Communication and internet technologies
Cambridge AS & A Level Computer Science 9618 · Paper 3 · Unit 14
Paper 3 · Unit 14. Protocols, the TCP/IP stack and circuit vs packet switching.
1. Why protocols are needed
A protocol is a set of rules for communication (format, timing, error handling) so devices from different manufacturers can communicate. Protocols are organised in a stack of layers; each layer has a specific job and passes data to the layer above or below.
2. The TCP/IP protocol suite
| Layer | Role | Examples |
|---|---|---|
| Application | Provides services to applications and users | HTTP, FTP, POP3, IMAP, SMTP, BitTorrent |
| Transport | Splits data into segments, adds port numbers, ensures reliable delivery and reassembly | TCP, UDP |
| Internet | Adds IP addresses and routes packets across networks | IP |
| Link | Transmits frames over the physical network using MAC addresses | Ethernet, Wi-Fi |
| Protocol | Purpose |
|---|---|
| HTTP / HTTPS | Transfers web pages between browser and web server (HTTPS is encrypted) |
| FTP | Transfers files between computers |
| SMTP | Sends (pushes) email to and between mail servers |
| POP3 | Downloads email to one device and usually deletes it from the server |
| IMAP | Keeps email on the server so it syncs across devices |
| BitTorrent | Peer-to-peer file sharing: a tracker coordinates peers; files are split into pieces downloaded from many peers (seeds and leechers) at once |
Try it: which TCP/IP layer?
3. Circuit and packet switching
| Circuit switching | Packet switching | |
|---|---|---|
| Path | A dedicated path is set up for the whole communication | Data is split into packets that may take different routes |
| Order | Data arrives in order | Packets may arrive out of order and are reassembled |
| Bandwidth | Reserved, even when idle; guaranteed quality | Shared, efficient use of the network |
| Failure | A broken link ends the call | Packets can be rerouted around failures |
| Use | Traditional phone calls | The internet, email, web |
A router reads the destination IP address of each packet and uses its routing table to forward it to the next hop on the best route.
4. Examiner tips
- Name the layer and its job, not just the protocol.
- POP3 vs IMAP: POP3 downloads, IMAP syncs and keeps mail on the server.
- BitTorrent: mention tracker, peers, seeds and pieces.
- Packet switching: packets contain a header with destination, sequence number and checksum.
5. Practice questions
Practice: answer and check instantly
Type a short answer, then press Check or Enter.
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The Internet layer.
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SMTP.
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IMAP.
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FTP.
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BitTorrent.
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Circuit switching.
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TCP.
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The Link layer.
Exam-style questions
9. Explain why a protocol stack is used. [3]
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Each layer has a specific, separate job [1] · layers can be changed/updated without affecting others [1] · makes design and troubleshooting simpler and allows different hardware/software to interoperate [1].
10. Compare circuit switching and packet switching. [4]
Show mark guide
Circuit: dedicated path set up first [1], bandwidth reserved [1] · packet: data split into packets sent on different routes [1], reassembled in order at the destination / efficient use of bandwidth [1].
6. Topic test: predict your grade
Topic test: predict your grade
15 questions, about 10 minutes. Answer them all, then press Mark my test.
Frequently asked questions
What are the layers of the TCP/IP model?
The four layers are application, transport, internet and link.
What is the difference between POP3 and IMAP?
POP3 downloads emails to one device and usually removes them from the server. IMAP keeps emails on the server so they stay in sync across devices.
What is the difference between circuit switching and packet switching?
Circuit switching sets up a dedicated path for the whole communication. Packet switching splits data into packets that can take different routes and are reassembled at the destination.