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Packets and packet switching Cambridge IGCSE Computer Science revision

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In plain words

A file is not sent across a network in one piece. It is chopped into small chunks called packets, and each packet makes its own way to the destination.

Think of sending a jigsaw through the post, one piece to an envelope. Each envelope carries the address, a piece number and a return address, so the puzzle can be rebuilt at the other end even if the envelopes arrive in the wrong order.

6 things to know

  1. Data is broken down into packets to be transmitted.
  2. A packet has three parts: the packet header, the payload and the trailer.
  3. The header contains the destination address, the packet number and the originator's address.
  4. The payload is the actual data being carried. The trailer marks the end of the packet and holds an error check.
  5. In packet switching, each packet can take a different route across the network. Routers control the route each packet takes.
  6. Packets may arrive out of order. Once the last packet has arrived, they are put back in order using the packet numbers.

Worked example

A photo is sent from a phone to a laptop using packet switching. Describe what happens.

  1. The photo is broken down into packets, and each is given a header with the destination address, a packet number and the sender's address.
  2. Each packet is sent on its own. Routers choose a route for each one, so different packets may go different ways.
  3. The packets may arrive in the wrong order. When the last one has arrived, the laptop uses the packet numbers to put them back in order.

Tips and tricks

  • Learn the three items in the header by heart. "Identify what is in a packet header" is a common three-mark question.
  • Do not say the packets "follow each other". The whole point is that each one can take a different route.
5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Packets and packet switching: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

  1. Which part of a packet holds the actual data being sent?
    • the header
    • the payload (the answer)
    • the trailer
    • the router

    The header and trailer exist to get the payload there safely.

  2. Which of these is stored in a packet header?
    • the payload
    • the sender's password
    • the destination address (the answer)
    • the size of the receiver's screen

    The header holds the destination address, the packet number and the originator's address.

  3. In packet switching, which device controls the route a packet takes?
    • a hard disk
    • a router (the answer)
    • a monitor
    • a keyboard

    Routers pass each packet on towards its destination.

  4. Why might packets arrive out of order?
    • They are sent in a random order.
    • Some packets are larger than others.
    • Each packet can take a different route. (the answer)
    • The receiver asks for them in reverse.

    Routes differ in length and in how busy they are.

  5. How does the receiving device put packets back in the right order?
    • by their size
    • by the packet number in each header (the answer)
    • by the time on its own clock
    • by the sender's address

    The packet number records each packet's place in the sequence.

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