Big picture
A computer network is a collection of interconnected devices that share data and resources. The internet is the largest network of all: a network of networks.
This chapter is mostly definitions and comparisons, but the board paper always includes a case study: a company has several buildings with given distances and numbers of computers, and you must suggest where to place the server, how to lay the cables, which device to use, and which kind of network it is. Learn the reasoning behind each choice.
Learning goals
Describe the evolution of networking from ARPANET to the internet.
Explain data communication terms: sender, receiver, medium, protocol, bandwidth, data transfer rate and IP address.
Distinguish circuit switching and packet switching.
Compare wired and wireless transmission media.
Explain the functions of network devices.
Compare bus, star and tree topologies, and PAN, LAN, MAN and WAN.
Explain common protocols and web terms.
Solve network design case studies.
Core ideas
1. Evolution of networking
ARPANET (1969): the first network, created by the US Department of Defense's Advanced Research Projects Agency, linking computers at universities and research centres.
NSFNET (mid-1980s): created by the National Science Foundation for academic research; more capable than ARPANET.
Internet: many networks joined together using common protocols (TCP/IP) to form a global network of networks.
2. Data communication terms
Term | Meaning |
|---|---|
Sender | Device that sends data |
Receiver | Device that receives data |
Message | The data being communicated |
Communication medium | The path through which data travels (cable or wireless) |
Protocol | A set of rules governing communication |
Bandwidth | The range of frequencies a channel can carry; the capacity of the channel, measured in hertz (Hz) |
Data transfer rate | Number of bits transferred per second: bps, Kbps, Mbps, Gbps (1 Kbps = 1,024 bps) |
IP address | A unique address assigned to each device on a network, e.g. IPv4 192.168.1.10 (32-bit, written as four numbers 0–255) |
MAC address | A permanent, unique physical address of a network interface card (48-bit) |
3. Switching techniques
Basis | Circuit switching | Packet switching |
|---|---|---|
Path | A dedicated physical path is set up for the whole communication | Message is broken into packets that travel independently, possibly by different routes |
Resource use | The path is reserved even when idle | Links are shared efficiently |
Example | Traditional telephone calls | The internet |
Reassembly | Not needed | Packets are reassembled in order at the destination |
4. Transmission media
Wired (guided) media
Medium | Features | Use |
|---|---|---|
Twisted pair cable | Pairs of insulated copper wires twisted together to reduce interference; cheap and easy to install; limited bandwidth and distance (about 100 m); UTP (unshielded) and STP (shielded) | LANs, telephone lines |
Coaxial cable | Central copper conductor with insulation, braided shield and outer cover; better shielding and bandwidth than twisted pair | Cable TV |
Optical fibre | Transmits data as light through glass or plastic fibres using total internal reflection; very high bandwidth, long distances, immune to electromagnetic interference, secure; expensive and hard to install and join | Backbone and long-distance links, links between buildings far apart |
Wireless (unguided) media
Medium | Features | Use |
|---|---|---|
Radio waves | 3 kHz to 3 GHz; travel long distances, penetrate walls; omnidirectional | FM radio, Wi-Fi, mobile phones |
Microwaves | 1 GHz to 300 GHz; line-of-sight transmission; towers must face each other | Long-distance telephone, satellite communication |
Infrared | 300 GHz to 400 THz; very short range; can't pass through walls | TV remote controls, short-range device communication |
Other wireless technologies: Bluetooth (short-range, connecting devices like headphones; forms a PAN) and satellite links.
5. Network devices
Device | Function |
|---|---|
Modem (modulator–demodulator) | Converts digital signals from a computer into analog signals for transmission over telephone lines, and back again |
Ethernet card / NIC | Network interface card that connects a computer to a network; has a unique MAC address |
RJ45 connector | Eight-pin connector at the end of an Ethernet (twisted pair) cable |
Repeater | Regenerates (amplifies) a weakened signal so it can travel further; used when a cable exceeds about 100 m |
Hub | Connects multiple computers in a LAN; broadcasts incoming data to all ports |
Switch | Connects multiple computers in a LAN; intelligently sends data only to the intended destination port, reducing traffic |
Router | Connects different networks and forwards data packets along the best path, using IP addresses |
Gateway | Connects networks that use different protocols, translating between them; the entry/exit point of a network |
Wi-Fi card | Enables a computer to connect to a wireless network |
6. Network topologies
Topology | Arrangement | Advantages | Disadvantages |
|---|---|---|---|
Bus | All devices share a single main cable (backbone) with terminators at both ends | Cheap, needs less cable, easy to extend | A fault in the backbone stops the whole network; hard to locate faults; performance drops with heavy traffic |
Star | Every device connects to a central hub or switch | Easy to install and manage; one node failing doesn't affect others; easy fault detection | Needs more cable; if the central device fails, the network fails |
Tree | A hierarchy of star networks connected to a bus backbone | Expandable; suits large organisations with departments | Depends on the backbone; more cable and configuration |
7. Types of networks by area
Type | Area covered | Example |
|---|---|---|
PAN (personal area network) | Around a person, about 10 m | Phone connected to wireless earphones and a laptop via Bluetooth |
LAN (local area network) | A building or campus, up to about 1 km | School computer lab, office |
MAN (metropolitan area network) | A city, up to about 50 km | Cable TV network, a city's bank branch network |
WAN (wide area network) | Countries and continents | The internet, a multinational company's network |
8. Protocols
Protocol | Purpose |
|---|---|
TCP/IP | The basic suite of internet communication. TCP divides messages into packets and reassembles them reliably; IP addresses and routes packets |
HTTP (HyperText Transfer Protocol) | Transfers web pages between web servers and browsers |
HTTPS | Secure HTTP, encrypting data between browser and server (used for banking and logins) |
FTP (File Transfer Protocol) | Transfers files between computers |
SMTP (Simple Mail Transfer Protocol) | Sends email |
POP3 (Post Office Protocol 3) | Receives (downloads) email from a mail server |
PPP (Point-to-Point Protocol) | Direct connection between two nodes, such as dial-up internet |
TELNET | Remote login to another computer over a network |
VoIP (Voice over Internet Protocol) | Voice calls over the internet |
9. Introduction to the web
Term | Meaning |
|---|---|
WWW (World Wide Web) | A system of interlinked web pages accessed over the internet using HTTP; invented by Tim Berners-Lee (1989) |
HTML (HyperText Markup Language) | Language used to display content on web pages, using predefined tags |
XML (eXtensible Markup Language) | Language used to store and transport data, with user-defined tags |
Domain name | Human-readable name of a website, e.g. |
URL (Uniform Resource Locator) | The complete address of a resource: protocol, domain and path, e.g. |
Website | A collection of related web pages under one domain |
Web browser | Software to access and view web pages (Chrome, Firefox, Edge) |
Web server | A computer that stores websites and delivers pages on request |
Web hosting | A service that stores a website on a server so it is available on the internet |
10. Solving a network design case study
Example. A company has four blocks: Admin (80 computers), Sales (30), Finance (20) and Store (10). Distances: Admin–Sales 70 m, Admin–Finance 120 m, Admin–Store 60 m, Sales–Finance 50 m, Finance–Store 140 m.
Question | Answer and reasoning |
|---|---|
Best block for the server | Admin, because it has the most computers, so most traffic stays local (the 80:20 rule) |
Cable layout | Star topology centred on Admin, connecting each block to Admin, so cable is short and each link independent |
Where to place a repeater | On any link longer than about 100 m, here Admin–Finance (120 m), if a direct cable is used |
Device to connect computers within each block | Switch (or hub), which sends data efficiently to the right computer |
Device to connect the company to the internet | Router (with a modem) |
Best wired medium for long links between blocks | Optical fibre (high speed, long distance, immune to interference) |
Type of network | LAN, since all blocks are within a campus; connecting to an office in another city would make a WAN |
A reliable approach to questions
For a device question, ask what it connects: computers in a LAN (switch or hub), different networks (router), different protocols (gateway), weak signal (repeater), analog line (modem).
For a protocol, match the task: web pages (HTTP/HTTPS), files (FTP), sending mail (SMTP), receiving mail (POP3), voice (VoIP), remote login (Telnet).
For a case study, put the server in the block with the most computers, use star layout from it, add repeaters on long cables, and choose optical fibre for long or fast links.
Connections that matter
Python–SQL connectivity sends data across a network to a database server.
Responsible computing covers cyber safety, threats and ethics on networks.
Informatics Practices covers the same networking terms from a user's angle.
Common mistakes
Saying a hub sends data only to the destination. A hub broadcasts; a switch sends selectively.
Confusing SMTP and POP3. SMTP sends; POP3 receives.
Confusing HTML and XML. HTML displays; XML stores and transports.
Calling a repeater an amplifier of data volume. It regenerates the signal.
Placing the server in the block nearest the gate rather than the one with the most computers.
Revision checklist
I can describe the evolution of networks.
I can define data communication terms and units.
I can compare circuit and packet switching.
I can compare all transmission media.
I can explain each network device.
I can compare topologies and network types.
I can match protocols to their purposes and explain web terms.
I can solve a network design case study.