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Cambridge IGCSE Computer Science · 0478

Topic 2: Data Transmission — Part 1

Types & Methods of Transmission

Packet-based transmission

Data is broken down into smaller, manageable chunks called packets for efficient transmission. Each packet consists of:

Packet header
Contains control information:
  • Destination address (e.g. IP address) — where the packet is going.
  • Packet number — used to reorder packets at the destination.
  • Originator's address (e.g. IP address) — where the packet came from.
Payload
The actual data being sent.
Trailer
Contains error-checking information (e.g. a checksum) to ensure data integrity.

Packet switching process

  1. Data is broken down into several packets.
  2. Each packet is sent individually across the network.
  3. Routers direct each packet along the most efficient path; packets may take different routes.
  4. Packets may arrive out of order.
  5. The receiving device uses packet numbers to reassemble the data in the correct order once the last packet arrives.
  6. Missing packets are requested for retransmission.
A message split into three numbered packets, traveling via different routers to a destination where they are reassembled into the original order 1, 2, 3
Diagram 1: A message split into three numbered packets, traveling via different routers to a destination where they are reassembled into the original order 1, 2, 3.

Exam Traps

  • Checksum / error checking belongs in the trailer, not the header — the header carries addresses and packet number.
Method Description Advantages Disadvantages
Serial Bits are sent one after another along a single wire. Less expensive (one wire); less susceptible to skew (data arriving at different times). Slower transmission speed.
Parallel Multiple bits are sent at once using multiple wires. Faster data transmission. More expensive; susceptible to skew, especially over long distances.
Simplex Data flows in one direction only (e.g. keyboard to computer). No chance of data collisions. Receiver cannot give feedback or request resend.
Half-duplex Data flows in both directions, but only one at a time (e.g. walkie-talkies). Two-way communication using fewer resources than full-duplex. Slower than full-duplex due to switching time.
Full-duplex Data flows in both directions simultaneously (e.g. phone call). Fast and efficient; no waiting for the line to be free. Most complex and expensive to implement; requires more bandwidth.

Universal Serial Bus (USB)

Definition
A standard interface for connecting peripherals to a computer using serial data transmission.
Transmission process
Data is broken into packets, the host computer initiates communication, and packets are sent serially. The receiver checks the trailer for errors and reassembles them.
Benefits
  • Plug-and-play: easy to use; recognized automatically by the OS.
  • Universal: supported by many modern devices.
  • Power transmission: can charge devices or power peripherals simultaneously with data transfer.
Drawbacks
  • Limited distance: maximum cable length is roughly 5 metres.
  • Physical durability: connectors can be prone to bending or physical damage.

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