Benefits:

Easy and cheap to install (less cable)
Good for small, temporary networks
Easy to extend by adding more cable
No specialised hardware needed (just cable and connectors)

Drawbacks:

If main cable fails, whole network fails
Limited cable length (signal degradation)
Performance degrades with more devices
Collisions are common (only one device can transmit at a time)
Difficult to troubleshoot
Security issues (all devices see all traffic)

When to use:

Small office/home networks (rarely used today)
Temporary network setups
Educational environments for learning

Star Topology

Structure:

All devices connect to a central switch or hub
Each device has its own dedicated cable
Central device manages all communication

How packets are transmitted:

Source sends packet to central switch
Switch examines destination address
Switch forwards packet only to the destination port
Other devices don’t see the traffic

Benefits:

Easy to install and manage
Fault tolerance (one cable failure affects only that device)
High performance (dedicated bandwidth per connection)
Easy to add/remove devices
Centralised management and monitoring
Better security (traffic only goes to destination)
Easy to troubleshoot

Drawbacks:

Requires more cable
Central switch is single point of failure
More expensive (needs switch/hub)
Limited by switch port count

When to use:

Most modern office networks
School computer labs
Home networks
Anywhere reliability and performance matter

Mesh Topology

Structure:

Devices interconnected with multiple redundant paths
Two types: full mesh and partial mesh

Full mesh: Every device connects directly to every other device
Partial mesh: Some devices connected to all, others to only a few

How packets are transmitted:

Source determines best path to destination
Packet forwarded through intermediate devices
If a path fails, alternative route used automatically
Packets may take different routes for load balancing

Benefits:

Excellent fault tolerance (multiple redundant paths)
No single point of failure
Can route around congestion or failures
Good performance (multiple paths = load balancing)
Scalable

Drawbacks:

Very expensive (many cables and ports)
Complex to set up and manage
Difficult to maintain
Requires intelligent routing protocols
Overkill for most small/medium networks

When to use:

Internet backbone
Critical infrastructure (military, emergency services)
Large data centres
Wireless mesh networks (community Wi-Fi)

Hybrid Topology

Structure:

Combination of two or more different topologies
Most large networks are hybrid
Example: Star-bus (multiple star networks connected by a bus backbone)

How packets are transmitted:

Depends on the specific combination
Usually hierarchical: packets travel up to backbone, across, then down

Benefits:

Flexible; can be designed for specific needs
Combines advantages of different topologies
Scalable and practical for large networks
Can be optimised for performance and cost

Drawbacks:

Complex design and implementation
Expensive
Difficult to troubleshoot

When to use:

Large organisations with multiple departments/buildings
University campuses
Corporate headquarters
WANs connecting multiple LANs

Topology Comparison Summary

Topology	Cost	Reliability	Performance	Scalability	Complexity
Bus	Low	Poor	Poor	Poor	Low
Star	Medium	Good	Good	Good	Medium
Mesh	Very High	Excellent	Excellent	Good	High
Hybrid	High	Good	Good	Excellent	High

Cloud Computing

What is Cloud Computing?

Definition: Delivery of computing services (servers, storage, databases, networking, software) over the internet (“the cloud”).

Types of Cloud

Public Cloud:

Services delivered over public internet
Shared infrastructure across multiple organisations
Owned and operated by third-party providers
Examples: Amazon Web Services (AWS), Microsoft Azure, Google Cloud

Private Cloud:

Dedicated to a single organisation
Can be on-premises or hosted
More control and security
Higher cost
Examples: Company’s internal private cloud, government private cloud

Hybrid Cloud:

Combination of public and private clouds
Data and applications can move between them
Best of both worlds

Cloud Service Models

Model	What you get	Example
IaaS (Infrastructure as a Service)	Virtual machines, storage, networking	AWS EC2, Google Compute Engine
PaaS (Platform as a Service)	Platform to develop/deploy applications	Google App Engine, Heroku
SaaS (Software as a Service)	Ready-to-use software	Google Workspace, Office 365, Dropbox

Benefits of Cloud Computing

Cost savings: No upfront hardware costs; pay for what you use
Scalability: Easily scale up or down based on demand
Accessibility: Access from anywhere with internet connection
Reliability: Providers offer high uptime guarantees (SLAs)
Automatic updates: Providers manage software updates
Disaster recovery: Data backed up across multiple locations
Global reach: Deploy services worldwide easily
Focus on core business: No need to manage infrastructure

Drawbacks of Cloud Computing

Internet dependency: Requires reliable internet connection
Security concerns: Data stored on third-party servers
Privacy: Provider may have access to your data
Vendor lock-in: Difficult to migrate to another provider
Limited control: Cannot control underlying infrastructure
Ongoing costs: Can be expensive long-term compared to owned hardware
Compliance issues: May violate data protection laws (data stored in other countries)
Latency: May be slower than local access

Justifying Cloud vs On-Premises

Choose Cloud when:

Variable or unpredictable workload
Need rapid scalability
Limited capital budget (prefer operational expenditure)
Want to avoid managing infrastructure
Need global deployment
Startup or small business

Choose On-Premises when:

Strict security/regulatory requirements
Predictable, stable workload
Data sovereignty concerns
Need complete control
Very large scale (may be cheaper long-term)
Poor internet connectivity

Wired vs Wireless Networks

Wired Networks

Characteristics:

Physical cables connect devices
Higher speeds and reliability
More secure (physical access needed)
Limited mobility
Installation requires cabling infrastructure

Implications:

Better performance for bandwidth-intensive applications
More suitable for fixed workstations
Higher installation cost but lower ongoing costs
Less flexible for changing layouts

Wireless Networks

Characteristics:

No physical cables; use radio waves
Mobility and flexibility
Easier to add new devices
Signals can be intercepted
Subject to interference

Implications:

Convenient for mobile devices
Lower installation cost
Potential security vulnerabilities
Performance affected by distance and obstacles
Great for guest access and temporary setups

Comparison Summary

Feature	Wired	Wireless
Speed	Faster (1 Gbps+)	Slower (typically <1 Gbps)
Reliability	Very reliable	Affected by interference
Security	More secure (physical access)	Less secure (can be intercepted)
Mobility	None	Full mobility
Installation	Difficult, requires cabling	Easy
Cost	Higher initial, lower ongoing	Lower initial, higher ongoing (maintenance)
Interference	None	Susceptible

Transmission Media

Copper Cable

Types:

Twisted Pair: Most common (Ethernet cables); pairs of wires twisted to reduce interference
Coaxial: Single copper core with shielding; used for cable TV and older networks

Characteristics:

Relatively cheap
Easy to install
Susceptible to electromagnetic interference (EMI)
Limited distance (100m for Ethernet)
Speeds up to 10 Gbps (short distances)

Applications:

LAN cabling (Cat5e, Cat6, Cat6a)
Telephone lines
Cable TV/internet

Fibre-Optic Cable

Characteristics:

Uses light pulses through glass/plastic fibres
Very high speeds (hundreds of Gbps)
Long distances (kilometres without repeaters)
Immune to electromagnetic interference
Very secure (difficult to tap)
More expensive than copper
Requires special equipment and skills to install

Applications:

Internet backbone
Long-distance telecommunications
High-speed connections between buildings
Data centres
FTTH (Fibre to the Home)

Radio Waves (including Wi-Fi)

Characteristics:

Use radio frequencies for transmission
Can penetrate walls and obstacles (reduced by distance)
Coverage area depends on power and frequency
Subject to interference from other devices
Shared medium (bandwidth shared among users)

Wi-Fi specifics:

Uses 2.4 GHz and 5 GHz frequencies (also 6 GHz with Wi-Fi 6E)
2.4 GHz: Better range, more interference
5 GHz: Higher speed, shorter range, less interference
Range typically 30-100m indoors

Microwaves

Characteristics:

Higher frequency than radio waves
Line-of-sight transmission required
Cannot penetrate buildings/obstacles well
Affected by weather (rain fade)
High bandwidth
Used for point-to-point links

Applications:

Backhaul links between cell towers
Satellite communications
Building-to-building connections

Satellites

Characteristics:

Cover vast geographical areas
High latency (signal travel time to space and back)
Expensive to launch and maintain
Weather affects signal quality
Bandwidth shared among many users

Types:

Geostationary: Fixed position; high latency; covers large area
Low Earth Orbit (LEO): Lower latency; multiple satellites needed (e.g., Starlink)

Applications:

Global communications
Television broadcasting
Internet in remote areas
GPS navigation

LAN Hardware

Switch

Connects devices within a LAN
Forwards data only to intended recipient (unlike hub)
Uses MAC addresses to make forwarding decisions
Creates dedicated communication paths
Full-duplex communication possible
Essential for modern star topology networks

Server

Powerful computer providing services to clients
Types: file server, print server, web server, database server
Usually has redundant components (power supplies, hard drives)
Runs server operating system
Central point for security and management

Network Interface Card (NIC)

Hardware that connects a device to a network
Has unique MAC address burned in
Converts data between computer and network
Can be wired (Ethernet port) or wireless
May be integrated into motherboard or add-on card

Wireless Network Interface Card (WNIC)

NIC for wireless networks
Contains antenna and radio transmitter/receiver
Connects to Wi-Fi networks
Handles encryption/decryption of wireless signals

Wireless Access Point (WAP)

Allows wireless devices to connect to wired network
Extends coverage of wireless network
Can support multiple wireless clients
May include routing capabilities
Often PoE (Power over Ethernet) powered

Cables

Connect devices in wired networks
Types: Cat5e, Cat6, Cat6a, Cat7, fibre optic
Terminated with RJ45 connectors (copper) or special connectors (fibre)
Maximum length depends on cable type (typically 100m for copper)

Bridge

Connects two separate network segments
Filters traffic based on MAC addresses
Reduces unnecessary traffic between segments
Can connect different media types (e.g., copper to fibre)
Modern switches incorporate bridging functions

Repeater

Amplifies and regenerates signals
Extends network distance beyond normal limits
Works at physical layer (doesn’t understand packets)
Can introduce latency
Modern networks use switches instead of repeaters

Router

Role and Function

Primary functions:

Connects different networks together (e.g., LAN to internet)
Forwards data packets between networks
Determines best path for data using routing tables and protocols
Performs Network Address Translation (NAT)
Provides firewall functionality
Assigns IP addresses via DHCP

How routers work:

Receives packet on one interface
Examines destination IP address
Looks up best path in routing table
Forwards packet out appropriate interface
Updates packet headers (TTL, checksums)

Routing table contains:

Destination networks
Next hop addresses
Interface to use
Metric (cost) for each route

Routing protocols:

Static routing: Manually configured routes
Dynamic routing: Automatically learn routes (RIP, OSPF, BGP)

Ethernet and CSMA/CD

Ethernet Basics

Most common LAN technology
Defines wiring and signalling standards
Uses CSMA/CD to manage access to shared medium
Originally used bus topology; now typically star with switches

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

How it works:

Carrier Sense: Device listens to see if network is free
Multiple Access: Multiple devices can attempt to transmit
Collision Detection: Device detects if another transmitted simultaneously

Process:

Device checks if network is idle
If idle, start transmitting
While transmitting, listen for collisions
If collision detected:

Stop transmitting
Send jam signal to ensure all devices know collision occurred
Wait random time (backoff)
Attempt to retransmit

Repeat until successful or maximum attempts reached

Important: CSMA/CD is only used in half-duplex connections. Modern switched networks use full-duplex, eliminating collisions.

Bit Streaming

Definition

Continuous transmission of digital data as a stream of bits, typically for audio or video.

Methods of Bit Streaming

Real-time streaming:

Data played as received; not stored permanently
Requires consistent data rate
Sensitive to delays and buffering
Examples: Live video broadcast, video conferencing
Uses protocols like RTP (Real-time Transport Protocol)

On-demand streaming:

User can start, pause, rewind
Data may be buffered ahead
More tolerant of network variations
Examples: Netflix, YouTube, Spotify
Uses protocols like HTTP with adaptive bitrate

Importance of Bit Rates and Broadband Speed

Bit rate: Number of bits transmitted per second (bps)

Required bit rates for different content:

Music streaming: 128-320 kbps
SD video: 3-5 Mbps
HD video (1080p): 5-8 Mbps
4K video: 15-25 Mbps
8K video: 50-100 Mbps

Impact of insufficient bandwidth:

Buffering/pausing
Reduced quality (automatic downscaling)
Lag/latency
Packet loss
Poor user experience

Broadband speed requirements:

Higher speeds allow higher quality streaming
Multiple simultaneous streams require more bandwidth
Consistent speed more important than peak speed
Latency affects real-time streaming

World Wide Web vs Internet

Internet

Global network of interconnected computer networks
The physical infrastructure
Hardware: cables, routers, servers, satellites
Uses TCP/IP protocols
Transports all kinds of data (email, files, web, VoIP)

World Wide Web (WWW)

Service that runs on the internet
Collection of web pages and resources
Accessed via web browsers
Uses HTTP/HTTPS protocols
Linked via hyperlinks
One of many internet services (others: email, FTP, VoIP)

Comparison

Feature	Internet	World Wide Web
Definition	Physical network infrastructure	Service on the network
Analogy	The road system	The cars and traffic
Protocols	TCP/IP	HTTP/HTTPS
Components	Cables, routers, servers	Web pages, browsers, links
Access method	Any internet application	Web browser
Example	Sending an email	Browsing a website

Internet Hardware

Modem (Modulator-Demodulator)

Converts digital signals to analogue and vice versa
Enables digital devices to use analogue lines (telephone, cable)
Types:
Dial-up modem: Over telephone lines (obsolete)
DSL modem: Over telephone lines, higher speeds
Cable modem: Over TV cable infrastructure
Fibre modem: For fibre optic connections

PSTN (Public Switched Telephone Network)

Traditional telephone network
Originally analogue, now largely digital
Used for dial-up internet (historically)
Now often used for DSL internet (uses frequencies above voice)

Dedicated Lines

Leased lines reserved for a single customer
Guaranteed bandwidth and availability
Expensive but reliable
Examples: T1, E1, fibre leased lines
Used by businesses for WAN connections

Cell Phone Network

Mobile network infrastructure
Divides area into cells, each with base station
Technologies: 3G, 4G/LTE, 5G
Used for mobile internet access
Components: Base stations, mobile switching centres, backhaul connections

IP Addressing

IP Address Format

IPv4 (Internet Protocol version 4):

32-bit address
Written as four decimal numbers (0-255) separated by dots
Example: 192.168.1.1
Approximately 4.3 billion unique addresses
Running out; solved by NAT and IPv6

IPv6 (Internet Protocol version 6):

128-bit address
Written as eight groups of four hexadecimal digits
Example: 2001:0db8:85a3:0000:0000:8a2e:0370:7334
Can be abbreviated (omit leading zeros, use :: for consecutive zeros)
Virtually unlimited addresses (340 undecillion)

Subnetting

Dividing a network into smaller subnetworks
Improves efficiency and security
Reduces broadcast traffic
Uses subnet mask to identify network portion of IP address
Example: 192.168.1.0/24 means first 24 bits are network address

Associating IP Addresses with Devices

Static allocation: IP manually assigned; never changes
Dynamic allocation: IP automatically assigned via DHCP (Dynamic Host Configuration Protocol)
DHCP process:

Device broadcasts DHCP discovery
DHCP server offers an IP address
Device requests the offered address
Server acknowledges and assigns the address (lease)

Public vs Private IP Addresses

Public IP addresses:

Globally unique
Routable on the internet
Assigned by ISP
Can be accessed from anywhere
Security risk if exposed directly

Private IP addresses:

Not routable on the internet
Used within private networks
Reserved ranges:
10.0.0.0 – 10.255.255.255 (10.0.0.0/8)
172.16.0.0 – 172.31.255.255 (172.16.0.0/12)
192.168.0.0 – 192.168.255.255 (192.168.0.0/16)
Must use NAT to access internet

NAT (Network Address Translation):

Router translates private IPs to public IP
Multiple devices share one public IP
Provides basic security (hides internal structure)

Static vs Dynamic IP Addresses

Static IP:

Manually configured; never changes
Required for servers (web, email, etc.)
More expensive
Easier to access remotely
Used for critical services

Dynamic IP:

Automatically assigned by DHCP
Changes over time (when lease expires)
Cheaper (standard for home users)
Good for client devices
More complex to access remotely

Feature	Static IP	Dynamic IP
Assignment	Manual	Automatic (DHCP)
Changes	Never	May change
Cost	Higher	Lower
Use case	Servers, remote access	Client devices, home users
Management	More work	Automatic

URLs and DNS

Uniform Resource Locator (URL)

Definition: Address used to locate a resource on the World Wide Web.

Structure:

https://www.example.com:443/products/item.html?id=123#section
\___/  \_______________/ \_/ \_________________/\_______/\_______/
protocol     domain      port        path        query    fragment

Components:

Protocol: How to access (http, https, ftp)
Domain name: Human-readable address (www.example.com)
Port: Specific service port (optional, default 80 for http, 443 for https)
Path: Specific file/directory on server (/products/item.html)
Query: Parameters for dynamic content (?id=123)
Fragment: Specific section within page (#section)

Domain Name Service (DNS)

Purpose: Translates human-readable domain names into IP addresses.

Why DNS is needed:

Humans remember names (google.com) better than numbers (142.250.185.46)
IP addresses can change while domain names stay the same
Enables load balancing (one name to multiple IPs)
Essential for email routing (MX records)

How DNS works:

User types www.example.com in browser
Computer checks local cache (hosts file, browser cache)
If not found, queries recursive DNS resolver (usually ISP)
Resolver queries root name servers
Root points to .com TLD (Top-Level Domain) servers
TLD servers point to example.com’s authoritative name servers
Authoritative servers provide IP address for www.example.com
IP address returned to browser
Browser connects to that IP address

DNS record types:

A: IPv4 address
AAAA: IPv6 address
CNAME: Canonical name (alias)
MX: Mail exchange (email server)
NS: Name server
TXT: Text information (often for verification)

DNS hierarchy:

Root (.)
  ├── .com TLD
  │     └── example.com
  │           ├── www.example.com
  │           └── mail.example.com
  ├── .org TLD
  └── .uk ccTLD
        └── .co.uk
              └── bbc.co.uk

Summary Checklist for Assessment Objectives

AO1 (Knowledge) – You should be able to:

✓ Define LAN, WAN, client-server, peer-to-peer
✓ Describe network topologies (bus, star, mesh, hybrid)
✓ Explain cloud computing types (public, private)
✓ Describe transmission media characteristics
✓ List LAN hardware (switch, NIC, WAP, etc.)
✓ Explain router functions
✓ Describe CSMA/CD process
✓ Define bit streaming methods
✓ Differentiate internet vs WWW
✓ Describe IP address formats (IPv4, IPv6)
✓ Explain DNS purpose and hierarchy

AO2 (Application) – You should be able to:

✓ Justify network model for given scenarios
✓ Justify topology for given situations
✓ Compare wired vs wireless for specific needs
✓ Explain packet transmission in different topologies
✓ Justify cloud vs on-premises for organisations
✓ Calculate bandwidth requirements for streaming
✓ Explain IP address allocation methods
✓ Trace URL resolution through DNS

AO3 (Design/Evaluation) – You should be able to:

✓ Evaluate network designs for organisations
✓ Compare and contrast different network models
✓ Assess security implications of network choices
✓ Design appropriate network solutions
✓ Evaluate cloud computing benefits/drawbacks for specific cases
✓ Analyse network performance factors
✓ Critically evaluate transmission media choices