Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 10 CMPE 150 Introduction to Computer Networks 1 Announcements No labs next week Except for make up sessions Homework 2 is up Graded homework 1 should be back next week CMPE 150 Introduction to Computer Networks 2 Today Layer 2 Tanenbaum chapter 3 CMPE 150 Introduction to Computer Networks 3 Data Link Layer Application Transport Network DLL PHY CMPE 150 Introduction to Computer Networks 4 What does it do What functions it performs Typically Handling transmission errors a k a error control Flow control Framing CMPE 150 Introduction to Computer Networks 5 Framing CMPE 150 Introduction to Computer Networks 6 DLL and the Stack a Virtual communication b Actual communication CMPE 150 Introduction to Computer Networks 7 DLL Between Routers CMPE 150 Introduction to Computer Networks 8 The DDL and PHY The PHY delivers raw sequence of bits Unreliable service The DLL must detect and in some cases correct errors CMPE 150 Introduction to Computer Networks 9 DLL s Error Control Break bit stream into frames Check if frames arrived correctly If not Discards frame In some cases also request retransmisssion CMPE 150 Introduction to Computer Networks 10 Framing Revisited Not trivial Different methods CMPE 150 Introduction to Computer Networks 11 Framing Using Counters a Without errors b With one error CMPE 150 Introduction to Computer Networks 12 Character Counter Main problem CMPE 150 Introduction to Computer Networks 13 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 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 What if flab bit pattern occurs in data CMPE 150 Introduction to Computer Networks 15 Byte Stuffing a A frame delimited by flag bytes b Four examples of byte sequences before and after stuffing CMPE 150 Introduction to Computer Networks 16 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 CMPE 150 Introduction to Computer Networks 17 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 destuffs CMPE 150 Introduction to Computer Networks 18 Bit Stuffing Example a Original data b Data as they appear on the line c Data after de stuffing CMPE 150 Introduction to Computer Networks 19 Error Control CMPE 150 Introduction to Computer Networks 20 Error Control Reliable delivery Hop by hop Detecting errors Detecting and correcting errors CMPE 150 Introduction to Computer Networks 21 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 CMPE 150 Introduction to Computer Networks 22 Reliable Delivery Timers Retransmission Duplicate detection CMPE 150 Introduction to Computer Networks 23 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 CMPE 150 Introduction to Computer Networks 24
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