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CMPE 150 Fall 2005 Lecture 10

CMPE 150 Fall 2005 Lecture 10. Introduction to Computer Networks . Announcements. No labs next week! Except for make-up sessions. Homework 2 is up. Graded homework 1 should be back next week. Today. Layer 2!!!! Tanenbaum chapter 3. Data Link Layer. Application. Transport. Network.

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CMPE 150 Fall 2005 Lecture 10

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  1. CMPE 150Fall 2005Lecture 10 Introduction to Computer Networks

  2. Announcements • No labs next week! • Except for make-up sessions. • Homework 2 is up. • Graded homework 1 should be back next week.

  3. Today • Layer 2!!!! • Tanenbaum chapter 3.

  4. Data Link Layer Application Transport Network DLL PHY

  5. What does it do? • What functions it performs? • Typically: • Handling transmission errors, a.k.a., error control. • Flow control. • Framing.

  6. Framing

  7. DLL and the Stack • (a) Virtual communication • (b) Actual communication

  8. DLL Between Routers

  9. The DDL and PHY • The PHY delivers raw sequence of bits. • Unreliable service. • The DLL must detect and, in some cases, correct errors.

  10. DLL’s Error Control • Break bit stream into frames. • Check if frames arrived correctly. • If not: • Discards frame. • In some cases also request retransmisssion.

  11. Framing (Revisited) • Not trivial. • Different methods.

  12. Framing: Using Counters (a) Without errors. (b) With one error.

  13. Character Counter • Main problem?

  14. Framing: Flag Byte • Each frame starts and ends with special bytes: flag bytes. • Two consecutive flag bytes indicate end of frame and beginning on new frame. • Problem?

  15. Framing: Flag Byte • Each frame starts and ends with special bytes: flag bytes. • Two consecutive flag bytes indicate end of frame and beginning on new frame. • Problem? • What if flab bit pattern occurs in data?

  16. Byte Stuffing • (a) A frame delimited by flag bytes. • (b) Four examples of byte sequences before and after stuffing.

  17. Byte Stuffing (Cont’d) • Single ESC: part of the escape sequence. • Doubled ESC: single ESC is part of data. • De-stuffing. • Problem: • What if character encoding does not use 8-bit characters?

  18. Bit Stuffing • Allows character codes with arbitrary bits per character. • Each frames begins and ends with special pattern. • Example: 01111110. • When sender’s DLL finds 5 consecutive 1’s in data stream, stuffs 0. • When receiver sees 5 1’s followed by 0, de-stuffs.

  19. Bit Stuffing: Example • (a) Original data. • (b) Data as they appear on the line. • (c) Data after de-stuffing.

  20. Error Control

  21. Error Control • Reliable delivery. • Hop-by-hop! • Detecting errors. • Detecting and correcting errors.

  22. Acknowledgments • Special control info (in the case of the DLL, control frame) acknowledging receipt of data. • Positive and negative ACKs. • ACKs. • NACKs. • Are ACKs sufficient?

  23. Reliable Delivery • Timers. • Retransmission. • Duplicate detection.

  24. Flow Control • Handles mismatch between sender’s and receiver’s speed. • Receiver’s buffer limitation. • Feedback-based flow control. • Explicit permission from receiver. • Rate-based flow control. • Implicit mechanism for limiting sending rate. • DLL typically uses feedback-based flow control.

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