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Chapter 7 Packets, Frames, and Error Detection

Chapter 7 Packets, Frames, and Error Detection. Packet small block of data eg. Ethernet limits to 1500 bytes of data per data packet ( does not include 18 bytes frame header and 8 byte preamble) Why packets? minimize retransmission fair access to shared medium via TDM (fig 7.2)

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Chapter 7 Packets, Frames, and Error Detection

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  1. Chapter 7 Packets, Frames, and Error Detection • Packet • small block of data eg. Ethernet limits to 1500 bytes of data per data packet ( does not include 18 bytes frame header and 8 byte preamble) • Why packets? • minimize retransmission • fair access to shared medium via TDM (fig 7.2) • minimize delay

  2. Frames • Frame format • definition of a packet used with specific type of network eg Ethernet frame vs. Token Ring frame • Frames begin with a header followed by data and a trailer (fig 7.3)

  3. Data stuffing • Inserting extra bits or bytes in data in order to distinguish between data being sent and control information such as frame delimiters (fig 7.4)(fig 7.5) • byte stuffing used in character-oriented hardware eg. PPP • bit stuffing used in bit-oriented hardware eg frame relay

  4. Transmission errors • Interference can cause random data to appear or transmitted data to be lost or changed. • Need for error detection

  5. Error Detection Techniques • Parity Check • involves use of a parity bit for each character being sent • Even/odd parity • allows detection of a change in one bit • cannot detect transmission errors that change an even number of bits. • Checksum • 16-bit checksum generated by adding 16-bit data and carry bits together (fig 7.6) • can fail to detect transmission errors (fig 7.7) • Cyclic redundancy checks • Detects more errors than checksum such as burst errors. • Uses exclusive or (fig 7.8) and shift registers (fig 7.9)(fig 7.10)

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