CMPE 150 – Winter 2009

Lecture 6

January 22, 2009

P.E. Mantey

CMPE 150 -- Introduction to Computer Networks

Instructor: Patrick Mantey mantey@soe.ucsc.edu http://www.soe.ucsc.edu/~mantey/

Office: Engr. 2 Room 595J Office hours: Tuesday 3-5 PM TA: Anselm Kia akia@soe.ucsc.edu Web site: http://www.soe.ucsc.edu/classes/cmpe150/Winter09/

Text: Tannenbaum: Computer Networks (4th edition – available in bookstore, etc. )

Syllabus

Assignment #3

Available on the web site:http://www.soe.ucsc.edu/classes/cmpe150/Winter09/

Due Thursday, January 29, 2009

Today’s Agenda Link Layer

Services Framing

kind, seq, ack, info Error Control (CRC) Flow Control

Protocols Standards

Data Link Layer Design Issues

Provide Services to Network Layer1.Framing2.Error Control3.Flow Control

Cyclic Redundancy Check

Cyclic Redundancy Check

Cyclic Redundancy Check

At Transmitter, with M = 1 1 1 0 1 1, compute

2rM= 1 1 1 0 1 1 0 0 0 with G = 1 1 0 1

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

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

Cyclic Redundancy Check

At the Receiver, compute:

Note remainder = 0 no errors detected

Polynomial Code

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

G = 1 1 0 1 x3 + x3 + 1 = G(x)R(x) = remainder of [2 3 M(x)/G(x)]

(recall r = 3)

T(x) = 23 M(x) + R(x)

Error-Detecting Codes

Calculation of the polynomial code

checksum.

Ref: Tannenbaum, Fig. 3-8

CRC Codes

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

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

CRC-CCITT G(x) = x16 + x12 + x5 + 1

CRC-32 G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x +

1

CRC Performance Errors go through undetected only if divisible by G(x) With “suitably chosen” G(x) CRC code detects:

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 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,

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

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

[adapted from Stallings, Chapter 7, page 205]

Logic implementation of Polynomial Encoder / Decoder

Ref: Stallings, Fig. 7.6

Ref: Stallings, Fig. 7.6

Data Link

Connects to (adjacent) computers(connected by a wire or equivalent)

Serial -- Bits delivered in order sent

Data Link Layer - Functions Provide service interface to the

network layer Dealing with transmission errors Regulating data flow

Slow receivers not swamped by fast senders

Data Link Layer - Functions

Relationship between packets and frames.

Services Provided to Network Layer

(a) Virtual communication. (b) Actual communication.

Link Layer Services

Unacknowledged Connectionless Service

Acknowledged Connectionless Service

Acknowledged Connection-oriented Service

Link Layer Services

Unacknowledged Connectionless Service Sender just sends, no acknowledgments No attempt to resend lost frames

Acknowledged Connectionless Service

Acknowledged Connection-oriented Service

Link Layer Services

Unacknowledged Connectionless Service Sender just sends, no acknowledgments No attempt to resend lost frames Used for

Real-time traffic (voice, video) Highly reliable LANS When error control is done at higher layers (e.g.

TCP) Acknowledged Connectionless Service

Acknowledged Connection-oriented Service

Link Layer Services

Unacknowledged Connectionless Service Sender just sends, no acknowledgments

Acknowledged Connectionless Service No logical connection Frames individually acknowledged Tannenbaum says “an option, not a

requirement” BUT:

Acknowledged Connection-oriented Service

Link Layer Services

Unacknowledged Connectionless Service Sender just sends, no acknowledgments

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

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

Connection-Oriented Service

Protocol steps 1. establish connection 2. reliably deliver data stream

Number frames Acknowledgments

3. free the connection

Link Layer Connection of Routers in WAN Using Connection-

Oriented Service

Ref: Tannenbaum, Fig. 3-3

Framing

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

stuffing.

Ref: Tannenbaum, Fig. 3-5

Framing: Bit Stuffing

Bit stuffing (a) 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.Ref: Tannenbaum, Fig. 3-6

Elementary Data Link Protocols

Manage data flow at Link Layer An Unrestricted Simplex Protocol

A Simplex Stop-and-Wait Protocol

A Simplex Protocol for a Noisy Channel

Protocol Definitions

Continued

Some definitions needed in the protocols to follow. These are located in the file protocol.h.

Protocol Definitions

(ctd.)

Some definitions needed in the

protocols to follow. These are located in the file protocol.h.

Unrestricted Simplex Protocol

From Stallings: Data and Computer Communications

Stop-and-Wait

• Simplest form of flow control.– Transmitter sends frame and waits.– Receiver receives frame and sends ACK.– Transmitter gets ACK, sends other frame, 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 propagation

delays.

Simplex Stop-and-

Wait Protocol

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

From Stallings: Data and Computer Communications

A Simplex Protocol for a Noisy Channel

A positive acknowledgement

with retransmission protocol.

Continued

A Simplex Protocol for a Noisy Channel (ctd.)

A positive acknowledgement with retransmission protocol.

Sliding Window Protocols

Sliding Window Protocols Supports bi-directional data transfer

Full-duplex “piggy backing” of acks

Sliding Window Protocols A One-Bit Sliding Window Protocol

A Protocol Using Go Back N A Protocol Using Selective Repeat

Sliding Window Protocols (2)

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

Sliding Window Allows multiple frames to be in transit at

the same time. Receiver allocates buffer space for n

frames. Transmitter is allowed to send n (window

size) frames without receiving ACK. Frame sequence number: labels frames.

Sliding Window

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

Cumulative ACK: ack’s multiple frames. Example: if receiver receives frames 2,3,

and 4, it sends an ACK with sequence number 5, which ack’s receipt of 2, 3, and 4.

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 they can range from 0 … 2k -1 , modulo 2k.

Transmission window shrinks each time frame is sent, and grows each time an ACK is received.

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

01

20 1 2 3 4 5 6 7 0 1 2 3 4

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

0 1 2 3 4 5 6 7 0 1 2 3 43456

RR40 1 2 3 4 5 6 7 0 1 2 3 40 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 0 1 2 3 4 5 6 7 0 1 2 3 4Stallings Figure 7-4

A One-Bit Sliding Window Protocol

Continued

A One-Bit Sliding Window Protocol (ctd.)

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+1• Sender keeps trying to send frame m until it is acknowledged• Sender won’t send m+1 until m is acknowledged • (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.

One-Bit Sliding Window Protocol

Two scenarios for protocol 4. (a) Normal case. (b) Abnormal case. The notation is (seq, ack, packet number). An asterisk indicates where a network layer accepts a packet. Tannenbaum Figure 3-15

Bit Length of a Link• Assume link is fully occupied

• 1st bit is just arriving at the receiver

• sender has sent continuously

• R = data rate (bps)

• d = distance (length of link) (meters)

• V = propagation velocity (meters / second)

• (~ 2 - 3 x 108 meters/second)

• Bit length = Rd / V bits