2 missions. Guess, learn, then check. Your place is saved.
Mission 1If networks spread viruses, why does every school connect its PCs?
Mission 2How do two computers that have never met agree how to talk?
The specification points and sources are listed under the lesson: see Sources.
Mission 1
If networks spread viruses, why does every school connect its PCs?
Make a guess: If networks spread viruses, why does every school connect its PCs?
Networks · why network and types of network
Why connect computers?
A computer network is two or more computers (or other devices) connected so they can share data and resources.
Joining them brings gains, and it brings costs and risks:
Gains
Costs and risks
Share files and data from any computer
Hardware (cables, switches, servers) costs money
Share printers and one internet connection
Needs skilled staff to set up and maintain
Communicate: email, messaging, shared documents
Malware can spread from one computer to others
Back up every computer from one place
Hackers can attack from anywhere on the network
Install software and updates centrally
If a central device or server fails, many users are hit
Quick check
?A small office has five stand-alone computers. Which change would a network bring?
Networks · why network and types of network
Types of network
A LAN (local area network) covers a small area, such as one school, and is usually owned and run by one organisation.
A WAN (wide area network) covers a wide geographical area and links LANs. It is often under collective or shared ownership.
The internet is the biggest WAN.
A PAN (personal area network) links one person's devices over a few metres, e.g. phone and earbuds, using Bluetooth.
Networks · why network and types of network
Wired or wireless?
Wired connections use cables; wireless connections use radio waves.
Copper cable is cheap for short runs. Fibre-optic cable carries more data over long distances with no electrical interference, but costs more.
Wired
Wireless
Speed and bandwidth
Usually higher
Usually lower, shared by nearby devices
Range
Longer, especially with fibre
Limited; walls and interference weaken it
Latency (delay)
Lower
Higher
Mobility and set-up
Devices fixed; cabling costs
Devices can move; cheap to add
Security
Must plug in to tap it
Signals can be picked up nearby
Quick check
?A college has 30 desktop PCs in one room, used to edit large video files stored on a server in the same building. Which connection suits them best?
Mission check
?If networks spread viruses, why does every school connect its PCs?
A network lets a school share files, printers and one internet connection, and back up and manage every PC from one place. The risks, such as malware spreading, hardware costs and a central failure, are real, so networks are protected and maintained rather than avoided.
Mission 2
How do two computers that have never met agree how to talk?
Make a guess: How do two computers that have never met agree how to talk?
Networks · protocols and layers
Protocols
A protocol is a set of rules for transferring data between devices.
It sets things like how data is formatted, how devices are addressed and how errors are spotted.
Both devices must follow the same protocol, whoever made them. Different jobs use different protocols.
Networks · protocols and layers
Protocols for the web and email
HTTP: a browser requests web pages from a web server, which sends them.
HTTPS: HTTP with encryption, so intercepted data can't be read.
SMTP: sends email from an app to a mail server, and between mail servers.
IMAP: reads email kept on the server, so every device shows the same inbox.
Networks · protocols and layers
Protocols that move the data
TCP splits data into numbered packets, checks they all arrive, has missing ones resent and puts them back in order.
IP gives each packet its source and destination IP addresses and routes it across networks.
Quick check
?Ama clicks Send in her email app. Which protocol carries the message to her mail server?
Networks · protocols and layers
Why use layers?
Sending data is split into layers. Each layer does one part of the job and each protocol belongs to one layer.
Sent data passes down the layers, each adding its own information. Received data passes back up.
Benefits: each layer is a smaller problem; one layer can change without changing the others (swap a cable for Wi-Fi and the browser still works); makers can build for one layer.
Networks · protocols and layers
The 4-layer TCP/IP model
Layer
What it does
Protocols and parts
Application
Where network applications, such as web browsers and email programs, work
HTTP, HTTPS, SMTP, IMAP
Transport
Sets up communication between the two hosts, which agree settings such as packet size; splits data into packets
TCP
Internet
Addresses and packages data; routes the packets across the network
IP
Link
The network hardware, such as the NIC, and the operating system's device drivers
NIC, device drivers
Quick check
?A router reads a packet's destination IP address to choose where to send it next. Which layer of the TCP/IP model is this?
Mission check
?How do two computers that have never met agree how to talk?
The two computers have never met, but they follow the same protocols, such as SMTP for email and TCP and IP for moving the data. The work is split into layers, each with its own protocols, so every device that follows the rules can talk to every other.
Recap
Big ideas
A network connects devices so they can share data and resources. It brings gains (sharing, central backup and management) and costs (hardware, maintenance, security risks).
A LAN covers a small area and is owned by one organisation; a WAN covers a wide area and links LANs. The internet is the biggest WAN.
A protocol is a set of rules for transferring data. Both devices must follow the same one, and different jobs use different protocols.
In the 4-layer TCP/IP model, HTTP, HTTPS and SMTP work at the application layer, TCP at the transport layer and IP at the internet layer.
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Mini-lesson complete!
⭐⭐⭐
You've worked through Computer networks for AQA GCSE Computer Science (8525). 🎉
Mission 1If networks spread viruses, why does every school connect its PCs?
Mission 2How do two computers that have never met agree how to talk?