eec-484/584 computer networks

EEC-484/584Computer NetworksLecture 7

Wenbing Zhao

[email protected](Part of the slides are based on Drs. Kurose & Rosss slides for their Computer Networking book, and on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall)

EEC-484/584: Computer Networks

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OutlineQuiz 1 resultData Link layerData Link Layer Design IssuesFramingError Detection and CorrectionIS&T Tour ScheduledDecember 6 (Wednesday) 2-3pm (MW session)December 7 (Thursday) 4-5pm (TTh session)


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EEC484 Quiz1 ResultHigh 89; Low 65; Average 80Average of Q1 25, Q2 27, Q3 18, Q4 - 11


Chart1

1

2

4

Grade Range

Number of Students

classroster_EEC_484_51

Title

Fall 06 - Class Nbr: 5125

Instructor

Wenbing Zhao

ClassDescrSectionSession

EEC484Computer Networks511

CSUIDLast NameFirst NameStatusPhoneEmail

First sessesion studentsLecture 3Lecture 4Lecture 5Lab1

2222324CriderHasaniEnrolled216/[email protected]

2339416RachedWaelEnrolled440/[email protected]

second session students

[email protected]

2292249FiskeRobertEnrolled440/[email protected]

2313661MatthewSlagleEnrolled330/[email protected]

2370723DavidDresslerEnrolled216/[email protected]

2392069DavidBojackEnrolled440/[email protected]

Quiz 1Q1Q2Q3Q4Total

RichardReed282620108460-691

FiskeRobert24261056570-792

MatthewSlagle28301758080-894

DavidDressler2628201589

DavidBojack2426201989

CriderHasani2228171077

RachedWael2022191576

24.626.617.611.380


Grade Range

Number of Students

*

EEC584 Quiz1 Result(MW Session)High 88, low 68, average 77Average of Q1 24, Q2 24, Q3 19, Q4 - 11


Chart1

2

6

6

Grade Range

Number of Students


Title


Instructor

Wenbing Zhao




2016451KassemSoufan216/[email protected]

2274861MrunalParikh440/[email protected]

2416269SreedeviGatiganti216/[email protected]

2418299SujanaSiruvolu614/[email protected]

2426371NiranjanKodati216/[email protected]

(withdraw) 2428739Vinay KumarChitla216/[email protected]

2429059HardikBhavsar216/[email protected]

2431658MalavShah216/[email protected]

2438958Vikas ReddyRamalingam216/[email protected]

2443159MaulikkumarGajjar(792) [email protected]

2444940Vamsi KrishnaNagalla216/[email protected]

2445263DhanashreeBora216/[email protected]

2445934AnuroopKatta732/[email protected]

2446009RavinderBellam216/[email protected]

2446290UchitShah216/[email protected]

2447451KetanPatel216/[email protected]

2448923Venkata DineshManne(944) [email protected]

2454215TirupathiMangalarapu(402) [email protected]

2460747SritambariThakur216/[email protected]

2449105RajamaniBhadriraju216/[email protected]

2451943SalamehElieEnrolled440/[email protected]

Session #2 students

2403255SandeepChittoor Krishna Reddy216/[email protected]

2415675MADHUKAR REDDYTHUMMA216/[email protected]

2423983HongleiZhangNo [email protected]

2435899IshaVedire216/[email protected]

2446092Sai KiranEde(402) [email protected]

2446506ShoebKhaja216/[email protected]

2446699Vinay DeepAyinapurapu(891) [email protected]

2446886SongCui(334) [email protected]

2447079SuryaAllena978/[email protected]

2447546Anil ReddyImmadi404/[email protected]

[email protected]

2448605AngaiSalankar404/[email protected]

2451221PavanKambhatla440/[email protected]

Attendance (session 1)+A77HomeworkLecture 3Lecture 4Lecture 5Lab1Lecture 6Lab2

2445934KattaAnuroop111111

2445263BoraDhanashree111family emergency10

2429059BhavsarHardik111111

2016451SoufanKassem111111

2447451PatelKetan111111

2431658ShahMalav111111

2443159GajjarMaulikkumar111111

2274861ParikhMrunal111111

2426371KodatiNiranjan111111

2449105BhadrirajuRajamani111111

2446009BellamRavinder111111

2416269GatigantiSreedevi111111

2460747ThakurSritambari11111

2418299SiruvoluSujana111111

2446290ShahUchit111111

2444940NagallaVamsi Krishna111111

2448923ManneVenkata Dinesh111111

2438958RamalingamVikas Reddy111111

(late registeration 2wks)2451943SalamehElie111

Attendance (session 2)HomeworkLecture 3Lecture 4Lecture 5Lab1Lecture 6quiz 1

2448605SalankarAngai111111

2447546ImmadiAnil Reddy111111

2448086ChenBo111111

2423983ZhangHonglei111111

2435899VedireIsha111111

2451221KambhatlaPavan111111

2403255Chittoor Krishna ReddySandeep111111

2446506KhajaShoeb111111

2446886CuiSong111111

2447079AllenaSurya111111

2446699AyinapurapuVinay Deep111111

2446092Sai KiranEde101111

MangalarapuTirupathi111111

THUMMAMADHUKAR REDDY111111

Quiz 1Q1Q2Q3Q4Total

SalankarAngai24241915826860-692

ImmadiAnil Reddy2026195706870-796

ChenBo24262010807080-896

ZhangHonglei283020108870

VedireIsha242619158471

KambhatlaPavan242019157876

Chittoor Krishna ReddySandeep242219208578

KhajaShoeb24241757078

CuiSong24201956880

AllenaSurya20261756882

AyinapurapuVinay Deep222619158282

Sai KiranEde242019157884



23.624.318.610.777.1

KattaAnuroop222820209098

BoraDhanashree9660-691

BhavsarHardik26241020809670-791

SoufanKassem28282020969480-894

PatelKetan22302020929490-10012

ShahMalav243020209493

GajjarMaulikkumar20202036392

ParikhMrunal242620209092

KodatiNiranjan242820209291

BhadrirajuRajamani222820209090

BellamRavinder283020209890

GatigantiSreedevi282820209690

ThakurSritambari223019209189

SiruvoluSujana262819209387

ShahUchit262018208484

NagallaVamsi Krishna203015107580

ManneVenkata Dinesh203019208975

RamalingamVikas Reddy243020209463

SalamehElie2426172087

24.027.418.618.488.5


Grade Range

Number of Students

Grade Range

Number of Students

*

EEC584 Quiz1 Result(TTh Session)High 98, low 63, average 88Average of Q1 24, Q2 27, Q3 19, Q4 - 18


Chart2

1

1

4

12

Grade Range

Number of Students


Title


Instructor

Wenbing Zhao




2016451KassemSoufan216/[email protected]

2274861MrunalParikh440/[email protected]

2416269SreedeviGatiganti216/[email protected]

2418299SujanaSiruvolu614/[email protected]

2426371NiranjanKodati216/[email protected]

(withdraw) 2428739Vinay KumarChitla216/[email protected]

2429059HardikBhavsar216/[email protected]

2431658MalavShah216/[email protected]

2438958Vikas ReddyRamalingam216/[email protected]

2443159MaulikkumarGajjar(792) [email protected]

2444940Vamsi KrishnaNagalla216/[email protected]

2445263DhanashreeBora216/[email protected]

2445934AnuroopKatta732/[email protected]

2446009RavinderBellam216/[email protected]

2446290UchitShah216/[email protected]

2447451KetanPatel216/[email protected]

2448923Venkata DineshManne(944) [email protected]

2454215TirupathiMangalarapu(402) [email protected]

2460747SritambariThakur216/[email protected]

2449105RajamaniBhadriraju216/[email protected]

2451943SalamehElieEnrolled440/[email protected]

Session #2 students

2403255SandeepChittoor Krishna Reddy216/[email protected]

2415675MADHUKAR REDDYTHUMMA216/[email protected]

2423983HongleiZhangNo [email protected]

2435899IshaVedire216/[email protected]

2446092Sai KiranEde(402) [email protected]

2446506ShoebKhaja216/[email protected]

2446699Vinay DeepAyinapurapu(891) [email protected]

2446886SongCui(334) [email protected]

2447079SuryaAllena978/[email protected]

2447546Anil ReddyImmadi404/[email protected]

[email protected]

2448605AngaiSalankar404/[email protected]

2451221PavanKambhatla440/[email protected]

Attendance (session 1)+A77HomeworkLecture 3Lecture 4Lecture 5Lab1Lecture 6Lab2

2445934KattaAnuroop111111

2445263BoraDhanashree111family emergency10

2429059BhavsarHardik111111

2016451SoufanKassem111111

2447451PatelKetan111111

2431658ShahMalav111111

2443159GajjarMaulikkumar111111

2274861ParikhMrunal111111

2426371KodatiNiranjan111111

2449105BhadrirajuRajamani111111

2446009BellamRavinder111111

2416269GatigantiSreedevi111111

2460747ThakurSritambari11111

2418299SiruvoluSujana111111

2446290ShahUchit111111

2444940NagallaVamsi Krishna111111

2448923ManneVenkata Dinesh111111

2438958RamalingamVikas Reddy111111

(late registeration 2wks)2451943SalamehElie111

Attendance (session 2)HomeworkLecture 3Lecture 4Lecture 5Lab1Lecture 6quiz 1

2448605SalankarAngai111111

2447546ImmadiAnil Reddy111111

2448086ChenBo111111

2423983ZhangHonglei111111

2435899VedireIsha111111

2451221KambhatlaPavan111111

2403255Chittoor Krishna ReddySandeep111111

2446506KhajaShoeb111111

2446886CuiSong111111

2447079AllenaSurya111111

2446699AyinapurapuVinay Deep111111

2446092Sai KiranEde101111



Quiz 1Q1Q2Q3Q4Total

SalankarAngai242419208760-693

ImmadiAnil Reddy20261906570-792

ChenBo242620108080-899

ZhangHonglei2830201088

VedireIsha2426191887

KambhatlaPavan2420192083

Chittoor Krishna ReddySandeep2422191580

KhajaShoeb242417570

CuiSong242019568

AllenaSurya202617063

AyinapurapuVinay Deep2226192087

Sai KiranEde2420192083



23.624.318.611.678.1

KattaAnuroop222820209098

BoraDhanashree9660-691

BhavsarHardik26241020809670-791

SoufanKassem28282020969480-894

PatelKetan22302020929490-10012

ShahMalav243020209493

GajjarMaulikkumar20202036392

ParikhMrunal242620209092

KodatiNiranjan242820209291

BhadrirajuRajamani222820209090

BellamRavinder283020209890

GatigantiSreedevi282820209690

ThakurSritambari223019209189

SiruvoluSujana262819209387

ShahUchit262018208484

NagallaVamsi Krishna203015107580

ManneVenkata Dinesh203019208975

RamalingamVikas Reddy243020209463

SalamehElie2426172087

24.027.418.618.488.5


Grade Range

Number of Students

Grade Range

Number of Students

*

Data Link Layer Design IssuesServices Provided to the Network LayerPoint-to-point, source-to-destinationFramingPhysical bit stream divided up into framesError ControlAcknowledgements (acks) and negative acks (nacks)Sender has timer within timeout get ack, or send againIf frame transmitted multiple times, receiver may get several copies - Use sequence numbers


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Data Link Layer Design IssuesFlow ControlSender may transmit frames faster than receiver can receive themThrottle sender so sends no faster than receiver can receive them


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Functions of Data Link LayerProvide service interface to the network layerVirtual source-to-destination communication channelDealing with transmission errorsRegulating data flowSlow receivers not swamped by fast senders


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Services Provided to Network Layer(a) Virtual communication(b) Actual communication


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Placement of Data Link Protocol


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Types of Services Provided to Network LayerUnacked connectionlessOk if low error rate, real time applicationsAcked connectionlessUnacked connection-orientedAcked connection-oriented


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FramingDL layer divides physical bit stream into framesChecksum computed by source included in frameChecksum recomputed by destination and checked against checksum included in the frameDiscard/recover bad frame, notify source


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How Does DL Layer Form Frames?Insert time gaps between frames: too riskyCharacter countFlag bytes with byte stuffingStarting and ending flags, with bit stuffingPhysical layer coding violationsExample: encode 1 bit with 2 bits1 => 100 => 01Can use 00 or 11 to delimit frames


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Framing Based on Character CountUse field in header to indicate number of characters in frame

Count might be garbled, not used


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Framing Based on Byte StuffingEach frame starts and ends with a special flag byteProblem: flag byte might appear in dataSolution: Source inserts ESC (DL escape) before each flag byte; ESC before each ESCDestination removes inserted ESC bytesDisadvantage: depends on 8-bits characters in ASCII


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Framing Based on Byte Stuffing


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Framing Based on Bit StuffingEach frame begins and ends with special bit patterns, 01111110 (in fact, a flag byte)When sources data contains 11111, stuff 0 When destination receives 111110, deletes 0Advantages: Allows arbitrary number of bits per frameAllows arbitrary number of bits per character


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Framing Based on Bit Stuffing: ExampleOriginal Frame (payload only)Frame after bit stuffingReceived Frame after taken stuffed bit out


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Error Detection and CorrectionCauses of errorsTransmission errors on phone lines due to thermal noiseData transmission errors due to impulse noiseSignals are separated, distorted, recombinedCrosstalk between physically adjacent wiresCompression and decompressionReceiver out of synch with senderErrors usually occur in bursts


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Error-Correcting Codesn-bit codeword an n-bit unit containing data and check bits m bits of data, r bits redundant/check bits (n = m+r)How to measure the differences between two codewords (num of different bits)Using exclusive OR and counting number of 1 bits in the result


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Error-Correcting CodesHamming distance number of bit positions in which two codewords differIf two codewords are a Hamming distance d apart, it will require d single-bit errors to convert one into the other


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Error-Correcting CodesComplete codeComplete list of all legal codewords: 2m possible data messagesRecall that there are m bits of dataHamming distance of the complete codeFind two codewords whose Hamming distance is minimum


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Error-Detection CodesA distance d+1 code can detect up to d errors, why?If there are d+1 errors, one valid codeword might be turned into another valid codeword d errors will change a valid codeword into an illegal codeword can be detected!


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Error-Correcting CodesTo correct d errors, need a distance 2d+1 codeLegal codewords are so far part that even with d changes, original codeword is still closer than any other codeword, so it can be uniquely determined


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Error-Correcting Codes: ExampleConsider a code with only four valid codewords0000000000, 0000011111, 1111100000, 1111111111This code has a distance 5 can correct double errorsIf 0000000111 arrives, receiver knows the original must have been 0000011111However, if triple error changes 0000000000 to 0000000111, the error will not be corrected properly


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Parity BitParity bit a single bit is appended to the dataParity bit is chosen so that number of 1 bits in the codeword is even or oddExample: Given 1011010With even parity 10110100With odd parity 10110101A code with a single parity bit has a distance 2Since any single-bit error produces a codeword with wrong parity can be used to detect single bit errors


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Error-Detecting CodesIf a single parity bit is appended to a block, error detecting probability is only 0.5 if burst error occurs (why?)This can be improved by treating a block as a matrix, n bits wide and k bits high, A parity bit is computed for each column and affixed to the matrix as the last rowThe matrix is transmitted one row at a timeProbability of accepting bad block is 2-n


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Error-Detecting Codes: CRCPolynomial code, also known as CRC (Cyclic Redundant Code)Treat bit string as polynomial with 0 and 1 coefficientsm-bit frame: M(x) = bm-1xm-1 + + b0E.g.: 11011010 => M(x) = x7 + x6 + x4 + x3 + x1Use modulo 2 arithmeticNo carries or borrows: XOR


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Cyclic Redundant CodeSender and receiver agree on generator polynomial G(x) (High & low order bits must be 1)For a frame with m bits corresponding to M(x), m > deg G(x) = rAppend checksum to end of frame so polynomial T(x) corresponding to checksummed frame is divisible by G(x)When receiver gets checksummed frame, divides T(x) by G(x)If remainder R(x) != 0, then transmission error


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Algorithm to Compute CRC ChecksumLet m = deg M(x), r = deg G(x)Append r 0 bits to lower-order end of frame: xrM(x)Divide bit string corresponding to xrM(x) by bit string corresponding to G(x)Subtract remainder R(x) from bit string corresponding to xrM(x), result is checksummed frame. Let T(x) be its polynomialxrM(x) = Q(x)G(x) + R(x)xrM(x) R(x) = Q(x)G(x) = T(x)


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Compute CRC ChecksumXOR


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International Standard PolynomialsCRC-12 G(x) = x12 + x11 + x3 + x2 + x1 + 1Used for 6-bit charactersCRC-16 G(x) = x16 + x15 + x2 + 1 CRC-CCITTG(x) = x16 + x12 + x5 + 1Used for 8-bit charactersCRC-32G(x) = x32 + x26 + x23 + x22 + x16 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1Used in IEEE 802Detects all bursts of length 32 or less and all bursts affecting an odd number of bits


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Optional HomeworkQ1. The following character encoding is used in a data link protocol: A: 01000111; B: 11100011; FLAG: 01111110; ESC: 11100000 Show the bit sequence transmitted (in binary) for the four-character frame: A B ESC FLAG when each of the following framing methods are used:(a) Character count.(b) Flag bytes with byte stuffing.(c) Starting and ending flag bytes, with bit stuffing


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Optional HomeworkQ2. The following data fragment occurs in the middle of a data stream for which the byte-stuffing algorithm described in the text is used: A B ESC C ESC FLAG FLAG D. What is the output after stuffing?Q3. A bit string, 0111101111101111110, needs to be transmitted at the data link layer. What is the string actually transmitted after bit stuffing?


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Optional HomeworkQ4. One of your classmates, Scrooge, has pointed out that it is wasteful to end each frame with a flag byte and then begin the next one with a second flag byte. One flag byte could do the job as well, and a byte saved is a byte earned. Do you agree?


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Optional HomeworkQ5. To provide more reliability than a single parity bit can give, an error-detecting coding scheme uses one parity bit for checking all the odd-numbered bits and a second parity bit for all the even-numbered bits. What is the Hamming distance of this code?


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Optional HomeworkQ6. A bit stream 10011101 is transmitted using the standard CRC method described in the text. The generator polynomial is x3 + 1. Show the actual bit string transmitted. Suppose the third bit from the left is inverted during transmission. Show that this error is detected at the receiver's end.


When frame arrives at routerhardware verifies checksumPasses frame to DL layer softwareDL layer software checks if frame is one expectedIf so, gives packet in payload of frame to routing softwareRouting software chooses appropriate outgoing line passes packet back down to DL layer softwareDL layer software transmits it

, but not all 2n possible codewords are used

No need to remember these generator polynomials

eec-484/584 computer networks

Documents

computer networkslecture

average of q1

computer networking

class nbr

eec484 quiz1 resulthigh

kurose rosss slides

pm tth sessioneec

pm mw sessiondecember