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

2. CMPE 150- 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. CMPE 150- Introduction to Computer Networks 3 Today Layer 2!!!! Tanenbaum chapter 3.

4. CMPE 150- Introduction to Computer Networks 4 Data Link Layer Application Transport Network PHY DLL

5. CMPE 150- Introduction to Computer Networks 5 What does it do? What functions it performs? Typically: –Handling transmission errors, a.k.a., error control. –Flow control. –Framing.

6. CMPE 150- Introduction to Computer Networks 6 Framing

7. CMPE 150- Introduction to Computer Networks 7 DLL and the Stack (a) Virtual communication (b) Actual communication

8. CMPE 150- Introduction to Computer Networks 8 DLL Between Routers

9. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 11 Framing (Revisited) Not trivial. Different methods.

12. CMPE 150- Introduction to Computer Networks 12 Framing: Using Counters (a) Without errors. (b) With one error.

13. CMPE 150- Introduction to Computer Networks 13 Character Counter Main problem?

14. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 16 Byte Stuffing (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

17. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 19 Bit Stuffing: Example (a) Original data. (b) Data as they appear on the line. (c) Data after de-stuffing.

20. CMPE 150- Introduction to Computer Networks 20 Error Control

21. CMPE 150- Introduction to Computer Networks 21 Error Control Reliable delivery. –Hop-by-hop! Detecting errors. Detecting and correcting errors.

22. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 23 Reliable Delivery Timers. Retransmission. Duplicate detection.

24. CMPE 150- Introduction to Computer Networks 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.