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Cambridge IGCSE GCSE Computer Science

Cambridge: Data transmission — Cambridge IGCSE GCSE Computer Science revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 8 minutes to read.

Packets and transmission modes

  • Data is split into packets, each with a header (source and destination addresses, packet number), payload and trailer (error check). Packets may take different routes and are reassembled in order at the destination.
  • Serial transmission sends one bit at a time down a single wire; parallel sends multiple bits simultaneously down several wires but risks skew over distance.
  • Simplex sends in one direction only, half-duplex in both directions but not at once, and full-duplex in both directions simultaneously. USB is serial, uses a standard connector, is plug-and-play and supplies power.

Error detection

  • Parity checks add a bit so the number of 1s is odd or even; they fail when two bits change. Parity blocks use row and column parity to locate the exact bit in error.
  • Checksums calculate a value from the data and compare it after transmission; checksums detect but do not correct errors.
  • Echo check sends the data back to the sender for comparison, and an automatic repeat request (ARQ) uses acknowledgements and timeouts to trigger retransmission.

Key terms

Packet
A unit of data with header, payload and trailer.
Packet switching
Sending packets independently across a network to be reassembled.
Serial transmission
Sending bits one after another on a single channel.
Full-duplex
Data travelling in both directions simultaneously.
Parity bit
Extra bit making the count of 1s odd or even for error checking.
Checksum
A calculated value compared before and after transmission.

Common exam traps

  • Saying encryption stops data being intercepted — it stops it being understood.
  • Claiming parity always detects errors; two flipped bits can cancel out.
  • Confusing half-duplex with full-duplex.

Practice questions with answers

  1. 1. Which part of a packet holds the destination address?

    • • Payload
    • • Header
    • • Trailer
    • • Checksum only

    Answer: Header

    Headers carry addressing and sequence information.

  2. 2. Explain why packets may arrive out of order.

    Answer: Each packet may take a different route depending on network congestion

    Packet numbers allow correct reassembly.

  3. 3. Serial transmission sends:

    • • Several bits at once
    • • One bit at a time
    • • Only text
    • • Only images

    Answer: One bit at a time

    It is reliable over long distances.

  4. 4. Which describes half-duplex?

    • • One direction only
    • • Both directions but not simultaneously
    • • Both directions at once
    • • No transmission

    Answer: Both directions but not simultaneously

    Like a walkie-talkie.

  5. 5. Give two advantages of USB.

    Answer: Plug and play with a standard connector, and it supplies power to devices

    It also detects devices automatically.

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