Networks and topologies

OCR J277 Paper 1 tests networks extensively. Expect 4–6 mark questions on topology advantages/disadvantages and comparisons between network types. Know the diagrams as well as the descriptions.

LAN vs WAN

Feature	LAN (Local Area Network)	WAN (Wide Area Network)
Geographic area	Small (building, campus)	Large (city, country, world)
Ownership	Usually owned by one organisation	Usually uses third-party telecommunications infrastructure
Speed	High (up to 10 Gbps on modern switches)	Slower (dependent on infrastructure)
Example	School network, home network	The internet, bank network across branches

Factors affecting network performance

1. Bandwidth (transmission capacity): higher bandwidth → more data per second.
2. Latency: time for data to travel from sender to receiver; lower = better.
3. Number of users: more users sharing a network → slower for each.
4. Type of transmission medium: fibre optic → fastest; twisted pair → slower; wireless → varies.
5. Error rates: data errors require retransmission → slows the network.

Network topologies

Star topology

Device - Switch/Hub - Device
              |
           Device

• All devices connect to a central switch or hub.
• Data passes through the switch; the switch routes it to the correct device.

Advantage	Disadvantage
If one cable/device fails, others still work	If the switch fails, ALL devices lose connection
Easy to add new devices	Requires more cable than a bus
Good performance (switch manages data flow)	Central switch is a cost

Mesh topology

• Full mesh: every device connected to every other device.
• Partial mesh: some devices have multiple connections.

Advantage	Disadvantage
Very reliable — multiple paths for data	Very expensive (more cables/hardware)
If one connection fails, data rerouted	Difficult to install and manage
Good for critical systems

Client–server vs peer-to-peer

Client–server

• One or more powerful servers (file server, web server, print server) provide services.
• Clients (user computers) request services from servers.
• Advantages: centralised security, easier management, shared data, backups managed centrally.
• Disadvantages: expensive servers; single point of failure; requires IT management.
• Used in: schools, businesses, online services.

Peer-to-peer (P2P)

• All computers have equal status — any can be both client and server.
• Each computer stores its own files; resources shared directly between computers.
• Advantages: cheap to set up; no central server required; each computer manages itself.
• Disadvantages: no central security; difficult to manage; slower if one computer is powerful and others are not.
• Used in: small offices, home networks, BitTorrent.

Common OCR exam mistakes

1. Saying the internet is a LAN — it is the world's largest WAN.
2. Confusing a hub with a switch: a hub broadcasts data to ALL devices; a switch directs data only to the correct device — much more efficient.
3. Saying "star topology fails if one cable breaks" — the network still works; only that device loses connection. Only if the switch fails does the whole network go down.
4. Forgetting that P2P has security disadvantages — no central control means no guaranteed security policy.