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Chapter 7

Chapter 7. Switching, Packets, Frames, Parity, Checksums, and CRCs. Smallest piece of information. When you send a message between two hosts, the smallest piece of information may be: Bit Character (e.g. RS232). Switching. When should this piece of information be sent?

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Chapter 7

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  1. Chapter 7 Switching, Packets, Frames, Parity, Checksums, and CRCs

  2. Smallest piece of information • When you send a message between two hosts, the smallest piece of information may be: • Bit • Character (e.g. RS232)

  3. Switching • When should this piece of information be sent? • As soon as it is available -> circuit switching • When the entire message is ready -> message switching • After X number of bits/characters -> packet switching

  4. Switching cont. Circuit sw. Message sw. Packet switching

  5. Connecting to the Network The Problem • Cannot afford individual network connection per pair of computers • Reasons • Installing wires consumes time and money • Maintaining wires consumes money (esp. long-distance connections)

  6. Solution • Network has • Shared central core • Many attached stations

  7. The Problem With Sharing • Demand high • Some applications have large transfers • Some applications cannot wait • Need mechanism for fairness

  8. Packet Switching Principle • Solution for fairness • Divide data into small units called packets • Allow each station opportunity to send a packet before any station sends another • Form of time-division multiplexing

  9. Illustration of Packet Switching • Acquire shared medium • Send one packet • Allow other stations opportunity to send before sending again

  10. Packet Details • Depend on underlying network • Minimum / maximum size • Format • Hardware packet called a frame

  11. Example Frame Format Used with RS-232 • RS-232 is character-oriented • Special characters • Start of header (soh) • End of text (eot)

  12. When Data Contains Special Characters • Translate to alternative form • Called byte stuffing • Example

  13. Illustration of Frame With Byte Stuffing • Stuffed frame longer than original • Necessary evil

  14. Handling Errors • Data can be corrupted during transmission • Bits lost • Bit values changed • Frame includes additional information to detect / correct error • Set by sender • Checked by receiver • Statistical guarantee

  15. Error Detection And Recovery Techniques • Parity bit • One additional bit per character • Can use • Even parity • Odd parity • Cannot handle error that changes two bits

  16. Error Detection And Recovery Techniques (continued) • Checksum • Treat data as sequence of integers • Compute and send arithmetic sum • Handles multiple bit errors • Cannot handle all errors

  17. Error Detection And Recovery Techniques (continued) • Cyclic Redundancy Check (CRC) • Mathematical function for data • More complex to compute • Handles more errors

  18. Example Checksum Computation • Checksum computed over data • Checksum appended to frame

  19. Illustration Of ErrorsA Checksum Fails to Detect • Second bit reversed in each item • Checksum is the same

  20. Building Blocks For CRC • Exclusive or • Shift register • a shows status before shift • b shows status after shift • Output same as top bit

  21. Example Of CRC Hardware • Computes 16-bit CRC • Registers initialized to zero • Bits of message shifted in • CRC found in registers

  22. Example CRC Computation • Input data is all 1 bits • CRC shown after 15, 16, and 17 bits shifted • Feedback introduces zeroes in CRC

  23. Illustration of Frame Using CRC • CRC covers data only

  24. Summary • Packet technology • Invented to provide fair access in shared network • Sender divides data into small packets • Hardware packets called frames • Can use packet-switching with RS-232 • Special characters delimit beginning and end of frame • Byte-stuffing needed when special characters appear in data

  25. Summary (continued) • To detect data corruption • Sender adds information to packet • Receiver checks • Techniques • Parity bit • Checksum • Cyclic Redundancy Check (CRC) • Provide statistical guarantees

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