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Understanding Switching, Packets, Frames, and Error Detection in Networking

This chapter explores fundamental concepts in data transmission, focusing on switching methods and the smallest information units, including bits and frames. It discusses different switching techniques—circuit, message, and packet switching—and highlights the importance of fairness in shared networks. Additionally, it covers packet details, including formats, special characters, byte stuffing, and error handling techniques such as parity bits, checksums, and cyclic redundancy checks (CRCs). These mechanisms are essential for ensuring data integrity during transmission.

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Understanding Switching, Packets, Frames, and Error Detection in Networking

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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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