Packet switching and transmission methods
Follow a message through packets and compare serial, parallel, simplex, half-duplex and full-duplex transmission.
Mapped to the Cambridge IGCSE Computer Science 0265 syllabus for exams in 2029. These original explanations and self-checks are independent study support, not Cambridge papers, official marks or grades. Practical programming is taught with worked code and trace tasks; an executable Python workspace and timed digital-exam simulation are not yet available.
Before you begin
- Use the 2029 syllabus for this course. Cambridge specifies Python 3.10 or higher for programming and digital exams for both components. The syllabus currently identifies this route for 2029 only; confirm later-year availability with Cambridge and your school.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before packet switching and transmission methods, which statement is the safest starting point?
What you will learn
- Describe packet structure and packet switching
- Distinguish transmission directions and formats
- Choose a transmission method for a context
Packet switching divides a message
A message is split into packets for transmission. A packet header carries control information such as source and destination addresses, sequencing or packet identification; its payload contains the part of the message being sent; a trailer can contain an error-checking value.
Routers make forwarding decisions, so packets from one message can take different routes. At the destination, sequence information is used to reassemble the packets. A missing or damaged packet can be detected and requested again depending on the protocol.
Transmission has direction and layout
Serial transmission sends bits one after another along a link; parallel sends multiple bits simultaneously over several lines. Simplex sends in one direction, half-duplex supports both directions but not at the same time, and full-duplex supports simultaneous two-way communication.
Suitability depends on distance, cost, interference, timing and whether responses are needed. Serial links use fewer wires and are practical over longer distances; parallel links can move several bits at once over short distances but can suffer timing differences between lines.
Pause and explain
Which habit best checks your understanding of packet switching and transmission methods?
Worked example
A walkie-talkie allows two people to speak, but only one can transmit at a time. Identify the direction method.
Show the worked solution
- Communication can travel both ways.
- The participants cannot transmit simultaneously.
- That matches half-duplex.
Answer Half-duplex
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
A 900-byte file is split into 300-byte payloads. How many packets are needed, ignoring headers and trailers?
Give me a hint
Divide the payload size by the packet payload capacity.
Compare my reasoning
- 900 ÷ 300 = 3.
- Each packet carries one 300-byte payload.
- Headers and trailers are excluded by the question.
3 packets
Look for these in your work
- Division shown
- Packet count correct
- Assumption stated
Compare simplex, half-duplex and full-duplex for a one-way TV broadcast, a walkie-talkie and a telephone call.
Give me a hint
Match whether communication is one-way, alternating or simultaneous.
Compare my reasoning
- A TV broadcast sends from station to viewer: simplex.
- A walkie-talkie alternates directions: half-duplex.
- A telephone call supports simultaneous speaking and listening: full-duplex.
TV broadcast: simplex; walkie-talkie: half-duplex; telephone: full-duplex.
Look for these in your work
- Each context matched
- Direction described
- Simultaneity distinguished
Packets 2 and 3 arrive before packet 1, and packet 4 is missing. Explain how a destination can still reconstruct the message and identify the problem.
Give me a hint
Use packet order information, reassembly and a missing-packet check.
Compare my reasoning
- Sequence details in the packet headers identify the intended order.
- The receiver can hold packets 2 and 3 while waiting for packet 1.
- It detects packet 4 is missing and may request retransmission before completing reassembly.
Use sequence information to reorder packets and detect/request the missing packet before reassembly is complete.
Look for these in your work
- Order tracked
- Reassembly explained
- Missing packet addressed
Common mistakes
- Confusing serial with simplex: one describes layout, the other direction.
- Assuming every packet follows the same route.
- Describing a packet trailer as the message payload.
What can you explain now?
Which method allows simultaneous two-way communication?
Compare with the explanation
Full-duplex
Full-duplex carries data in both directions at the same time.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What information do packet parts carry, and how are serial/parallel and simplex/duplex different?
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.