IGCSE Computer Science - Data Transmission and Encryption

Data Packets

Packets are small data chunks for internet transmission, managed by TCP, containing a header, payload, and trailer.

  • Header: Source IP, Destination IP, Payload, Packet number, Error checker.

  • Payload: The actual data.

  • Trailer: Additional error checks, End of packet notification.

Packet switching uses routers to find the best path for each packet, minimizing interference and efficiently retransmitting corrupted packets.

Data Transmission

Wired connections can be serial (one bit at a time) or parallel (multiple bits simultaneously, but skewing can occur).

Transmission modes include:

  • Simplex: Unidirectional.

  • Half-duplex: Bidirectional, not simultaneous.

  • Full-duplex: Bidirectional and simultaneous.

Different combinations vary in speed and cost.

USB (Universal Serial Bus)

USB is an asynchronous, serial method with automatic device detection and driver loading.

Advantages:

  • Automatic device detection.

  • Standardized connections.

  • Supports varied data transmission rates.

Disadvantages:

  • Limited cable length (approximately 5 meters).

  • Older versions have slow transmission rates.

Error Checking

Errors occur when received data differs from expected data, often due to interference.

Common methods:

  • Parity Check: Detects corrupted bits using odd or even numbers of 1s.

  • Checksums: Custom algorithms calculate values on data blocks; mismatch flags errors.

  • Check Digits: Algorithms validating identification numbers.

  • Echocheck: Transmits data back for comparison (unreliable).

  • Automatic Repeat Request (ARQ): Retransmission requested upon error detection.

Encryption

Encryption encodes data to prevent unauthorized access.

Types:

  • Symmetric Encryption: Uses the same key for encryption and decryption.