CMPE 150 Fall 2005 Lecture 10

1CMPE 150- Introduction to Computer Networks

CMPE 150

Fall 2005Lecture 10

Introduction to Computer Networks

2CMPE 150- 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.

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

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Data Link Layer

Application

Transport

Network

PHY

DLL

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What does it do?• What functions it performs?• Typically:– Handling transmission errors, a.k.a., error

control.– Flow control.– Framing.

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Framing

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DLL and the Stack

• (a) Virtual communication• (b) Actual communication

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DLL Between Routers

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The DDL and PHY• The PHY delivers raw sequence of bits.– Unreliable service.

• The DLL must detect and, in some cases, correct errors.

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

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

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Framing: Using Counters

(a) Without errors. (b) With one error.

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Character Counter• Main problem?

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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?

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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?

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Byte Stuffing

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

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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?

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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.

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Bit Stuffing: Example

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

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Error Control

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Error Control• Reliable delivery.– Hop-by-hop!

• Detecting errors.• Detecting and correcting errors.

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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?

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

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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.