Ghep Report 3.6

8/22/2019 Ghep Report 3.6

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I HC QUC GIA TP.H CH MINHTRNG I HC BCH KHOA

KHOA ININ T

CHNG TRNH O TO KS CHT LNG CAOVIT PHPPFIEV

LUN VN TT NGHIP

KHO ST V CI THIN CHT LNG

GIAO THC TCP TRONG MI TRNG

WIRELESS LAN IEEE 802.11

GVHD: ThS. TTR NGHA

SVTH: HONG GIA MINHMSSV 20801248

NGUYN NH KHANH MSSV 20800918

TP. H CH MINH, THNG 06 NM 2013


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Li cm n GVHD: Ths. TTr Ngha

i

LI CM N

Chng ti mun by t lng bit n su sc v chn thnh ti ging vin hng dn ca

chng ti, Thc s TTr Ngha v nhng nh hng ca thy v c bit l nhng khch lv tinh thn chng ti chn ra hng i ny v c n lc thc hin lun vn ny. Chng ti

b qu ti v nhiu yu ttc ng, c bit l khong thi gian thc tp tt nghip song hnh

cng lun vn. Vi nhng tng v cc nghiu chnh ca thy gip chng ti ci

thin ni dung v hnh thc lun vn rt nhiu.

Chng ti cng mun gi li cm n c bit ti Gio s Ljiljana Trajkovic, khoa Khoa Hc

ng Dng, i hc Tng hp Simon Fraser, Canada. Chnh s nhit tnh v chn thnh gip

ca b i vi hai sinh vin n t Vit Nam chng ti, c bit l qua th in t, lm

xoay chuyn rt ln qu trnh hin thc giao thc Snoop ca chng ti.

Chng ti cm nhn su sc v tnh yu m ba m ca chng ti dnh cho mnh trong qu

trnh hon thnh lun vn, i vi chng ti v c bn h, lun vn ny l mt ct mc quan

trong cho cuc i mnh. Li thm hi hng tun l ng lc cho chng ti vt qua nhng

p lc v tinh thn v trch nhim ca mnh i vi lun vn ny.

Cui cng, chng ti cm n nhng ngi bn trong din n NS quc t gip chng

ti v cch thc tm hiu v lp trnh OTcl, d rng h khng thy c nhng dng ny.

Tp. H Ch Minh, ngy 15 thng 6nm 2013.

Sinh vin

Hong Gia Minh

Nguyn nh Khanh


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Lun vn tt nghip GVHD: Ths. T TrNgha

ii

TM TT LUN VN

Giao thc iu khin truyn dn (TCP) l giao thc iu khin giao vn cho trn Internet

v h trnhiu ng dng trn Internet, nh World Wide Web (WWW), truyn ti tp tin, thin t Gii thut kim sot nghn tng lng nghebng thng mng v c tnh tr

hon ca mt mng ng thi c gng thch nghi hiu nng ca n theo sthay i trong lu

lng mng cng nh trnh sp mng.

TCP c thit k hot ng trong mng hu tuyn truyn thng vi gi thit rng mt gi

d liu ch yu l do nghn mng v t l li bit (BER) ngu nhin l thp. Tuy nhin, nhng

thc th mng vi lin kt v tuyn li hng chu vic mt gi d liu do BER l rt cao v do

chuyn giao kt ni, cc hin tng thng tn ti trong mng v tuyn. Do , TCP trong

mng v tuyn hot ng rt km hiu qu v tc truyn ti rt chm, hoc lm cho pha

ngun pht phi ti truyn khng cn thit, xa hn na l gim hiu nng ca mng.

Rt nhiu loi gii php c ngh ci thin hiu nng TCP trn mng v tuyn. Vic

cy ghp mt proxy tng cng hiu nng (PEP), nh giao thc Snoop, l mt trong nhng

phng php tip cn mt TCP mnh mhn trn mng khng dy.

Mc tiu ca lun vn ny l nghin cu cch m giao thc Snoop ci thin hiu nng TCP

trn mng v tuyn nh th no, c th l IEEE 802.11b. Giao thc Snoop c chng ti xy

dng trn phn mm m phng NS2 c pht trin bi UC Berkely. Nhng nhng kho st

v c ch hot ng ca TCP v 802.11 c chng ti tin hnh trc bng cng c m

phng mng OPNET Modeler 14.5. Tn dng OPNET tin hnh m phng cch thc hot

ng c bn ca TCP trn chun 802.11 mang li nhng li thvt tri hn so vi NS2 v

gip chng ti hiu r v TCP v 802.11 nhanh hn trc khi bt tay vo xy dng m hnh

Snoop trong NS2. Cui cng, nhng nh gi kt qu m phng giao thc Snoop c tinhnh song song vi trnh by ra nhng im cn hn ch ca lun vn trong vic xy dng m

hnh ny. Vi thi gian chng ti c c, hng ci tin ca Snoop cng c chng ti

xem xt ti cho cc hng i tip theo ca mi c nhn trong tng lai.


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iii

DANH SCH THUT NGVIT TT

ACK Acknowledgement

AIMD Additive-Increase, Multiplicative-DecreaseAIRMAIL Asymmetric Reliable Mobile Access In Link-Layer

AP Access Point

ARQ Automatic Repeat Request

A-TCP Adaptive TCP

ATM Asynchronous Transfer Mode

BER Bit Error Rate

BS Base Station

BSS Basic Service Set

BW Bandwidth

CA Congestion Avoidance

CID Connection ID

CE Congestion Experienced

CFP Contention Free Period

CP

CRC

Contention Period

Cyclic Redundancy CheckCSMA/CA Carrier Sense Multiple Access with Collision Avoidance

CTS Clear To Send

CWND Congestion Window

CWR Congestion Window Reduction

DCF Distributed Coordination Function

DIFS

DS

Distributed Interframe Space

Direct Sequence

DS Distributed System

E2E End-To-End

ECE Ecn-Echo

ECN Explicit Congestion Notification

ELN Explicit Loss Notification

ELN-ACK Explicit Loss Notification with Acknowledgement

ESS Extended Service Set

ELNR Explicit Loss Notification To ReceiverFACK Forward Acknowledgment


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iv

FEC Forward Error Control

FER Packet Error Rate

FH Fixed Host

FR Fast RetransmitFTP File Transfer Protocol

GBN Go-Back-N

HL Higher Layer

HTTP Hypertext Transfer Protocol

IEEE Institute Of Electronics And Electrical Engineers

IMCP Internet Control Message Protocol

IP Internet Protocol

ISM Industrial, Scientific, Medical

I-TCP Indirect TCP

LAN Local Area Network

MAC Medium Access Control

METP Mobile-End Transport Protocol

MH Mobile Host

MIMO Multi-Input and Multi-Output

MSDU MAC Service Data UnitMSS Maximum Segment Size

MTU Maximum Transfer Unit

NAK Negative Acknowledgement

NAV Networl Allocation Vector

NS Network Simulator

OFDM Orthogonal Frequency Division Multiplexing

OSI Open Systems Interconnection Reference Model

PCF Point Coordination Function

PEP Performance Enhancement Proxy

PIFS PCF Interframce Space

PHY Physical

PPP Point To Point

QoS Quality of Service

RED Random Early Detection

RF Radio Frequency


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v

RFC Request For Comment

RLP Radio Link Control

RTO Retransmit Timeout

RTS Request To SendRTT Round Trip Time

SACK Selective Acknowledgement

SIFS Short Interframce Space

SCMTP Split-Connection Mobile Transport Protocol

SMART Simple Method To Aid Retransmission

SPR Selective Repeat Protocol

SS Slow-Start

SSTHRESH Slow-Start Threshold

SYN Synchronization

TCP

TDM

Transmission Control Protocol

Time Division Multiplexing

TDMA Time-Division Multiple Access

TULIP Transport Unaware Link Improvement Protocol

UDP User Datagram Prototol

UMTS Wireless Asynchronous Transfer ModeWATM Universal Mobile Telecommunications System

WLAN Wireless Local Area Network

WEP Wired Equivalent Privacy

WTCP Wireless Transmission Control Protocol

ZWA Zero Window Advertisement


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vi

MC LC

CHNG 1. GII THIU ..................................................................................................................... 1

1.1 t vn v ng lc cho hng i.................................................................................. 1

1.2 Mc tiu ............................................................................................................................. 1

1.3 Cc cng vic lin quan ....................................................................................................... 2

1.4 Phng php v ni dung ................................................................................................... 2

CHNG 2. GIAO THC LP VN CHUYN TCP ................................................................................. 5

2.1 Qu trnh pht trin ca TCP ............................................................................................... 5

2.2 Cc c trng ca giao thc TCP ......................................................................................... 7

2.2.1 TCP l mt giao thc nh hng kt ni .................................................................... 7

2.2.2 TCP l mt giao thc truyn tin tin cy ........................................................................ 8

2.2.3 TCP l mt giao thc nh hng byte ........................................................................ 9

2.3 nh ng gi tin TCP.......................................................................................................... 9

2.4 C chkim sot lung ca TCP .........................................................................................11

2.5 C ch tiot c TCP .....................................................................................................13

2.5.1 C ch tiot c TCP ngn gc v ci tin c cononr ...........................14

2.5.2 Gii tht ca Karn.....................................................................................................14

2.6 C chkim sot nghn ca TCP ........................................................................................15

2.6.1 Slow-start ..................................................................................................................15

2.6.2 Congestion avoidance (CA) .........................................................................................16

2.6.3 Fast recovery .............................................................................................................17

2.7 TCP Tahoe .........................................................................................................................18

2.8 TCP Reno ...........................................................................................................................19

2.9 TCP New Reno ...................................................................................................................20

2.10 TCP Selective Acknowledgement .......................................................................................22

2.11 TCP Forward Acknowledgement ........................................................................................23

2.12 TCP Vegas ..........................................................................................................................24


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2.13 nh gi hi nng TCP .....................................................................................................25

2.13.1 Gii tht c Ng...................................................................................................26

2.14 TCP cho knh truyn v tuyn ...........................................................................................27

2.15 t n .............................................................................................................................27

CHNG 3. HI QUT IEEE 802.11 .................................................................................................28

3.1 Tng qut v WLAN ...........................................................................................................28

3.2 Kin trc ca chun 802.11 ................................................................................................29

3.3 Giao thc MAC ca 802.11 ................................................................................................31

3.4 Khung d liu IEEE 802.11 .................................................................................................33

CHNG 4. M PHNG NH GI HOT NG C BN CA TCP, TCP TRN 802.11 V CC CUHNH HOT NG CA 802.11 .........................................................................................................35

A. M PHNG NH GI C CHQUN L NGHN CA TCP TRONG OPNET MODELER 14.5 .............35

4.1 Thit lp m phng ............................................................................................................36

4.1.1 M hnh m phng ....................................................................................................36

4.1.2 Cu hnh ng dng .....................................................................................................37

4.1.3 C hnh TCP c bn ..................................................................................................37

4.2 Kt qu m phng .............................................................................................................40

4.2.1 TCP Tahoe ..................................................................................................................40

4.2.2 TCP Reno ...................................................................................................................41

4.2.3 TCP New Reno ...........................................................................................................42

4.2.4 TCP SACK ...................................................................................................................42

B. M PHNG NH GI THROUGHPUT CA IEEE 802.11b HOT NG VI TCP TAHOE, TCP RENO,TCP SACK TRONG OPNET MODELER 14.5...........................................................................................44

4.3 Thit lp m phng ............................................................................................................44

4.3.1 M hnh m phng ....................................................................................................44

4.4 Kt qu m phng .............................................................................................................47

C. M PHNG HOT NG CA WIRELESS LAN ...............................................................................47

4.5 Wireless LAN 802.11 vi cc thng s phc tp hn ..........................................................47


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viii

4.5.1 M hnh m phng ....................................................................................................48

4.5.2 Cu hnh ng dng .....................................................................................................48

4.5.3 Cu hnh profile cho ng dng ...................................................................................49

4.5.4 Tin hnh m phng ..................................................................................................49

4.6 Kt lun .............................................................................................................................68

CHNG 5. XEM XT CC PHNG PHP CI THIN TCP CHO MNG V TUYN .............................69

5.1 Giao thc phn chia kt ni TCP ........................................................................................70

5.1.1 Giao thc Indirect TCP (I-TCP) ....................................................................................70

5.1.2 Giao thc Selective Repeat (SRP) ...............................................................................71

5.1.3 Giao thc Mobile-End Transport ................................................................................71

5.1.4 Giao thc Mobile-TCP ................................................................................................72

5.1.5 Giao thc Split-Connection Mobile Transport (SCMTP) ..............................................72

5.1.6 Kt lun .....................................................................................................................73

5.2 Giao thc lp lin kt d liu (link layer protocol) .............................................................73

5.2.1 Giao thc Snoop ........................................................................................................74

5.2.2 Giao thc Adaptive-TCP .............................................................................................74

5.2.3 Giao thc Asymmetric Reliable Mobile Access In Link-Layer (AIRMAIL) ......................75

5.2.4 Giao thc Radio Link ..................................................................................................75

5.2.5 Giao thc TULIP .........................................................................................................75

5.2.6 Kt lun .....................................................................................................................76

5.3 Giao thc thng bo hin (Explicit Notification) .................................................................76

5.3.1 Giao thc Explicit Congestion Notification..................................................................76

5.3.2 Giao thc Explicit Loss Notification ............................................................................77

5.3.3 Giao thc truyn tin IMCP ..........................................................................................78

5.3.4 Giao thc Syndrome ..................................................................................................78

5.3.5 Giao thc Mutiple Acknowledgements ......................................................................79

5.3.6 Kt lun .....................................................................................................................79


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ix

5.4 Giao thc end-to-end (E2E) ...............................................................................................79

5.4.1 Giao thc End-To-End Smart ......................................................................................79

5.4.2 Giao thc TCP Westwood...........................................................................................80

5.4.3 Giao thc Freeze-TCP .................................................................................................80

5.4.4 Giao thc Delayed Duplicate Acknowledgement ........................................................81

5.4.5 Giao thc Wireless Transmission Control (WTCP) .......................................................81

5.4.6 Giao thc TCP Eiffel ...................................................................................................82

5.4.7 Giao thc TCP Real .....................................................................................................82

5.4.8 Kt lun .....................................................................................................................82

5.5 Tng kt cc phng php ti cp trong chng 5 .............................................82

5.6 Gii php ngh thc hin ...............................................................................................84

CHNG 6. GIAO THC SNOOP ........................................................................................................85

6.1 Tng n hot ng c gio thc noop ........................................................................85

6.2 Snoop_data() .....................................................................................................................86

6.3 Snoop_ack() .......................................................................................................................88

6.4 Kt lun .............................................................................................................................89

CHNG 7. CU HNH M PHNG GIAO THC SNOOP ...................................................................90

7.1 M hnh 1FH-1MH .............................................................................................................90

7.1.1 M hnh m phng trong NS2 ....................................................................................90

7.1.2 Cu hnh cc thng s chc nng ...............................................................................91

7.1.3 M hnh to mt gi trong mng v tuyn .................................................................95

7.1.4 Biu din mng trong NAM (Network Animator) ..............................................98

7.2 nh gi hot ng ca giao thc noop khi th i cc thng s....................................99

7.2.1 M hnh nFH-nMH .....................................................................................................99

7.2.2 M hnh tng xc t li bit ....................................................................................100

CHNG 8. T QU M PHNG GIAO THC NOOP I NS-2.35 ................................................102

8.1 M hnh 1FH-1MH ...........................................................................................................102


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x

8.1.1 Congestion window .................................................................................................102

8.1.2 Throughput ..............................................................................................................105

8.1.3 Sequence number ....................................................................................................106

8.1.4 tr end-to-end (Delay E2E) ..................................................................................110

8.2 M hnh nH-nMH ...........................................................................................................111

8.3 M hnh tng xc t li bit ............................................................................................112

CHNG 9. T LUN HNG PHT TRIN CA TI ............................................................116

9.1 Kt lun ...........................................................................................................................116

9.2 Cc hng pht trin c ti .......................................................................................116

9.2.1 Th i thng s hot ng ca giao thc Snoop ...................................................116

9.2.2 Tch hp th c chACK gi mo vo snoop module .............................................116

9.2.3 Hin thc gii thut Snoop trong thc ttrn nn tng hiu hnh BSD 4.3 .........118

PH LC A. TI LIU THAM KHO ...................................................................................................120

PH LC B. CU TRC FILE TRACE TRONG NS2-2.35 .......................................................................129

B1. Wired Network Trace.......................................................................................................129

B2. Old Wireless Network Trace.............................................................................................129

PH LC C. CC THAM NH HNG TI HOT NG TCP........................................................131

C1. Cc tham s ng vo ca TCP ...........................................................................................131

C2. Cc th ng r ca TCP .............................................................................................133

PH LC D. THNG S HOT NG CA IEEE 802.11 ....................................................................135

D1. Cc tham s ng vo ca WLAN .......................................................................................135

D2. Cc tham s ng ra ca WLAN .........................................................................................138

D3. Cc thng s thng k WLAN ...........................................................................................139

PH C E. COE M PHNG .........................................................................................................141

E1. Tcl script ..........................................................................................................................141

E2. Perl script ........................................................................................................................146


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xi

DANH SCH CC HNH

Hnh 2.1-1 Tin trnh pht trin ca TCP ............................................................................................. 6

Hnh 2.1-2 Tin trnh pht trin ca TCP (tip theo)............................................................................ 6

Hnh 2.2-1 Qu trnh m kt ni v ng kt ni ca TCP [Kenvin R. Fall & W. Richard Stevens] ......... 7

Hnh 2.3-1 TCP x ng bt i rr . Ptron Brc . vi ........................................ 9

Hnh 2.3-2 TCP header [Kenvin R. Fall & W. Richard Stevens] ............................................................10

Hnh 2.4-1 Kt ni TCP theo 2 chi, 1 u cui truyn d li, u cui cn li gi xc nhn kmtheo received window [Kenvin R. Fall & W. Richard Stevens] ............................................................11

Hnh 2.4-2 Received window (rwnd) v bm pha thu (RcvBuffer) [Kurose & Ross, 2013]..............12

Hnh 2.4-3 C chca strt trong TCP [Fred Halsall, 2005]..........................................................13

Hnh 2.6-1 Hot ng c c chslow-start [Kevin R. Fall W. Richard Stevens]..................................16

Hnh 2.6-2 Sth i congestion window khi b timeout [Fred Halsall, 2005] ..................................17

Hnh 2.6-3 Sth i ca congestion window khi nhn c duplicate ACK [Fred Halsall, 2005] .....18

Hnh 2.8-1 trng thi hot ng ca TCP Reno [Kurose & Ross, 2013] ..........................19

Hnh 2.9-1 Mt nhi gi trong TCP Rno Intrntorking Rcittion 9 i, pring tr2009, Dept. of Computer Science, Technion] .....................................................................................21

Hnh 2.9-2 Mt nhiu gi trong TCP New Reno [Internetworking Recitation #9 slides, Spring Semester2009, Dept. of Computer Science, Technion] .....................................................................................21

Hnh 2.10-1 Qu trnh truyn d liu dng TCP vi ty chn AC c kch hot [Jeremy Stretch,2010] ................................................................................................................................................22

Hnh 3.1-1 Chun 802 ca IEEE ..........................................................................................................28

Hnh 3.1-2 Tc truyn ti ti v phm vi hot ng ca cc chun khng dy ca IEEE [Kurose &Ross, 2013] .......................................................................................................................................29

Hnh 3.2-1 Cc thnh phn c bn ca IEEE 802.11 [Kurose & Ross, 2013] ........................................30

Hnh 3.3-1 Cc c im khi qut ca cc chun IEEE 802.11 ...........................................................31

Hnh 3.3-2 CSMA/CA .........................................................................................................................32

Hnh 3.4-1 Khung d liu 802.11 .......................................................................................................33

Hnh 4.1-1 hnh m phng TCP c bn .........................................................................................37Hnh 4.1-2 C hnh ng FTP ....................................................................................................38
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xii

Hnh 4.1-3 Thit lp thi gian m t thi khong xy ra mt gi ........................................................39

Hnh 4.2-1 C chslow-start ca TCP Tahoe ......................................................................................40

Hnh 4.2-2 Hot ng c TCP Tho i vi nhiu loi mt gi ........................................................41

Hnh 4.2-3 Congestion window ca TCP Reno khi c xy ra mt gi ...................................................41

Hnh 4.2-4 So snh congestion window ca hai TCP Reno v TCP New Reno ......................................42

Hnh 4.2-5 So snh congestion window ca TCP SACK, TCP Reno v TCP New Reno ...........................43

Hnh 4.3-1 hnh m phng IEEE 802.11 ........................................................................................45

Hnh 4.3-2 Thit lp ng FTP ....................................................................................................45

Hnh 4.3-3 Thit lp thng s cu hnh 802.11b .................................................................................46

Hnh 4.4-1 So snh throughput ca TCP Reno, TCP SACK v TCP Tahoe..............................................47

Hnh 4.5-1 Kin trc ca chun IEEE 802.11 WLAN ............................................................................47

Hnh 4.5-2 mng trn OPNET Modeler 14.5 .............................................................................49

Hnh 4.5-3 Profile ng dng ca Employee ........................................................................................50

Hnh 4.5-4 Profile ng dng ca Manager .........................................................................................51

Hnh 4.5-5 Hot ng ca Employee v Manager trong sut thi gian m phng ..............................51

Hnh 4.5-6 ng ng dng FTP v Web ca Employee ...............................................................52

Hnh 4.5-7 ng ng dng Remote Login ca Employee theo 2 chiu........................................53

Hnh 4.5-8 () ng ng dng FTP ca Manager theo 2 chi; (b) ng ng dng Web caManager theo 2 chiu .......................................................................................................................54

Hnh 4.5-9 ng ng dng Remote Login ca Manager theo 2 chiu .........................................54

Hnh 4.5-10 m phng tng my truy cp: (a) 2 my truy cp mng, (b) 10 my truy cpmng, (c) 50 my truy cp mng .......................................................................................................55

Hnh 4.5-11 tr E2E khi sng my truy cp vo mng ln t l 2, 10, 50 my .......................55

Hnh 4.5-12 tr truy cp ng truyn khi sng my truy cp vo mng ln t l 2, 10, 50

my ..................................................................................................................................................56

Hnh 4.5-13 mng m phng cc thng s vt l ca WLAN .....................................................57

Hnh 4.5-14 Throughput ca ton mng ............................................................................................58

Hnh 4.5-15 tr truy cp ng truyn ........................................................................................59


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xiii

Hnh 4.5-16 tr E2E ......................................................................................................................60



Hnh 4.5-19 Throughput khi quan st trong ca s ng t460,000 bp n 520,000 bps .........61


Hnh 4.5-21 Throughput quan st trong ca s ng t470,000 bp n 530,000 bps ..............63



Hnh 4.5-24 Throughput quan st trong ca sthng ng t495,000 bp n 520,000 bps ..........65


Hnh 4.5-26 tr E2E ......................................................................................................................66



Hnh 4.5-29 Throughput ti AP ..........................................................................................................67

Hnh 5.1-1 M hnh phn chia kt ni TCP .........................................................................................70

Hnh 5.2-1 Giao thc lp lin kt d liu ci thin hi nng TCP .................................................73

Hnh 5.3-1 TCP header ......................................................................................................................77

Hnh 6.1-1 Hnh minh ha hot ng c bn ca giao thc Snoop .....................................................86

Hnh 6.2-1 gii thut ca khi snoop_data() [Balakrishnan, Padmanabhan, Seshan, & Katz,1997] ................................................................................................................................................87

Hnh 6.3-1 gii thut ca khi snoop_ack() [Balakrishnan, Padmanabhan, Seshan, & Katz,1997] ................................................................................................................................................88

Hnh 7.1-1 mng m phng giao thc Snoop trn NS2.............................................................91

Hnh 7.1-2 c hnh ng (traffic) dng trong m phng ..................................................94

Hnh 7.1-3 M hnh GibrtEiott (Mrkov 2 trng thi) m phng qu trnh gy li ........................96

Hnh 7.1-4 Trnh din hnh mng trong NAM ................................................................................98

Hnh 7.1-5 Theo di qu trnh truyn d liu trong NAM ...................................................................99

Hnh 7.2-1 hnh mng cho trng hp n = 3 ...............................................................................100


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xiv

Hnh 8.1-1 So snh congestion window ca TCP Tahoe....................................................................103

Hnh 8.1-2 So snh congestion window ca TCP Reno .....................................................................103

Hnh 8.1-3 So snh throughput ca TCP Tahoe ................................................................................105

Hnh 8.1-4 So snh throughput ca TCP Reno ..................................................................................105

Hnh 8.1-5 Throghpt trng bnh cho 4 trng hp theo thi gian ................................................106

Hnh 8.1-6 Sequence number c H trong trng hp khng s dng Snoop trn nn TCP Tahoe .107

Hnh 8.1-7 Sequence number c H trng hp s dng Snoop trn nn TCP Tahoe ....................109

Hnh 8.1-8 Bi ct gi tr trung bnh ca cc Delay E2E .............................................................111

Hnh 8.2-1 Tng throughput ca tt c cc nt theo sng MH ...................................................112

Hnh 8.3-1 Congtion ino trong trng hp m hnh li l 3.10-5............................................113

Hnh 8.3-2 Throughput ng vi trng hp BER = 3.10-5 .................................................................113

Hnh 8.3-3 Throughput trung bnh ng vi trng hp BER=3.10-5..................................................114

Hnh 9.2-1 Cch thc hot ng ca ACK gi [Furat Asmat, Moojid et al] ........................................117

Hnh 9.2-2 khi chi tit cho phn x l snoop_data() [Indradeep Biswas & A. L. Ananda] .......118

Hnh 9.2-3 khi chi tit cho phn x l snoop_ACK() [Indradeep Biswas & A. L. Ananda]........119


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xv

DANH SCH CC BNG

Bng 2.3-A Ch thch cc trng trong TCP header ...........................................................................10

Bng 2.13-A Mt s v d tnh BDP [www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factors] .............................................................................................................................................25

Bng 3.1-A Tm tt cc thng s vt l ca 802.11 ............................................................................29

Bng 3.2-A Bng o nh c im ng dng ca BSS Ad hoc v BSS infrastucture ............................30

Bng 4.1-A Bng cc thnh phn trong m phng TCP c bn ...........................................................37

Bng 4.1-B Cu hnh chung cho 3 phin bn TCP ...............................................................................37

Bng 4.1-C Tm tt cc c chca 4 loi TCP m phng ...................................................................39

Bng 4.3-A Cc thnh phn s dng trong m phng IEEE 802.11 .....................................................44

Bng 4.5-A Cc thit b s dng trong m hnh ..................................................................................48

Bng 4.5-B Bng cc ng dng chn dng trong m phng ...............................................................49

Bng 4.5-C Cu hnh loi v c tnh ng dng ..................................................................................50

Bng 4.5-D Bng cu hnh kho st cc thng s vt l ca WLAN .....................................................57

Bng 4.5-E Bng cu hnh RTS ...........................................................................................................58

Bng 4.5-F Bng cu hnh fragmentation ...........................................................................................59

Bng 4.5-G Bng cu hnh RTS + fragmentation .................................................................................62

Bng 4.5-H Bng cu hnh PCF ...........................................................................................................64

Bng 4.5-I Bng cu hnh PCF + fragmentation ..................................................................................66

Bng 7.1-A Bng a ch cho cc nt mng.........................................................................................92

Bng 7.1-B Bng cu hnh thng scho ng truyn v nt mng v tuyn ....................................92

Bng 7.1-C Bng cc thng s cu hnh cho mng LAN ......................................................................93

Bng 7.1-D Bng c hnh cc ng dy lin kt..............................................................................94

Bng 8.1-A Kt qu throughput trung bnh c 4 trng hp ..........................................................106

Bng B1-A Cu trc tp tin ghi li s kin m phng trong mng c dy [nsnam.isi.edu] .................129

Bng B2-A Cu trc tp tin ghi li s kin m phng mng khng dy [nsnam.isi.edu].....................129

Bng B2-B Cu trc tp tin ghi li s kin m phng mng khng dy [nsnam.isi.edu] (tip theo) ...130


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Bng C1-A Bng cc thng s thit lp cho hot ng TCP...............................................................131

Bng C2-A Bng cc thng s TCP thng k ng ra ..........................................................................134

Bng D1-A Bng cc thng s cu hnh cho WLAN ...........................................................................135

Bng D2-A Cc thng s thng k WLAN ton cc ...........................................................................138

Bng D3-A Thng s ti cc nt mng .............................................................................................139


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1

CHNG 1. GII THIU

S ph bin ngy cng rng ca cng ngh v tuyn dng truy cp Internet trthnh 1

phn ca cuc sng hin i ngy nay. Xut hin rng ri ni lm vic, hgia nh, trng

hc, caf, sn bay v cc khu phWireless Local Area Network (WLAN) ngy nay tr

thnh 1 trong nhng cng ngh truy cp mng quan trng nht. WLAN l mt cng ngh

truyn thng khng dy. Sra i ca n gii quyt c 1 s vn ca mng Local Area

Network (LAN) truyn thng v ang l xu th nghin cu pht trin. Mng WLAN da trn

sng v tuyn truyn d liu, d liu c iu ch vo mt sng mang lan truyn trong

khng gian. Cht lng truyn dn trn knh trn v tuyn bin ng theo thi gian v nh

hng ng kn cc giao thc h tr truyn ti d liu ng tin cy nh Transmission

Control Protocol (TCP) vn chim hn 90% lu lng mng Internet hin nay [GeoffHuston, Telstra, 2000].

1.1 t vn v ng lc cho hng i

TCP l giao thc truyn tin tin cy c thit k hot ng tt trn mng c xc sut li bit

thp, nh l mng hu tuyn. TCP cho rng tt c cc li truyn tin l do nghn mng ch

khng phi do li gi tin. Khi gp nghn, TCP iu chnh kch thc ca s chng tc nghn

v truyn li cc gi tin b mt. C ch ny hot ng tt trn mng hu tuyn. Tuy nhin,trong mng v tuyn vi c trng c xc sut li bit cao, round-trip time (RTT) ln v bin

ng, ng truyn bt i xngDo TCP gim ca s tc nghn (congestion window),

thc hin truyn li hay b timeout dn n hiu nng km. Mc ch nghin cu ci thin

hiu nng ca TCP trn mi trng v tuyn nhm em li tri nghim tt hn khi s dng

cc dch vInternet khng dy ng thi cng ph hp vi xu th nghin cu hin nay v c

trin vng pht trin di trong tng lai.

1.2 Mc tiu

Nhiu phng php c ngh nng cao hiu nng TCP trong mi trng v tuyn.

Mt trong nhng phng php l TCP Snoop. Giao thc ny ch yu tp trung chnh sa

v phn mm ti lp lin kt d liu ti im truy cp (access point, AP) v bo ton tnh cht

end-to-end ca TCP. tng chnh ca phng php ny l sao lu gi tin ti AP v thc

hin truyn li cc bphn on v tuyn.


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2

Trong lun vn ny, chng ti tin hnh phn tch v m phng hot ng ca giao thc

Snoop ci thin hiu nng TCP trn mi trng mng v tuyn m chng ti chn l IEEE

802.11b Wireless LAN. Cc nghin cu v m phng hot ng c bn c thc hin trn

OPNET Modeler 14.5 v phn ci thin nng cao c thc hin trn Network Simulator

2.35 (NS2.35). Snoop module c tch hp vo im truy cp tin hnh x l cc gi

tin i qua. Phng php ny tp trung vo vic ti truyn li gi tin cc b trong mng v

tuyn, che giu s mt gi trong phn khc mng ny khi lp vn chuyn. Mc tiu chnh

ca phng php ny l ci thin hiu nng ca TCP trong mng v tuyn m khng thc

hin vic ti truyn bn pht dn n vic gim ca s tc nghn v gim thng lng

(throughput).

1.3 Cc cng vic lin quan

Mt trong nhng kh khn, m theo chng ti, khi vic s dng phn mm m phng NS2 l

x l d liu th c c ti u ra m phng t cc tp tin ghi du cc s kin m phng

[ph lc A]. Ngoi kin thc lp trnh C++ v Tcl l bt buc, c mt kt qu quan st

hon ton sng v r rng th chng ti phi dng cc ngn ngscript x l cc gi d liu

th ny, chng ti ang ni ti Perl v AWK. Hai ngn ng ni ting x l tp tin s liu

ny c chng ti nghin cu song song vi qu trnh x l kt qu, chng ti khng trnh

by v hai ngn ng ny trong lun vn bi chng ti ngh n khng kh to nn s khc

bit.

Hiu hnh nn tng cho cc phn mm v cc gi ng dng chng ti s dng l hiu

hnh Linux, c th l Ubuntu 12.04 LTS. Vic ci t cc thnh phn b tr c th s

dng cc thnh phn cn thit ca Perl, AWK, Gnuplot, NS2, NAM lm tng kinh nghim

nghin cu cho chng ti.

1.4 Phng php v ni dung

Phng php tip cn vn v tin trnh cng vic c th hin qua th tcc chng

trong lun vn ny.

Lun vn bao gm cc chng sau:

Chng 1: Gii thiuChng mu nu ln mc ch, t vn , xc nh bi ton v

mc tiu lun vn hng ti gii quyt. Chng ny cng tm tt ni dung thc hin trong

lun vn ny.


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3

Chng 2: Giao thc lp vn chuyn TCP Trong phn ny, u tin, chng ti trnh by

cc loi giao thc TCP ph bin v kho st khi qut cc tnh nng c trng ca tng loi.

Cc u im v nhc im ca tng loi TCP c xem xt v 2 loi TCP Tahoe v TCP

Reno c chn kho st v pht trin trong lun vn ny. Tip theo, cc phn tch su v

c ch trnh nghn, c chiu khin v tc ng ca mi trng truyn dn nhm phc v

cho vic a ra gii php nng cao hiu nng ca TCP trn ng truyn v tuyn.

Chng 3: Chun IEEE 802.11 Wireless LANChng ny cung cp cc khi nim c bn

ca chun IEEE 802.11 bao gm tng quan, kin trc, lp vt l, giao thc MAC, cu trc

khung tin

Chng 4: M phng cc c tnh c bn ca TCP v chun IEEE 802.11b Wireless LAN

Chng ny gip hiu r hn v hot ng ca giao thc TCP v 802.11b WLAN, cc thng

snh hng cng nh cc thng s kt qugip nh gi hiu nng ca giao thc TCP v

802.11b WLAN. Cng c s dng trong phn ny l OPNET Modeler 14.5 c pht trin

bi OPNET Technology.

Chng 5: Gii php ci thin hiu nng TCP trn mi trng v tuynChng ny trnh

by cc gii php ci hiu nng TCP trn mi trng v tuyn c xut. Cc u im,

nhc im, phm vi ng dng ca cc gii php c tm tt. Cui chng l gii phpgiao thc TCP Snoop m chng ti ngh v thc hin trong bi lun vn ny ng thi a

ra cc vn pht trin tip hng i.

Chng 6: L thuyt giao thc Snoop Chng ny trnh by chi tit v giao thc Snoop

bao gm tng, s hot ng v phn tch cc trng hp c bit.

Chng 7: M phng giao thc Snoopy l phn chnh ca lun vn ny. Chng ny s

trnh by cc m hnh thc hin m phng giao thc Snoop nhm mc tiu ci thin hiu

nng TCP trn mi trng IEEE 802.11b v nh gi kt qu m phng. Cng c s dng

trong phn ny l Network Simulator 2.35 c pht trin bi UC Berkeley.

Chng 8 : Kt qu m phng giao thc Snoop trong NS2 Chng ny trnh by kt qu

m phng, so snh, kim nghim li l thuyt, nhn xt v nh gi hot ng ca giao thc

Snoop.


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Chng9: Kt lun v cc hng pht trin ca tiChng ny a ra kt lun tng th

v cc hng pht trin ti bao gm cc hng pht trin tip v giao thc v cch thc

hin thc trn thc t.


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5

CHNG 2. GIAO THC LP VN CHUYN TCP

Hu ht cc ng dng trong Internet s dng TCP bi tnh tin cy v cch thc iu khin

dng d liu bo m d liu c phn phi an ton qua ton b lp mng (IP). Bn thn

lp IP ch lm dch vng gi d liu v nh tuyn ch khng h trbt k khi nim

phin giao vn hay kt ni. Mt khi mt gi d liu c nhn hay gi i, lp IP lp tc gii

phng b nhca gi ny, do khnng ti gi li hoc kim tra l khng tn ti trong

dch v chuyn gi tin.

Chng ny trnh by mt cch ngn gn v tng th ca TCP v nhng c im chung ca

n. Tt c cc loi TCP thng dng v tnh nng c bit ca n, tp trung nhn mnh vo c

ch trnh nghn v kim sot nghn. im mnh v im yu ca mi phin bn TCP, m s

c ng dng trong mi trng mng khng dy, sc tho lun v nh gi. Cui cng,

cc phn tch su v cc c ch trnh nghn v kim sot nghn sc tin hnh nhm h

trcho gii php ci tin TCP nhng chng sau.

2.1 Qu trnh pht trin ca TCP

TCP c th xem l cuc cch mng trong giao thc truyn ti bi tnh phc tp trong kin

trc v tnh n gin trong tng. Chc nng chnh ca TCP c nh ngha trong [RFC

793] v c cng bnm 1981. T, cc ci tin t ph c cc nh khoa hc nghin

cu thnh cng. Nhng yu cu i vi cc thc th tham gia Internet [RFC 1122] lc ra cc

yu cu ca giao thc TCP cn phi c i vi cc thc th ny. Cc loi TCP mrng c

nh ngha trong [RFC 1323], [RFC 2018] v [RFC 2481]. TCP kim sot tc nghn [RFC

2581], mt trong nhng loi TCP quan trng nht, m t nhng cp nht hon thin c ch

x l nghn.

Nghn xy ra khi m nhu cu ln hn kh nng p ng, chng hn nh bng thng cang truyn, bm v khnng x l ca cc nt (node) mng trung gian (router). Kim

sot nghn lin quan ti phn b ti nguyn trong mt mng mng ny c th hot ng ti

mt mc hiu nng chp nhn c khi m yu cu v truyn ti vt qu khnng p ng

ca mng. iu ny i hi vic thit k giao thc phi rt thn trng c thp ng cc

dch v tt trong trng hp ti ln. Hay ni cch khc, sp mng do nghn lm lng ph

ti nguyn mng v gy ra cc hu qu khng mong mun.


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Sp mng do nghn ln u xut hin trn Internet gia nhng nm 1980 [RFC 896]. Nguyn

nhn l do cc kt ni TCP ti truyn li gi tin khng cn thit. TCP nguyn bn [RFC 793]

iu khin dng d liu da trn kch thc ca s nhm ti u hot ng ca bm v s

dng c ch ti truyn Go-Back-N, nhng cha cp ti qun l kch thc ca khung ca

s nhm thch nghi khi nghn xy ra.

Cc loi sp mng do nghn c trnh by chi tit trong [Fall & Floyd, 1996]: classify

congestion collapse, nh ni l do l t vic ti truyn khng cn thit [Nagle, 1984] v

c chnh sa bng bnh thi ti truyn v thut ton kim sot nghn [Jacobson, 1988],

fragmentation-based congestion collapse, c ngh bi [Kent & Morgul, 1987] v c

kt hp vi maximum transmission unit(MTU) trong [RFC 1063, 1988] cng vi congestion

collapse from undelivered packets, xy ra khi mng qu ti dn ti cc gi tin b loi btrckhi ti c ch [S.Floyd & Fall, 1999].

Hnh 2.1-1 Tin trnh pht trin ca TCP

Hnh 2.1-2 Tin trnh pht trin ca TCP (tip theo)


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S pht trin ca Internet lm sinh si ny nca cc phin bn TCP. Nhiu trong s chng

tht bi trong ci tin do li logic, hay hiu sai cc c im ca TCP [RFC 2525]. S khc

c th thn trng hn khi cy ghp cc thut ton kim sot nghn nhng cha ti u nn li

s dng hao kit ti nguyn ca mng.

S pht trin ca TCP phi trnh lm thay i cc c trng gc ca n v phi trnh chy

ua v trangca cc thut ton qun l nghn ca cc giao thc [RFC 2914]. TCP thay

i nhiu ln trong cu trc nguyn thy ca n, trong qu trnh pht trin sut 3 thp k qua.

Stng trng theo hm m ca ti nguyn t Internet lm gia tng nhng vn nghn. H

qu, nhiu phin bn TCP tn ti. Hin ti, tt c cc loi TCP ph bin u s dng cc c

chslow-start (SS), congestion avoidance (CA),fast retransmit(FR) vfast recovery.

2.2 Cc c trng ca giao thc TCP

2.2.1 TCP l mt giao thc nh hng kt ni

TCP l giao thc nh hng kt ni. Trc khi bt k u cui no truyn ti d li, kt ni

phi c thit lp gia chng. Trong khi kt ni c thit lp, hai u cui trao i cc

thng s kt ni. Mt vi thng s chc trao i trong lc thit lp kt ni, mt vi thng

khc c trao i sau kt ni.

Mt kt ni TCP thng tri qua 3 giai on: mkt ni, truyn d liu v ng kt ni.

Hnh 2.2-1 sau minh ha mt phin kt ni TCP vi giai on mkt ni v ng kt ni

(trng hp khng c d liu truyn)

Hnh 2.2-1 Qu trnh mkt ni v ng kt ni ca TCP [Kenvin R. Fall & W. RichardStevens]

Mt kt ni TCP tri qua cc bc sau:


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Client gi i mt gi SYN (gi tin TCP/IP c bit SYN c bt ln trong TCP header) xcnh a ch cng (port) ca u cui bn kia m n mun kt ni v s tun t (sequence

number)ban u ca client l ISN(c). y l gi th 1 ca qu trnh thit lp kt ni.

Server tr li bng 1 gi SYN cha s tun t ban u ca n l ISN(s). Trong gi tin ny,server cng ng thi xc nhn gi SYN t client bng cch t trng ACK = ISN(c) +

1. y l gi th 2.

Client xc nhn gi SYN trn bng cch gi 1 gi th 3 vi trng ACK = ISN(s) + 1 Sau khi 3 bc trn c thc hin thnh cng, kt ni c thit lp, qu trnh ny cn

c gi l qu trnh bt tay 3 bc (3-way handshake)

ng mt kt ni TCP tri qua cc bc sau

Mt bn u cui gi 1 gi FIN vi s tun t hin ti m bn nhn mong mun (gi s lK nh hnh 2.2-1). Gi FIN ng thi cng cha ACK cho gi d liu nhn c sau cng

(ACK= L nh hnh 2.2-1)

u cui cn li tr li bng cch t gi tr ACK= K + 1 xc nhn rng nhn thnhcng gi FIN. Ti thi im ny, lp ng dng c thng bo rng u cui bn kia

mun ng kt ni v gi 1 gi FIN vi s sequence number L.

hon tt vic ng kt ni, gi cui cng cha ACK cho gi FIN sau cng. Nu giFIN b mt, thc hin truyn li cho n khi nhn c ACK ca n.

2.2.2 TCP l mt giao thc truyn tin tin cy

TCP h trcc c chnh m checksum, pht hin d liu trng lp, nh s tun t, truyn

li gi v cc bnh thi (timer) gip cho TCP l 1 giao thc truyn tin tin cy. Cc gi tin

TCP u cha trng checksum mc ch gip cho bn nhn pht hin d liu b sai. TCP

m bo cc byte d liu c nhn theo ng th t v loi b cc gi trng nhau do trong

mng chuyn mch theo gi, cc gi c thn u thu sai th t. TCP chuyn cc gi tin

nhn cho lp ng dng (Application) theo ng trnh tthng qua c chnh s gi tin. Gi

tin b li hay mt c bn pht truyn li m bo bn thu lun nhn c. TCP s dng

ACK thng bo nhn d liu thnh cng, nu khng nhn c ACK trong 1 khong thi

gian gi l timeout, TCP thc hin vic truyn li gi tin.


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2.2.3 TCP l mt giao thc nh hng byte

TCP l 1 giao thc nh hng byte ngha l bn pht chuyn cc byte d liu vo kt ni

TCP v bn nhn c tng byte nhn c t kt ni. Mc d TCP cung cp dch vnh

hng byte cho tng ng dng, TCP khng truyn tng byte ring l vo mng. Thay vo bn gi lu trlng byte d liu trong bm n khi dung lng ca 1 gi ri mi

truyn i. Bn nhn sau tr ni dung ca gi tin vo bm, v qu trnh c d liu xy

ra sau .

2.3 nh ng gi tin TCP

Gi tin TCP (TCP segment) gm TCP header theo sau l payload. Payload bao gm thng tin

d liu ca lp ng dng cao hn. TCP header bao gm thng tin a ch v tt c cc thngtin cn thit cho cc c ch ca TCP. Mt trng Options trong TCP header cha thng tin

cc kt ni TCP ring bit cho mc ch no . Mc nh ca kch thc TCP header l 20

byte. Tuy nhin, c th ln n 0 byte k c trng Options. Gi tin TCP sau c

chuyn xung lp IP nh l mt gi d liu (datagram) ca lp ny.

Hnh 2.3-1 TCP x l dng byte d liu [Larry L. Peterson & Bruce S. Davie]


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Hnh 2.3-2 TCP header [Kenvin R. Fall & W. Richard Stevens]Trong hnh trn, cc thng tin tng ng ca cc trng c tm tt theo bng di y

Bng 2.3-A Ch thch cc trng trong TCP header

Trng thng tin it Chi tit

Source Port 16S hiu cng ngun ca lp ngdng bn gi.

Destination Port 16S hiu cng ch ca lp ngdng bn nhn.

Sequence number 32 S tun t ca gi tin c gi ica phin kt ni ang din ra.

Acknowledgment Number 32

Nu bit iu khin ACKc btth trng ny cha gi tr ca sth t ca gi tin m bn nhnmong i ln tip theo.

Data offset 4

Trng qui nh di header(tnh theo n v t 32 bit)Trng thng tin ny ch ra nid liu bt u k t bit u tinca Source Port.

Reserved 6 bit cha s dng, dnh chocc mc ch trong tng lai.

Control 6

Gm cc c cho qu trnh 3-wayhandshake khi thit lp kt ni.URG: ccho trng UrgentPointer.ACK: cbo y l gi ACKPSH: Hm PushRST: Thit lp li ng truynSYN: ng b li s tun t

FIN:bn pht truyn ht dliu


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Windows 16S byte bn gi c th nhnc.

Checksum 16Chui bit dng kim tra li trongd liu ca gi tin.

Urgent Pointer 16 Ch xt ti khi c RG c bt

Option Ty binDng thc hin ty chn citin cho TCP (TCP, timestamp,TCP SACK)

Padding Ty binCc bit 0 thm vo nhm m

bo chiu di ca TCP headerls nguyn ln ca 32 bit.

2.4 C ch kim sot lung ca TCP

Kim sot lung l mt thnh phn quan trng ca giao thc TCP, kim sot lung s kimsot tc truyn gi tin b thu lun c b nhm lu trchng trc khi x l.

TCP cung cp kim sot lung s dng c ch ca strt (sliding window). Gi tin ACK

c gi bi pha nhn s km thm thng tin v ca s nhn (received window) gip bn

pht gii hn lu lng gi tin c gi i.

Hnh 2.4-1 Kt ni TCP theo 2 chiu, 1 u cui truyn d liu, u cui cn li gi xc nhnkm theo received window [Kenvin R. Fall & W. Richard Stevens]

Gi trreceived window c tnh ton da trn bm ca pha thu.


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Hnh 2.4-2 Received window (rwnd) v bm pha thu (RcvBuffer) [Kurose & Ross, 2013]

bm bn thu khng b trn

LastByteRcvdLastByteRead RcvBuffer

trong :

LastbyteReadl byte cui cng ca dng d liu c lp ng dng c t bm

LastByteRcvdl byte mi nht m bn nhn a vo bm

K hiu rwndl phn bm cn trng v RcvBufferl kch thc bm (Hnh 2.4-2), do

rwnd = RcvBuffer[LastByteRcvdLastByteRead]

Bng cch gi cho bn pht gi trrwnd, bn pht bit c rng bn nhn cn bao nhiu

dung lng bm trng cho kt ni. Ban u rwnd = RcvBuffer.Da vo thng sRcvBuffer, bn pht m bo sao cho

LastByteSentLastByteAcked rwnd

Hai thng sLastByteSentv LastByteAckedny ln lt cn c gi l cnh ca s trn

UWE (Upper Window Edge) v cnh ca sdi LWE (Lower Window Edge). Khi gi 1

gi mi th cnh ca s trn stng thm 1 n v. Tng t khi mi gi c xc nhn cnh

ca sdi tng ln 1 n v (Hnh 2.4-3)


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Hnh 2.4-3 C ch ca strt trong TCP [Fred Halsall, 2005]

S khc bit gia UWE v LWE chnh l ca s gi. Ca s ny s trt trn ton b cc

khung ang c truyn do cn gi l ca strt. Kch thc ca strt ny b gii

hn khng ch bi rwndm cn bi ca s tc nghn sc gii thiu trong phn sau.

2.5 C ch tiot ca TCP

Nhm trnh cho thi gian tr hon di khi khng c hi p t bn nhn; trong kt ni TCP,

mt c ch timeout c ngh [RFC 298]. Do , sau mi mt gi tin c truyn i,

mt b nh thi c bt ln v m li, trong mt phin kt ni TCP ch s dng duy nht

mt b nh thi timeout cho tt c cc gi tin, b nh thi ny ti khi ng, ngay khi nhn

c ACK trong khi thi gian timeout thit lp trc cho gi tin ny vn cha ht hn, cho

gi tin lin k sau. Nu bn gi khng nhn c ACKtrc thi im ht hn ca b nh

thi, n s gi s rng hoc gi tin hoc ACKb mt, v ti gi li gi tin. Gi tr timeout

(RTO) ny phi c chn mt cch cn thn. Nu RTO qu nh, vic ti gi s xy ra sm

v khng cn thit. Mt khc, nu RTO qu ln, TCP s p ng chm vi vic mt gi. Do

, gi tr RTO phi c ti u ha.

Khi mt gi tin gi qua kt ni TCP, bn gi phi phi ghi nhn li thi gian bao lu gi

tin ny c xc nhn, qu trnh ny l ly mu round-trip time (RTT). Phin bn TCP c ch

o c thi gian RTT ca mt gi tin trong mt ca s truyn, trong khi cc ci tin mi


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dng timestamp [RFC 1323] cho php ly mu tng gi tin trong mt ca s truyn, chnh

xc cng c ci thin theo phng php dng ty chn timestamp ny.

Chng ta quan tm so snh tc ly mu RTT v tc d liu c gi i lp vt l.

Nu t s ca tc ly mu RTT (s mu RTT o c / tr trung bnh RTT) vi tc gid liu xp xbng 1, gi tr o cng chnh xc.

2.5.1 C ch tiot ca TCP ngn gc v ci tin ca acononKarel

TCP nguyn bn s dng c ch thch nghi c lng RTT, theo [Kurose & Ross, 2013]

th vi:

l gi tr timestamp bn gi ghi nhn lc gi gi tin

l gi tr timestamp bn gi ghi nhn lc nhn c ACKtng ng vi gi tin ny

H s l hng s gia 0 v 1 iu khin nhp c lng RTT v gi tr 0.125 c sdng trong [acobson, 1988]. Phng php ny cn c gi l tng tr trung bnh theo hmm (Expotential Weighted Moving Average).

Thm vo , nhm thch nghi theo bin thin ca RTT, aconson/Karel [RFC 2988] nh

ngha DevRTTnh sau:

Hng s c kin ngh l 0.25 theo [acobson, 1988].V cui cng,

2.5.2 Gii tht ca Karn

[hang, 198], [ain, 1987], v [Karn, 1987] khng nh rng khi ly mu RTT i vi cc

gi tin c ti gi l khng chnh xc bi v khng th xc nh c ACKgi v tng


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ng l ca gi tin c gi ln u tin hay ca gi tin c truyn li. Karn ngh 2 bc

nh gi v kim tra cho vic ly mu RTT .

Khng ly mu vi cc gi tin c ti truyn. i vi mi ln ti truyn, thit lp thi gian timeout mi bng hai ln gi tr trc .2.6 C ch kim sot nghn ca TCP

2.6.1 Slow-start

Gii thut slow-start c thc hin khi 1 kt ni TCP va c thit lp xong hoc l do

mt gi b pht hin bi retransmission timeout (RTO).

C ch slow-start nh ngha thm 1 thng s vo giao thc TCP ca bn gi gi l ca s tcnghn (congestion window) hay gi tt l cwnd. Khi 1 kt ni c thit lp vi 1 u cui

hay vi 1 mng khc, cwndc t gi trban u l 1 Maximmun Segment Size (MSS).

C mi ln nhn c ACK, cwndtng thm 1 MSS. Bn pht c th truyn ti gi tr nh

nht trong 2 gi trcwndv rwndhay

LastByteSentLastByteAcked min{cwnd, rwnd}

Congestion window cung cp c ch kim sot p t bi bn pht trong khi receivedwindow cung cp c ch kim sot p t bi bn nhn. cwndda vo pht hin nghn mng

ca bn pht, rwndda vo bm cn trng ca bn nhn.

Bn gi truyn 1 gi tin ri i ACK. Khi nhn c ACK, cwndtng thm 1 t 1 ln 2 v 2

gi tin c truyn i. Nu 2 gi tin gi thnh cng v c ACK trv, cwndtng ln 4. Quy

lut tng cwnd ging hm m mc d trong 1 vi tnh hung khng hon ton l hm m do

bn nhn c th lm tr ACK, thng thng gi 1 ACK cho 2 gi nhn thnh cng.


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Hnh 2.6-1 Hot ng ca c ch slow-start [Kevin R. Fall W. Richard Stevens]

Trong trng hp n gin l khi ACK khng b delay, c mi ln nhn c ACKng th

bn pht gi i 2 gi (hnh 2.6-1 tri). Do ca s ca bn gi tng theo quy lut hm m

theo thi gian (hnh 2.6-1 phi, th pha trn). Khi ACK bdelay, nh trng hp 1 ACK

cho nhiu gi, quy lut tng vn l hm m nhng chm hn (hnh 2.6-1 phi, th pha

di)

2.6.2 Congestion avoidance (CA)

C ch slow-start va trnh by phn trn cho php cwnd tng nhanh n gi trngng m

lc ny mng c th nghn k hiu l ssthresh. Tuy nhin vn c kh nng mng cn ti

nguyn cho php kt ni tng cwnd qu gi trssthresh. trnh tng cwnd t ngt c th

lm nh hng cc kt ni TCP khc. C ch congestion avoidance ra i k gip bn pht

thm d ti nguyn mng bng cch tng cwnd khng qu t ngt nh slow-start.

Congestion avoidance thng s dng km vi slow-start s dng cc thng s cwnd v

ssthreshcho kt ni c thc hin nh sau:

Vi mi kt ni TCP, t gi trban u cho cwnd = 1MSSv ssthresh = 65535bytes TCP gi cc gi tin khng vt qu cwndv rwnd Khi nghn xy ra (pht hin bi timeout hay duplicate ACK)

ssthresh = max[min(cwnd, rwnd)/2, 2MSS]

Nu mt gi xy ra do timeout th cwnd = 1MSStrv trng thi slow-start


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Khi ACK l mi, tng cwnd, quy lut tng ph thuc vo TCP lc thc hin slow-starthay congestion avoidance.

Nu cwnd ssthresh, TCP thc hin slow-start; nu khng TCP thc hin congestionavoidance. Qu trnh slow-start tip tc khi cwnd ssthreshbc 3, sau chuynsang congestion avoidance.

Congestion avoidance tng cwnd theo quy lut sau c mi khi nhn ACK micwnd = cwnd + MSSMSS/cwnd (bytes)

Nhn thy y l quy lut tng theo hm tuyn tnh so vi hm m ca slow-tart. Quy lut

tng ny ti a l 1MSStrong khong 1 RTT bt chp slng ACK nhn c trong 1 RTT

so vi tng cwndda vo s ACK trong 1 RTT ca slow-start.

2.6.3 Fast recovery

Khi thc hin fast retranmit pht hin mt gi do duplicate ACK, thay v s dng c ch

slow-start, c ch congestion avoidance c thay vo. y l gii thut fast recovery gip

ci thin throughput do lc ny cwnd khng b gim mnh t ngt xung 1 nh trng hp

b timeout.

Hnh 2.6-2 Sthay i congestion window khi b timeout [Fred Halsall, 2005]


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Fast recovery thng c dng vi fast retransmit nh sau:

Khi nhn c gi duplicate ACK th3, tssthresh = max[min(cwnd, rwnd)/2, 2MSS]

Truyn li gi b mt, t cwnd = ssthresh + 3MSS, tng cwndthm 3 tnh sgi ri mng v c bn nhn lu trong bm.

Trong lc ny, nu nhn c thm 1 duplicate ACK, th tng cwnd thm 1MSS. Truynli gi b mt nhng phi tun theo gi trcwndmi

Khi c ACK mi, t cwnd = ssthresh (gi trbc u), ACK ny l xc nhn chogi b mt bc 1. Bc ny chnh l c ch congestion avoidance do TCP gim 1 na

tc khi b mt gi.

Hnh 2.6-3 Sthay i ca congestion window khi nhn c duplicate ACK [Fred Halsall,2005]

2.7 TCP Tahoe

Ngi i nhng bc u tin xy dng cc c trng c bn cho TCP l Van acobson trong

[Jacobson, 1988] khi ng tin hnh xy dng c ch nn tng cho tnh tin cy ca TCP bao

gm trnh nghn, kim sot nghn. l slow-start, trnh nghn (congestion avoidance) v

ti truyn nhanh (fast retransmit) c cng b nm 1988 vi tn gi 4.3 BSD Tahoe TCP v


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c ci tin sau bi chnh ng [acobson, 1990]. Mt trong nhng tng chnh ca ng

l mt phin kt ni TCP khi trng thi hot ng n nh s dng ton b ca s truyn

nn tun theo quy lut sau: mt gi tin mi khng c vo mng cho ti khi c mt gi tin

khc ra khi mng. iu ny m ch rng mt gi tin ch c th vo mng nu bn gi nhn

c mt gi ACKcho gi trc vo mng. Do vy, mt phin kt ni ti thi im

cn bng s c tc gi bng tc nhn.

Khuyt im ca TCP Tahoe xy ra trong tnh hung mt nhiu hn mt gi trong mt ca

s truyn. Throughput ca phin kt ni s gim do mi ln pht hin mt gi n phi gim

kch thc cwnd v 1. Thm vo , nu nh cwnd nh hn 4 th khng bao gi n c 3

duplicate ACK. im yu ny c khc phc trong TCP Reno [Jacobson, 1990] bi gii

thut ast recovery.

2.8 TCP Reno

TCP Reno hot ng ging nh TCP Tahoe giai on bt u truyn d liu v khi timeout

xy ra. Tuy nhin, khng ging nh TCP Tahoe, khi m fast retransmit xy ra, TCP khi

ng mt qu trnh mi l fast recovery. Theo [RFC 2851], TCP Reno s:

Hnh 2.8-1 S trng thi hot ng y ca TCP Reno [Kurose & Ross, 2013]


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t ssthresh = max(flightsize/2, 2MSS)khi nhn c 3 ACK trng nhau Thc hin gi li gi tin b mt v t cwnd = ssthresh + 3MSS Tng cwndthm 1MSScho ln nhn c thm 1 ACK trng Truyn cc gi d liu mi trong phm vi gii hn ca gi trcwndmi v rwndca bn

nhn

Khi ACK k l mi, t cwnd = ssthreshxc nh bc 1Ghi ch:flightsizel tng scc gi tin c gi nhng cha c xc nhn (data in flight)

Bc 2 v bc 3 c gi l Fast Recovery nh trnh by phn 2.6.3.

2.9 TCP New Reno

TCP New Reno [S. Floyd & Henderson, 1999] a ra mt ci tin nh cho TCP Reno. Ni

mt cch n gin, TCP Reno bn gi s thot khi trng thi ast recovery ngay khi nhn

c ACKca gi tin va truyn li. S khng c vn g nu l ch mt mt gi tin

trong mt ca s truyn. Tuy nhin, khi mt nhiu hn mt gi, li c th ti truyn li

tng gi tin b mt ny bn gi phi i y 3 duplicate ACKcho mt gi tin b mt,

khong thi gian i ny tr hon tc gi v gii hn kch thc ca s ng k nu mt

nhiu gi nh vy (hnh 2.9-1).

TCP New Reno, khc vi TCP Reno, khng thot khi trng thi ast recoveryngay khi nhn

c ACKmi (khng phi duplicate ACK), bn gi s phi nh s th t ca gi tin cui

cng gi i trong trng thi fast recovery, gi l partial ACK, v so snh vi ACKmi. S c

hai tnh hung:

1. S tun t ca ACKmi nhn c ln hn s tun t ca gi tin cui cng gi.2.Ngc li, tc l ton b cc gi gia hai s tun t ny b mt.Tnh hung 1 ny xy ra khi khng c gi no mt trong khong hiu s ca hai s tun t

ny. TCP bn gi lp tc thot khi ast recovery v tr li trng thi trnh nghn. Mt khc,

tnh hung 2 TCP phabn gi lp tc ti truyn ln lt cc gi tin b mt ny.


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Hnh 2.9-1 Mt nhiu gi trong TCP Reno [Internetworking Recitation #9 slides, SpringSemester 2009, Dept. of Computer Science, Technion]

Hnh 2.9-2 Mt nhiu gi trong TCP New Reno [Internetworking Recitation #9 slides, SpringSemester 2009, Dept. of Computer Science, Technion]


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2.10 TCP Selective Acknowledgement

TCP SACK l mt bn TCP chnh sa cho php bn nhn xc nhn cc gi tin b gi sai th

t mt cch chn lc thay v s dng kiu xc nhn tch ly truyn thng l xc nhn gi tin

ng cui cng.

Khi m bn nhn lu tr cc gi tin sai th t trong bm v gi xc nhn chn lc, bn thu

c th b qua vic truyn li cc gi ny khi timeout xy ra v n ch cn truyn li cc gi

m cha c xc nhn.

Bc 1: Gi s gi th 2 b mt.

Bc 2: Client pht hin ra 1 gi

b mt nm gia gi th 1 v th

3. N lin gi 1 duplicate ACK

thng bo l nhn c gi th

1, ng thi trong ty chn SACK

n km thm thng tin l nhn

c gi th 3.

Bc 3: Client nhn gi th 4 vgi 1 duplicate ACK khc cho gi

th1 v trong cha SACK cho

gi th 3 v c gi th 4.

Bc 4: Lc ny server nhn c

duplicate ACK cho gi th1 ng

thi cha thng tin SACK cho gi

th 3. Ty, server bit rng gi

th 2 b mt v truyn li gi s

2. Gi SACK k tip m server

nhn thng bo client nhn gi

th 4 thnh cng, ch cn truyn li gi s 2.

Bc 5: Client nhn c gi tin th2 c truyn li v gi xc nhn cho bit n nhn

d liu n gi th 4.

Hnh 2.10-1 Qu trnh truyn d liu dng TCP vi tychn SACKc kch hot [Jeremy Stretch, 2010]


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s dng ty chn SACK th cbn pht v bn thu u phi h trtnh nng ny v phi

c kch hot bng cch m phn s dng ty chn SACK-Permitted trong trng Options

ca gi tin SYN trong qu trnh thit lp kt ni TCP. S dng ty chn SACK lm cho TCP

hot ng gn tng t vi c ch selective repeat v ci thin hiu nng ca giao thc TCP.

Mt th nghim m phng so snh gia TCP Tahoe, TCP Reno v TCP SACK [Fall & Floyd,

1996] ch ra rng TCP SACK phc hi khi b mt gi chm nhanh v tt hn. Mt th nghim

thi gian thc trn Internet [Bruyeron, Hemon, & hang, 1988] cng cho thy, ty vo xc

sut li ng truyn m TCP SACKc throughput c ci thin t10% n 45% so vi

TCP Reno.

2.11 TCP Forward Acknowledgement

Gii thut Forward Acknowledgement (FACK) c Mathis & Mahdavi [Mathis & Mahdavi,

199] ngh ci thin qu trnh ng x ca TCP i vi mt nhiu hn mt gi trong

mt ca s truyn. Trong FACK, TCP duy tr ba bin s:

snd_fackch gi tin gn nht c xc nhn bi bn nhn thngbo cho bn gi thng qua

trng Option SACK trong gi tin TCP.

snd.nxti din cho s tun t bn nhn mong mun c nhn ln k.

retrans_datach lng gi tin c ti truyn nhng vn cha c xc nhn nhm ti u

thng s xc nh s lng gi tin truyn i ca bn gi bao gm cc gi tin c truyn

li ca TCP FACK.

Gii tht TCP FACK. M t trng thi khi nhn c mt ACKWhile (nhn c gi ACKbt k)

Kim tra trng Option

while (){Gi gi tin();

}Kt thc gii thut


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2.12 TCP Vegas

TCP Vegas c gii thiu vo nm 1994 vi c ch kim sot nghn da vo bn pht mi.

Tri ngc vi gii thut TCP Reno da vo nghn xc nh lu lng cho php ca

mng, TCP Vegas s don thi im bt u nghn mng bng cch theo di s bin thin

ca tc truyn m giao thc mong mun vi tc m n xc nh c. Chin lc ca

TCP Vegas l iu chnh tc truyn (congestion window) c gng duy tr 1 s lng

cc gi tin khng qu ln trong bm ca cc router trung gian trn ng truyn.

TCP Vegas theo di thi im m cc gi truyn i. Khi ACKn, n c lng RTT bng

cch tnh s sai khc ca thi im hin ti so vi thi im c ghi nhn ca 1 gi tin. Vi

mi kt ni, TCP Vegas nh ngha 1 thng s l BaseRTT = minRTT. Thng lng thc

c tnh nh sau:

vi W l ca s truyn d liu

Thng lng mong mun

TCP Vegas so snh 2 gi trtrn iu chnh congestion window bng cch tnh

ng thi nh ngha 2 tr sv sao cho , nu Diff , TCP Vegas tng tuyn tnh

congestion window RTTk tip. Nu Diff> , TCP gim truyn tnh congestion window

RTTk. Trng hp Diff , TCP Vegas gi nguyn gi tr congestion window.

TCP Vegas gim congestion window theo hm tuyn tnh tri vi c ch gim theo hm cp

snhn thng thng ca cc loi TCP khc m bo sn nh. TCP Vegas ch gim

congestion window theo cp s nhn khi c timeout. S gim tuyn tnh chl bc gim ban

u xy ra trc khi nghn tht s v gi tin bt u b loi b.


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2.13 nh gi hi nng TCP

Hiu nng TCP ch yu xc nh bng hai tiu chun nh gi l throughput v airness. Tuy

nhin, cn c cc yu t khc nh mc s dng kt ni (link utiliation) v t l mt gi.

Throughput c nh ngha tc cc gi tin m bn thu nhn c. Biu thc ton hc ca

throughput cc i ca mt phin kt ni xc nh da trn cc thng s bng thng, tr,

kch thc ca s truyn. Gii hn ca lun vn ny ch kho st yu t throughput.

Trong mt phin kt ni TCP, mc tiu cn phi t c l cc i gi tr throughput. Th

th bng cch no Thut ng keeping pipe ull m ch ti cc phng php c gng lp

y phin kt ni ti a c th nhng vn bo m tin cy ca truyn thng tin [RFC

1323]. Ti mt thi im bt k ca phin kt ni, i lngBandwidth Delay Product(BDP)

bng tch s ca bng thng v tr cho php bn gi xc nh c lng thng tin thm

vo t cc i thng tin truyn ti trong ng kt ni. n gin ch l nhn bng thng

vi round-trip delay. B m bn nhn dng BDP xc nh dung lng cn thit nhm

keeping pipe ull, bn gi dng BDP xc nh dung lng thng tin gi gia cc ACK.

Bng bn di cho mt vi tnh ton tng trng.

Bng 2.13-A Mt s v d tnh BDP [www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factors]

BW(Mbps) RT Delay(ms) Bytes/sec Delay(sec) BDP(bytes)1.533 5 191625 0.005 958

10 5 1250000 0.005 625045 5 5625000 0.005 28125

100 5 12500000 0.005 625001000 5 125000000 0.005 6250001.533 20 191625 0.020 38331000 20 125000000 0.020 2500000

Nhng throughput khng ch bi nh hng ca BPD. i vi kch thc ca s truyn

(bytes), nh hng ca n ln throughput ph thuc vo tng quan gia n v BDP. Nu

nh kch thc ca s truyn ln hn BDP th throughput ph thuc vo BDP (bng thng),

ngc li th throughput ph thuc vo ca s truyn v cng thc xc nh throughput trong

ng cnh ny s khc i cht nh sau:
http://www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factorshttp://www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factorshttp://www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factorshttp://www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factorshttp://www.netcraftsmen.net/resources/blogs/tcp-ip-performance-factors


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Tnh ton theo biu thc ny b rng buc bng iu kin l khng c mt mt. Mathis v cc

ng nghip ngh mt cng thc ni ting trong [Mathis et al, 1997]

trong :

MSS(maximum segment sie) l kch thc ti a ca mt gi tin khng k TCP header

l xc sut mt giGii hn trong phng trnh Mathis l cha xem xt ti nh hng ca timeout i vi

throughut, bi do gi tr timeout l c th ty chnh c trong TCP socket. Trong nghin

cu ca mnh, [Padhye et al] ra cng thc xc nh throughput da trn m hnh AIMD

thi gian timeout nh sau:

(

,

min, )

trong l sgi tin c xc nhn bi 1 gi ACK2.13.1 Gii tht ca Nag

Nhm gii quyt vn keeping the pipe ull, gii thut gii quyt trng hp khi d liu

t lp trn xung lp TCP khng chim y mt MSS [RFC 89]. Nagle ngh gii

php sel-cloking, dng cc ACKv d liu t lp trn a ra quyt nh khi no truyn giTCP i.

Gii tht Nag.M t phng php SelclockingWhen (lp Application y dng d liu xung lp TCP)

If (d liu sn c & kch thc ca s truyn) MSSGi gi d liu i ()

elseIf (d liu cha c xc nhn vn cn)

(m d liu mi ny cho ti khi nhn c ACKmi)

else Gi d liu i ();


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2.14 TCP cho knh truyn v tuyn

C ch ca TCP thc hin vic truyn li gi tin b mt hay bsai trn ng truyn t bn

pht n bn thu. Trong mng v tuyn, mt mt c th do nghn mng (trn b m ti

router, access point), do t l li bit cao ca mng v tuyn hay do qu trnh chuyn giao

(handoff). Trong tt ccc trng hp, ACK t bn nhn gi cho bn pht ch ch ra rng gi

tin b mt, bn pht khng bit nguyn nhn xy ra mt gi do nghn, handoff hay li bit.

TCP truyn li cc gi . ng thi c ch kim sot nghn; trong tt ccc trng hp;

gim kch thc ca s tc nghn. Bng cch gim ca s tc nghn ny, TCP cho rng mt

gi l do nghn bt chp c th do li bit hay handoff. Li bit rt d xy ra trong knh v

tuyn. Do , khi li bit xy ra hay do handoff, vic gim ca s tc nghn (gim tc

truyn) l khng ph hp. Hn th, trng hp bm ca cc router trng m mt gi doknh tuyn dn n gim tc truyn s lng ph ti nguyn mng.

Do , i vi knh truyn v tuyn, vic nghin cu 1 giao thc TCP ci tin nng cao

hiu nng truyn ti l vn vn cn ang c nghin cu.

2.15 Kt n

Trong chng ny, chng ta tho lun v s pht trin ca TCP v m t cc c trng vi

c bit nhn mnh c ch iu khin nghn v ti truyn khi mt gi. Hu ht cc phin bn

TCP c phn tch v nh gi, chng ta thy cch m mt gi c pht hin, ti

truyn li v window kim sot nghn c tnh ton qua cc tham s c trng ca TCP.

c bit, fast retransmitvfast recovery ca TCP Reno v TCP New Reno c m t

chi tit.


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CHNG 3. KHI QUT IEEE 802.11

Cng ngh mng truyn thng lun vn ng, trong hn 20nm trli y chng ta chng

kin c mt cuc cch mng truyn dn v tuyn lm thay i tp tnh cng vic v cuc

sng ca con ngi. Hu ht chng ta qu quen thuc vi chun IEEE 802.11, bit n vi

tn gi Wi-Fi. Wi-Fi l mt k thut c thng mi ha, c qun l bi lin minh Wi-Fi

[Kurose & Ross, 2013]. Cng ngh mng v tuyn khc bit vi cng ngh mng hu tuyn

cin mt vi hng quan trng, trong khi chia s vi nhau nhiu c tnh chung. Ging

nh cc lin kt trong hu tuyn, vn li bit l vn c bit c quan tm nhng mng

v tuyn li c bit hn chmi trng truyn ca n l khng th don trc c.

Hn na, cng ngh v tuyn cn phi ch ti vn cng sut truyn dn tn hiu, n b

gii hn bi tnh di ng vn thun tin trong mt phm vi hp hn nhiu so vi mng hutuyn. Mi trng truyn v tuyn cng tha kc tnh a truy cp ca mng hu tuyn

nhng c ci tin ph hp vi khnng bin ha ca mi trng truyn dn.

3.1 Tng qut v WLAN

WLAN hin ti l mt trong nhng cng ngh mng quan trng nht trong Internet hin nay

Mc d c nhiu cng ngh v tiu chun cho WLAN c pht trin trong nhng nm

90, chun IEEE 802.11 c xem l bc i u cho WLAN [Kurose & Ross, 2013].

802.11 l thnh vin ca nhm IEEE 802, nhm chun ha c trng cho cng ngh LAN.

802.2 Logical Link Control

802.3MAC

802.3PHY

802.5MAC

802.5PHY

802.11 MAC

FHSS DSSS OFDM

Hnh 3.1-1 Chun 802 ca IEEE

C nhiu lp con ca chun 802.11 (bng 3.1-1), chng chia s nhiu c tnh cng s dng

giao thc truy cp, s dng cng mt cu trc khung d liu, cng h trnhiu ch truyn

dn (infrastructure hay ad hoc) c thvn ra khong cch truyn dn xa hn.


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Bng 3.1-A Tm tt cc thng s vt l ca 802.11

Chun Ph vi ng tn(GHz)Tc truyn ti ti a

(Mbps)802.11a 5.1-5.8 54

802.11b 2.4-2.485 11802.11g 2.4-2.485 54802.11n 2.4-5 248

Hnh 3.1-2 Tc truyn ti ti a v phm vi hot ng ca cc chun khng dy ca IEEE

[Kurose & Ross, 2013]WLAN 802.11a hot ng bng tn 5 GHz v c th truyn ti ln ti 54Mbps, nhng

chng c khong cch truyn ngn vi mc cng sut pht cho trc v chu nhiu nh

hng ca hin tng ading a ng. WLAN 802.11b c tc truyn ti ln ti 11Mbps

v hot ng bng tn khng cn giy php ng k 2.4-2.485 GHz. WLAN 802.11g c

cng bng tn hot ng vi WLAN 802.11b nhng li c tc truyn ti d liu ti a cao

hn 802.11a, chun ny s dng OFDM, m hnh iu ch tng t nh 802.11a, t

c tc gii hn ln hn. WLAN 802.11n ra i sau v h trMIMO v nhiu c tnhmi khc.

3.2 Kin trc ca chun 802.11

Hnh 3.2-1 m t cc thnh phn c bn ca kin trc WLAN 802.11. Mt 802.11 WLAN

da trn nn tng kin trc tbo trong c h thng sc phn chia thnh cc t bo.

Mi t bo, gi l Basic Service Set (BSS), c iu khin bi Base Station (BS) gi l

Access Point (AP). Khi mt WLAN c kt t nhiu BSS, cc AP thng qua mt mng li,


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thng l Ethernet, gi l Distribution System (DS). Ton b mng WLAN bao gm cc BSS

khc nhau cng vi AP ca n v mng DS th c gi l Extended Service Set (ESS).

Bng 3.2-A Bng so snh c im ng dng ca BSS Ad hoc v BSS infrastucture

IEEE 802.11 ad hoc IEEE 802.11 infrastructure WLANKch thc mng nh, thi gian tn ti ngnv mc ch s dng, bi ton quan trng lcng sut.

Kch thc ca loi hnh ny ph thucvo cng sut pht ca AP pht i.

S lng nt mng phi nh v yu t cngsut.

H trnhiu nt mng hn.

Nhng mng WLAN s dng cc AP thng c gi l infrastucture WLAN. Khi cc nt

mng lin kt vi nhau khng qua mt AP th gi l mng ad hoc, mng ny khng c kt ni

Internet. Trong mt mng ad hoc, cc nt mng lin kt vi nhau bng thit lp lin lc nganghng. Mi nt mng lin lc trc tip vi nhau trong cng mt mng ad hoc. Ad hoc c

thit k ch c cc nt mng trong phm vi truyn dn ca nhau mi c th lin lc vi

nhau, do , khi cc trm con ca cc ad hoc khc nhau mun lin lc th phi thng qua mt

gateway v qu trnh nh tuyn. Loi mng ny khng i hi cc quyn qun tr v chia s

ti nguyn mng khng cn thng qua mt server. So snh hai loi ad hoc v loi

inrastructure nh bng 3.2-1.

Hnh 3.2-1 Cc thnh phn c bn ca IEEE 802.11 [Kurose & Ross, 2013]


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3.3 Giao thc MAC ca 802.11

Cc c trng ca 802.11 u tp trung vo 2 lp thp nht trong m hnh OSI bi n tch

hp cc thnh phn vt l v lin kt d liu. Tt c cc mng ca 802.11 u c lp MAC v

lp PHY.

MAC: t hp cc quy tc nhm xc nh cch thc truy cp v gi d liu. PHY: chi tit v truyn dn v thu thp d liu.Chun 802.11 nh ngha lp MAC v lp PHY cung cp nhiu chc nng h trhot ng

ca WLAN 802.11. Lp MAC qun l v duy tr lin lc gia cc nt mng 802.11 bng cch

phn phi cc truy cp trn mi trng truyn dn chung. Nh minh ha trong hnh 3.3-1, lp

PHY c tri ph nhy tn (FHSS), tri ph trc tip (DSSS) v hng ngoi (IR). Lp MAC

tng tc vi lp PHY bng cc khung MAC.

802.2 Logical Link Control

CSMA/CA

802.112Mbps

S-band ISMFHSS

802.11b11Mbps

S-band ISMDHSS

802.11a54Mbps

C-band ISMOFDM

802.11g54 Mbps

S-band ISMOFDM

Hnh 3.3-1Cc c im khi qut ca cc chun IEEE 802.11

Bn cnh cc chc nng ca lp MAC ging nh lp MAC ca mng hu tuyn truyn

thng, 802.11 MAC cn c nhng chc nng khc bit nh phn on gi tin, ti truyn li

v xc nhn. MAC cung cp cc quyn truy cp vo mi trng truyn chia s thng qua cc

chc nng phi hp (CF) h tr cc lu lng bt ng b v b rng buc thi hn;

Distributed Coordination Function (DCF) v Point Coordination Function (PCF).


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Hnh 3.3-2 CSMA/CA

DCF da trn c sl Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

Trong CSMA/CA, cc trm con mun thc hin truyn d liu trn mi trng chung phi

thc hin cc bc sau nh minh ha trong hnh 3.3-2.

1. Lng nghe ng truyn. Nu ng truyn ang rnh, n s ch trong thi lng bnggi trDIFS cho trc th sau chuyn sang giai on Contention Window. Trong giaion ny, khi ng bm back-off ngu nhin. Kim tra tnh trng ng truyn lnna, nu bn th gi nguyn gi tr bm back-o, cn ngc li nu trng th gim b

m back-off. Tin trnh kim tra tnh trng mi trng truyn c trng hay khng din ralin tc cho ti khi bm back-off v 0 th n s gi RTS.2. Bn nhn khi nhn c RTS, si mt khong SIFS sau truyn i gi CTS.3. Bn gi nhn c CTS, n bt u gi d liu. Cn i vi cc nt khc, khi nhn c

CTS th chuyn sang ch ch, v thit lp gi tr NAV da trn thng tin thu c tkhung CTS m n nhn c.NAV c vai tr nh bm li ca cc trm con, khng cquyn truy cp mi trng truyn, cho ti khi mi trng truyn rnh.

im quan trng ca DCF l bm back-off ngu nhin, iu ny lm gim khnng ng

khi m c nhiu nt ng thi lng nghe c knh truyn ang rnh. Cc gi RTS/CTS

chnh l phn CA ca 802.11 MAC nhm mc ch thng bo cho cc nt mng khc v tnh

trng knh truyn s b chim dng. Ngoi ra, mc ch nm sau hai loi gi tin ny nhm

trnh hin tng hidden node v exposed node [Kurose & Ross, 2013].

Ch hot ng PCF khng c s hin din ca bm back-off, quyn chim dng knh

truyn s do AP kim sot v phn b cho cc nt v tuyn. AP snh ngha hai giai on l

contention free period (CFP)v contention period (CP). Trong CP, DCF c s dng bnh

thng. Trong CFP, AP gi cc khung beacon sau mt PIFS sau khi xc nhn rng knhtruyn hon ton trng. chc rng AP c quyn u tin truy cp knh truyn nn PIFS


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DIFS, cc gi tin beacon hi xem nt mng no c khung d liu cn gi, cc nt khc

khng c khung d liu gi sau khi nhn c beacon s thit lp NAV cho mnh v

khng c php gi bt k gi tin no trong thi on CFP.

3.4 Khung dliu IEEE 802.11

Khung d liu 802.11 (hnh 3.4-1) cha cc trng (ield) c trng s dng cho knh v

tuyn. Trng d liu c th cha ti a 2312 byte. Tuy nhin, thng thng trng ny

c s dng t hn 1500 byte, cha gi tin IP hay l gi tin ARP. Tng t nh khung

Ethernet, khung 802.11 cha m kim tra vng (cyclic redundancy check, CRC) do bn

nhn c th pht hin ra li trn khung tin nhn c. M vng rt l hu dng trong mng

Wireless LAN, do trong mi trng ny xc sut li bit rt cao.

Trong khung d liu, c 4 trng a ch, mi trng a ch c th cha ti byte a ch

MAC

a ch th1 l a ch MAC ca bn pht khung tin. a ch th2 l a ch MAC ca bn nhn khung tin. a ch th3 l a ch MAC ca giao din (interface) router m BSS kt ni vo. a ch th 4 ch s dng trong mng ad hoc.Bn gi c th gi li khung tin nhiu ln nu ACK b mt. Trng s tun t (sequencenumber) gip bn nhn phn bit gia khung tin mi v khung tin c c ti truyn li. Giao

thc 802.11 cho php trm pht chim knh truyn trong 1 khong thi gian cn truyn

khung tin, bao gm thi gian truyn khung d liu v thi gian gi tr ACK. Khong

thi gian ny c quy nh trong trng Duration.

Hnh 3.4-1 Khung d liu 802.11

Khung iu khin h trcho vic truyn khung tin gm nhiu trng con (subield). Trng

Type v Subtype c s dng phn bit cc lin kt, RTS, CTS, ACK v khung d liu.

Trng To AP v From AP c dng nh ngha cc trng a ch khc nhau. Trng


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More ragments c dng thng bo cn nhiu phn on thuc cng 1 khung d liu s

c truyn tip theo sau phn on hin ti. Trng ny sc bn thu dng pht hin

vic truyn li cc khung tin trng nhau. Cui cng, trng Wired equivalent privacy (WEP)

thng bo rng vic mt m ha d liu c s dng hay khng. Cc m ty v khung

d liu MAC c trnh by chi tit trong [OHara & Petrick].


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CHNG 4. M PHNG NH GI HOT NG C BN CA

TCP, TCP TRN 802.11 V CC CU HNH HOT NG CA 802.11

A. M PHNG NH GI C CH QUN L NGHN CA TCP TRONG OPNET

MODELER 14.5

Ht nhn ca giao thc truyn ti TCP l c ch kim sot nghn ca n, do quan st

v kim chng cch vn hnh ca c ch ny th cng c h trc cung cp trong OPNET

Modeler 14.5 l rt thun li. Li ca ht nhn ny gm 3 thnh phn nh gii thiu trong

chng 2, l cc giai on slow-start, congestion avoidance, fast recovery. Chi tit hn,

cc giai on hot ng ca TCP gm nhiu cc tham snh hng v c trnh by chi tit

trong ph lc B, cc nh gi ban u m chng ti tin hnh cha th hin ht c s phctp trong cch hot ng ca TCP trong thc t.

Trong mi trng mng hu tuyn, mi trng truyn dn c t l li bit thp nhng l

mng Long Fat Network [RFC 1323] nn s kin mt gi xy r

Ghep Report 3.6

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Transcript of Ghep Report 3.6