CMPE 150 Winter 2009

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CMPE 150 – Winter 2009

Lecture 6

January 22, 2009 y ,

P.E. Mantey

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CMPE 150 -- Introduction to Computer Networks

Instructor: Patrick Mantey [email protected]

htt // d / t /

http://www.soe.ucsc.edu/~mantey/

Office: Engr. 2 Room 595J

Office hours: Tuesday 3-5 PM

TA: Anselm Kia [email protected]

Web site:

http://www.soe.ucsc.edu/classes/cmpe150/Winter09/

Text: Tannenbaum: Computer Networks p

(4^th edition – available in bookstore, etc. )

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Syllabus

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Assignment #3

Available on the web site:

http://www.soe.ucsc.edu/classes/cmpe150/Winter09/

Due Thursday, January 29, 2009

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Today’s Agenda

Link Layer

Services

Framing

kind, seq, ack, info

Error Control (CRC)

Flow Control

Protocols

Standards

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Data Link Layer Design Issues

Provide Services to Network Layer y

1.

Framing

E C l

2.

Error Control

3.

Flow Control

3.

Flow Control

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Cyclic Redundancy Check

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l d d h k

Cyclic Redundancy Check

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Cyclic Redundancy Check

At Transmitter, with M = 1 1 1 0 1 1, compute 2^rM= 1 1 1 0 1 1 0 0 0 with G = 1 1 0 1 2 M 1 1 1 0 1 1 0 0 0 with G 1 1 0 1

T = 2^rM + R [note G starts and ends with “1” ]

Æ R = 1 1 1 Transmit T= 1 1 1 0 1 1 1 1 1

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Cyclic Redundancy Check

At the Receiver, compute:

Note remainder = 0 ÅÆ no errors detected

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Polynomial Code

M = 1 1 1 0 1 1 ÅÆ x⁵ + x⁴ + x³ + x + 1 = M(x)( )

G = 1 1 0 1 ÅÆ x³ + x³ + 1 = G(x)

R(x) = remainder of [2³ M(x)/G(x)]

(recall r 3) (recall r = 3)

T(x) = 2³ M(x) + R(x)

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Error-Detecting g Codes

Calculation of the polynomial code

checksum.

Ref: Tannenbaum, Fig. 3-8

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CRC Codes

CRC-12 G(x) = x¹² + x¹¹ + x³ + x² + x + 1

CRC-15CRC 15 G(x) xG(x) = x¹⁶ + x x¹⁵ + x x² + 1 1

CRC-CCITT G(x) = x¹⁶ + x¹² + x⁵ + 1

CRC-32 G(x) = x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹

0 8 2

+ x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + x + 1

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CRC Performance

Errors go through undetected only if divisible by G(x)

With “suitably chosen” G(x) CRC code detects:

All i l bit

All single-bit errors

All double-bit errors, as long as G(x) has at least three “1s”

Any odd number of error as long as G(x) has a factor (x+1)

Any odd number of error, as long as G(x) has a factor (x+1)

Any burst error for which the length of the burst is less than the length of the G(x) (i.e. “r”)

Most larger burst errors

For equally-probable errors, with burst of length r+1, b bilit f d t t d i 1/2^{r 1}

probability of an undetected error is 1/2^r-1

For longer burst, probability of undetected error is 1/2^r

[adapted from Stallings, Chapter 7, page 205]

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Logic implementation of Logic implementation of

Polynomial Encoder / Decoder

Ref: Stallings, Fig. 7.6

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Ref: Stallings, Fig. 7.6

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Data Link

Connects to (adjacent) computers

(connected by a wire or equivalent) (connected by a wire or equivalent)

Serial -- Bits delivered in order sent

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Data Link Layer - Functions

Provide service interface to the network layer

network layer

Dealing with transmission errors

Regulating data flow

Slow receivers not swamped by fast senders

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Data Link Layer - Functions

Relationship between packets and frames.

frames.

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Services Provided to Network Layer

( ) Vi t l i ti

(a) Virtual communication.

(b) Actual communication.

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Link Layer Services

Unacknowledged Connectionless Service

Acknowledged Connectionless Service

Acknowledged Connection-oriented Service g

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Link Layer Services

Unacknowledged Connectionless Service

Sender just sends no acknowledgments

Sender just sends, no acknowledgments

No attempt to resend lost frames

Acknowledged Connectionless Service

Acknowledged Connection-oriented Service

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Link Layer Services

Unacknowledged Connectionless Service

Sender just sends, no acknowledgmentsj , g

No attempt to resend lost frames

Used for

Real-time traffic (voice video)

Real-time traffic (voice, video)

Highly reliable LANS

When error control is done at higher layers (e.g.

TCP) TCP)

Acknowledged Connectionless Service

Acknowledged Connection-oriented Service

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Link Layer Services

Unacknowledged Connectionless Service

Acknowledged Connectionless Service

No logical connection

Frames individually acknowledged

Tannenbaum says “an option, not a requirement”

BUT:

Acknowledged Connection-oriented Service

Acknowledged Connection oriented Service

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Link Layer Services

Unacknowledged Connectionless Service

Acknowledged Connectionless Service

No logical connection

Frames individually acknowledged

Tannenbaum says “an option, not a requirement”

BUT: large packets become multiple (fixed size) frames

Some frames get through, others lost, then resend of packet is very inefficient vs. resend of lost frames

Acknowledged Connection-oriented Service

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Link Layer Services

Unacknowledged Connectionless Service

Acknowledged Connectionless Service

Acknowledged Connection-oriented Service

Connection established before data sent

Each frame numbered

Data link guarantees error-free delivery

Each frame received once and only once

(vs. connectionless, where a packet can be received several time – if errors or timeouts occur

Provides reliable bit stream to Network Layer

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Connection-Oriented Service

Protocol steps

1 establish connection

1. establish connection

2. reliably deliver data stream

Number frames

Acknowledgments

3. free the connection3 ee t e co ect o

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Link Layer Connection of Routers in WAN Using Connection

in WAN Using Connection- Oriented Service

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Framing

(a) ( )

A frame delimited by flag bytes.y g y

(b) Four examples of byte sequences

before and after stuffing. Ref: Tannenbaum, Fig. 3-5

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Framing: Bit Stuffing

Bit stuffing

(a)(a) The original data.The original data.

(b) The data as they appear on the line.

(c) The data as they are stored in receiver’s memory after destuffing.

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Elementary Data Link Protocols

Manage data flow at Link Layer

A U t i t d Si l P t l

An Unrestricted Simplex Protocol

A Simplex Stop-and-Wait Protocol p p

A Simplex Protocol for a Noisy Channel

Channel

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Protocol Definitions Protocol Definitions

Continued Æ

Some definitions needed in the protocols to follow Some definitions needed in the protocols to follow.

These are located in the file protocol.h.

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Protocol Definitions Definitions

(ctd.) ( )

Some definitions d d i h

needed in the

protocols to follow.

These are located in the file protocol.h.

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Unrestricted Simplex

Protocol

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From Stallings: Data and Computer Communications

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Stop-and-Wait Stop and Wait

• Simplest form of flow control

• Simplest form of flow control.

– Transmitter sends frame and waits.

– Receiver receives frame and sends ACK.

– Transmitter gets ACK, sends other frame,

d it til f t d

and waits, until no more frames to send.

• Good when few frames.

• Problem: inefficient link utilization.

In the case of high data rates or long – In the case of high data rates or long

propagation delays.

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Simplex

St d

Stop-and- Wait

Wait

Protocol

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Animations Animations

• http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/go- back-n/go-back-n.html

http://www.humboldt.edu/%7Eaeb3/telecom/SlidingWindow.html

• http://netbook.cs.purdue.edu/othrpags/page15.htm

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From Stallings: Data and Computer Communications

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A Simplex Protocol for a Noisy

Ch l

Channel

A positi e A positive

acknowledgement with retransmission

protocol.

Continued Æ

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A Simplex Protocol for a Noisy A Simplex Protocol for a Noisy Channel (ctd.)

A positive acknowledgement with retransmission protocol.

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Sliding Window Protocols

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Sliding Window Protocols

Supports bi-directional data transfer

transfer

Full-duplex

Full duplex

“piggy backing” of acks

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Sliding Window Protocols

A One-Bit Sliding Window Protocol

Protocol

A Protocol Using Go Back N

A Protocol Using Selective

Repeat

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Sliding Window Protocols (2) Sliding Window Protocols (2)

• A sliding window of size 1, with a 3-bit sequence number.

• (a) Initially

• (a) Initially.

• (b) After the first frame has been sent.

• (c)( ) After the first frame has been received.

• (d) After the first acknowledgement has been received.

Tannenbaum Figure 3-13

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Sliding Window

Allows multiple frames to be in transit at the same time.

Receiver allocates buffer space for n

frames frames.

Transmitter is allowed to send n (window size) frames without receiving ACK.

Frame sequence number: labels frames

Frame sequence number: labels frames.

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Sliding Window

Receiver ack’s frame by including sequence number of next expected frame

number of next expected frame.

Cumulative ACK : ack’s multiple frames.

E l if i i f 2 3 d 4

Example: if receiver receives frames 2,3, and 4, it sends an ACK with sequence number 5,

hi h k’ i t f 2 3 d 4

which ack’s receipt of 2, 3, and 4.

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Sliding Window

Sender maintains sequence numbers it’s

allowed to send; receiver maintains sequence number it can receive.

These lists are sender and receiver windows.

Sequence numbers are bounded; if frame

reserves k-bit field for sequence numbers, then reserves k bit field for sequence numbers, then they can range from 0 … 2

^k

-1 , modulo 2

^k

.

Transmission window shrinks each time frame is

sent, and grows each time an ACK is received.

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Example: 3-bit sequence number and window size 7

• A (sender) (receiver)( ) ( ) B

• 0 1 2 3 4 5 6 7 0 1 2 3 4... 0 1 2 3 4 5 6 7 0 1 2 3 4 0

0 1

0 1 2 3 4 5 6 7 0 1 2 3 4 2 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 RR3

0 1 2 3 4 5 6 7 0 1 2 3 4

3 4 RR4 5

0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 6 0 1 2 3 4 5 6 7 0 1 2 3 4

Stallings Figure 7-4

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A One-Bit Sliding Window Protocol

Continued Æ

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A One-Bit Sliding Window Protocol ( d )

(ctd.)

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1-bit Sequence Number 1 bit Sequence Number

• Distinguish between “original” transmission and “retransmission”

• Sequence number will do this

• Extra long sequence field wasteful of header / channel

• Minimum size = 1 bit !

• Need to distinguish between frame m and frame m+1Need to distinguish between frame m and frame m+1

• Sender keeps trying to send frame m until it is acknowledged

• Sender won’t send m+1 until m is acknowledged

(S d ’t d 2 til 1 i k l d d hi h

• (Sender won’t send m+2 until m+1 is acknowledged – which means m was correctly received and acknowledged )

• Receiver knows what sequence number to expect next

• Arriving frame with wrong sequence number is rejected as a duplicate

• Correctly received frame – with right sequence number – sent to network layer and expected sequence number incremented by 1 modulo 2.

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O Bit Slidi Wi d P t l One-Bit Sliding Window Protocol

Two scenarios for protocol 4. (a) Normal case. (b) Abnormal

Th t ti i ( k k t b ) A t i k

case. The notation is (seq, ack, packet number). An asterisk indicates where a network layer accepts a packet.