Mission 1 · Spec 1.3.1
Types of network and performance
LANs and WANs, and the factors that make a network fast or slow.
- Starter 5 min
- Learn 10 min
- Lab 10 min
- Quiz 10 min
- Exam 10 min
Why is it so slow?
At 4pm the school Wi-Fi crawls; at 8am it's fast. Give as many reasons as you can.
Reveal
More users sharing the same bandwidth; heavy use like video streaming; wireless interference and walls causing errors that need re-sending; distance from the access point. These are the factors that affect network performance.
Key ideas
LAN
Local area network: covers a small geographical area (one building or site). The hardware is usually owned by the organisation.
WAN
Wide area network: covers a large geographical area, connecting LANs together. Uses infrastructure owned by third parties, e.g. telecoms companies. The internet is the biggest WAN.
Factors affecting performance
- Bandwidth: the amount of data that can be transmitted per second. More bandwidth, faster transfer.
- Number of users: more devices sharing the bandwidth means less each.
- Transmission media: fibre optic is faster than copper cable; wired is usually faster than wireless.
- Error rate: errors mean data must be re-sent, wasting time.
- Latency: the delay before data starts to arrive.
Faster or slower?
Exam-style questions
1. Describe two differences between a LAN and a WAN.
[4 marks]Mark scheme
- A LAN covers a small geographical area (1); a WAN covers a large area (1)
- LAN hardware is owned by the organisation (1); a WAN uses third-party infrastructure (1)
2. Explain two factors that could affect the performance of a network.
[4 marks]Mark scheme
- Bandwidth (1): more bandwidth means more data can be sent per second (1)
- Number of users (1): more users share the bandwidth so each gets less (1)
- Transmission media (1): e.g. fibre is faster than copper / wired faster than wireless (1)
- Error rate (1): errors mean data must be re-transmitted (1)
- Max 4: two factors, each explained
TUTOR NOTES
- Misconception: bandwidth is speed. It's the amount of data per second the connection can carry.
- Exam habit: for "explain", give the factor and its effect.
Mission 2 · Spec 1.3.1
Network models and hardware
Client-server versus peer-to-peer, and the hardware needed to build a network.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
Who's in charge?
In school, you log in on any computer and your files appear. At home, you might share a file straight from your laptop to your sister's. What's the difference?
Reveal
School uses a client-server network: a central server stores files and manages logins. At home, devices share directly as equals: peer-to-peer.
Key ideas
Client-server
A central server manages access, security and files; clients request services from it.
✓ Central backups, security and updates; users can log in anywhere
✗ Expensive; needs expert staff; if the server fails, clients lose access
Peer-to-peer
All computers are equal and share resources directly with each other.
✓ Cheap and easy to set up; no single point of failure
✗ No central backup or security; files are spread across machines; a computer sharing files may slow down
| Hardware | Purpose |
|---|---|
| Wireless access point (WAP) | Lets wireless devices connect to a wired network |
| Router | Forwards data packets between different networks, e.g. a LAN and the internet |
| Switch | Connects devices on a LAN and sends data only to the device it is addressed to |
| NIC (network interface controller/card) | Allows a device to connect to a network |
| Transmission media | The cables (copper, fibre optic) or radio waves that carry the data |
Hardware match
Client-server or peer-to-peer?
Exam-style questions
1. A small business is choosing between a client-server and a peer-to-peer network. Give one advantage of each.
[2 marks]Mark scheme
- Client-server: centralised backups / security / user management / software updates (1)
- Peer-to-peer: cheaper / easier to set up / no reliance on one server (1)
2. State the purpose of a router and a switch.
[2 marks]Mark scheme
- Router: forwards data packets between networks (1)
- Switch: connects devices on a LAN and sends data to the correct device (1)
TUTOR NOTES
- Misconception: a home "router" is just a router. It usually combines a router, switch, WAP and modem.
- Misconception: a switch connects networks together. That's a router.
Mission 3 · Spec 1.3.1
The internet
The internet as a worldwide network of networks, the Domain Name System, hosting, the cloud, and how data travels in packets.
- Starter 5 min
- Learn 10 min
- Lab 20 min
- Quiz 10 min
- Exam 10 min
The phone book
You type bbc.co.uk but computers find each other using numbers like 151.101.0.81. How does your computer find the number?
Reveal
It asks a DNS server, which works like a phone book: it looks up the domain name and returns the matching IP address.
Key ideas
The internet
A worldwide collection of computer networks connected together.
DNS
The Domain Name System translates domain names (URLs) into IP addresses so data can be sent to the right server.
Hosting
Storing a website's files on a web server that is permanently connected to the internet, so others can access it.
The cloud
Remote servers that store data or run applications, accessed over the internet.
✓ Access anywhere, easy to scale, provider handles backups
✗ Needs an internet connection; relies on the provider's security
Web servers and clients
A web server stores web pages and sends them when requested. A client (browser) requests the pages and displays them.
Packets
Data is split into packets, each with the destination address and a sequence number. Packets may take different routes and are reassembled at the destination.
DNS lookup, in order
Packets on the move
Exam-style questions
1. Describe how the Domain Name System is used when a user visits a website.
[4 marks]Mark scheme
- The URL / domain name is sent to a DNS server (1)
- The DNS server looks up the domain name (1)
- and returns the corresponding IP address (1)
- The browser then uses the IP address to request the page from the web server (1)
2. Give one advantage and one disadvantage of storing files in the cloud.
[2 marks]Mark scheme
- Advantage: accessible from anywhere / on any device / easy to share / provider handles backups (1)
- Disadvantage: needs an internet connection / security depends on the provider / ongoing cost (1)
TUTOR NOTES
- Misconception: the internet and the web are the same. The web is a service that runs on the internet.
- Misconception: DNS stores websites. It only stores name-to-address mappings.
Mission 4 · Spec 1.3.1
Star and mesh topologies
How star and mesh networks are laid out, and what happens when something fails.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
Roadworks
If the only road into your town is closed, you're stuck. If there are five roads, you just take another. How does that apply to networks?
Reveal
A mesh network has several routes between devices, so data can go around a failure. A star depends on one central switch.
Two topologies
Star
Every device connects to a central switch.
✓ If one cable fails, only that device is affected
✓ Fast, with few data collisions
✓ Easy to add devices
✗ If the switch fails, the whole network fails
✗ Lots of cable, so costly
Mesh
Devices connect to each other. In a full mesh every device connects to every other; in a partial mesh only some do.
✓ No single point of failure: data can take another route
✓ Can handle a lot of traffic
✗ Very expensive (many connections)
✗ Complex to set up and maintain
Break the network
Cut cables and fail devices in each topology.Exam-style questions
1. Draw a mesh network with four devices.
[2 marks]Mark scheme
- Four devices shown (1)
- Each device connected to every other device (6 connections) / multiple connections for a partial mesh (1)
2. A hospital network must keep working even if a cable is damaged. Recommend a topology and justify your choice.
[3 marks]Mark scheme
- Mesh (1)
- There are multiple routes between devices (1)
- so if one connection fails, data can be re-routed and the network keeps working (1)
TUTOR NOTES
- Lab prompt: how many cables can you cut in the full mesh before PC1 loses the server?
- Note: OCR J277 specifies star and mesh (not bus).
Mission 5 · Spec 1.3.2
Connections, addresses and protocols
Ethernet, Wi-Fi and Bluetooth; encryption; IP and MAC addresses; standards; the key protocols; and why networks use layers.
- Starter 5 min
- Learn 15 min
- Lab 20 min
- Quiz 10 min
- Exam 10 min
Two addresses
A letter needs a house address. Why might a network device need two different addresses?
Reveal
A MAC address is fixed into the hardware and identifies the device on its local network. An IP address identifies it on the wider internet and can change when it joins a different network.
Key ideas
Wired: Ethernet
Fast, reliable and secure, but needs cables.
Wireless: Wi-Fi and Bluetooth
Wi-Fi connects devices to a LAN over radio waves. Bluetooth connects devices over short distances (a PAN). Convenient, but slower, less reliable and easier to intercept, so encryption is used.
IP address
Identifies a device on a network so data can be routed to it. IPv4: 4 numbers from 0 to 255, e.g. 192.168.0.12. IPv6: 8 groups of hex, e.g. 2001:0db8:85a3:0000:0000:8a2e:0370:7334.
MAC address
A unique address assigned to a device's network interface when it's made, e.g. 3C:22:FB:1A:07:9E (12 hex digits). Used to route data within a LAN.
Standards
Agreed rules for hardware and software, so products from different manufacturers work together.
Layers
Protocols are grouped into layers, each with a specific job. Layers can be developed and changed independently, making networking easier to design and fix.
| Protocol | Purpose |
|---|---|
| TCP/IP | TCP splits data into packets and checks they arrive; IP addresses and routes packets |
| HTTP / HTTPS | Transfers web pages; HTTPS is encrypted |
| FTP | Transfers files between computers |
| POP | Downloads email from a server to one device, then usually deletes it from the server |
| IMAP | Accesses email that stays on the server, so it syncs across devices |
| SMTP | Sends email to and between mail servers |
Which kind of address?
Protocol match
Layers in action
Exam-style questions
1. Describe the difference between an IP address and a MAC address.
[2 marks]Mark scheme
- An IP address identifies a device on a network / the internet and can change (1)
- A MAC address is unique, assigned to the hardware when it is made and does not change (1)
2. Give two benefits of using layers in networking.
[2 marks]Mark scheme
- Each layer can be developed / changed independently without affecting others (1)
- Makes the network easier to understand / troubleshoot (1)
- Allows hardware and software from different manufacturers to work together / encourages standards (1)
- Max 2
3. Explain why a user with a phone and a laptop might prefer IMAP to POP.
[2 marks]Mark scheme
- IMAP keeps email on the server (1)
- so the same messages can be accessed and stay in sync on both devices (1)
TUTOR NOTES
- Misconception: MAC addresses are for Apple Macs. MAC stands for Media Access Control.
- Misconception: POP and IMAP send email. They retrieve it; SMTP sends.