14 Communication and internet technologies
Use protocol layers and compare circuit switching with packet switching for different communication needs.
Section Paper 3 · Advanced Theory · Sections 13–20
Mapped to Cambridge International AS & A Level Computer Science 9618, 2027–2029 Version 2. These original lessons and self-checks are independent study support; they are not Cambridge papers, official marks or grades. The course does not yet provide a timed four-paper simulation or a locked-down Paper 4 programming environment.
Before you begin
- Use the 2027–2029 syllabus for your exam year. AS candidates study Sections 1–12; A Level candidates study Sections 1–20. Paper 4 uses a programming language chosen from Python, Java or Visual Basic in console mode.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which protocol is associated with sending email between mail servers?
What you will learn
- Explain the key representations, structures or processes in this syllabus section
- Apply the method to a new computer-science example
- Justify a design choice and check its limits
Protocols make layered communication interoperable
A protocol defines agreed message formats, actions and error handling. Layering divides communication responsibilities so one layer can use the service below it and offer a service above it.
The four TCP/IP layers are Application, Transport, Internet and Link. Application protocols support services; Transport manages end-to-end communication; Internet addressing and routing move packets across networks; Link transfers frames over a local medium. Examples include HTTP, FTP, POP3, IMAP, SMTP and BitTorrent for peer-to-peer file sharing.
Switching methods make different trade-offs
Circuit switching reserves a path or channel for the communication session. It can provide predictable capacity after setup, but reserved capacity may be wasted when no data is sent and setup can add delay.
Packet switching divides data into packets that can be routed independently. It uses shared links efficiently and can route around failures, but packets may experience variable delay, loss or reordering. Routers forward packets using network information; the receiver reassembles them as required by the protocol.
Pause and explain
What is a common advantage of packet switching?
Worked example
A browser requests a secure web page. Identify a protocol layer for the web request and a layer responsible for end-to-end transport.
Show the worked solution
- HTTP or HTTPS is an application-layer protocol.
- TCP operates at the transport layer in the TCP/IP model.
- Internet and link layers carry packets across networks and local media.
Answer The web request uses an application protocol; TCP provides transport between endpoints.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Place HTTP, TCP, IP and Ethernet in their TCP/IP layer, from application down to link.
Give me a hint
Map application, transport, internet and local-link responsibilities.
Compare my reasoning
- HTTP supports web requests at the Application layer.
- TCP provides transport; IP provides Internet-layer addressing and routing.
- Ethernet is a Link-layer technology for local frames.
HTTP—Application; TCP—Transport; IP—Internet; Ethernet—Link.
Look for these in your work
- Four protocols placed
- Layer order correct
- Functions distinguished
A live video call sends data over a shared network. Compare one advantage and one drawback of packet switching for the call.
Give me a hint
Consider shared capacity and variable arrival times.
Compare my reasoning
- Shared links can carry traffic from many users efficiently.
- Congestion may cause variable delay or packet loss.
- Real-time applications may need buffering and recovery strategies.
It shares capacity efficiently, but congestion can create delay and loss that affect playback.
Look for these in your work
- Benefit stated
- Drawback stated
- Effect connected
A remote factory control link needs predictable bandwidth for a continuous session. Compare why circuit switching might help and what it costs.
Give me a hint
Think about reservation during idle periods and setup.
Compare my reasoning
- A reserved circuit can provide predictable capacity once established.
- Setting it up can add time before communication begins.
- Capacity may remain reserved when the control system sends little data.
Circuit switching can offer predictable capacity but uses reserved resources and requires setup.
Look for these in your work
- Predictability explained
- Setup cost identified
- Idle-capacity cost identified
Common mistakes
- Treating TCP/IP layers as four physical devices.
- Assuming packet switching guarantees that packets arrive in order.
- Calling BitTorrent a circuit-switched service.
What can you explain now?
Which TCP/IP layer handles IP addressing and routing between networks?
Compare with the explanation
The Internet layer
The Internet layer moves packets among networks; the Link layer handles local frame transfer.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How does each TCP/IP layer contribute, and what trade-offs distinguish circuit and packet switching?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.