Nghien Nghien cuu van de an ninh mang Internet khong day va ung dungCuu Van de an Ninh Mang Internet Khong Day Va Ung Dung

S ha bi Trung tm Hc liu i hc Thi Nguyn http://www.lrc-tnu.edu.vn

I HC THI NGUYN

KHOA CNG NGH THNG TIN

..*..

BI PHI LONG

NGHIN CU VN AN NINH MNG

INTERNET KHNG DY V NG DNG

LUN VN THC S KHOA HC MY TNH

THI NGUYN - 2009


I HC THI NGUYN

KHOA CNG NGH THNG TIN

..*..

BI PHI LONG

NGHIN CU VN AN NINH MNG

INTERNET KHNG DY V NG DNG

Chuyn nghnh: KHOA HC MY TNH

M s : 60.48.01

LUN VN THC S KHOA HC MY TNH

Ngi hng dn khoa hc: PGS.TS NGUYN VN TAM

THI NGUYN - 2009

THI NGUYN - 2009

i


MC LC

Trang

TRANG PH BA.

LI CM N....

LI CAM OAN......

MC LC.i

DANH MC CC K HIU, CH CI VIT TT....v

DANH MC CC BNG.....ix

DANH MC CC HNH....x

M U..1

CHNG 1. TNG QUAN V MNG INTERNET....3

1.1. Gii thiu cng ngh mng Internet khng dy v ng dng ...3

1.1.1. Cng ngh mng Internet khng dy3

1.1.2. u v nhc im ca cng ngh mng Internet khng dy....4

1.1.2.1. u im...4

1.1.2.2. Nhc im.5

1.2. Kin trc c bn ca mng LAN khng dy.5

1.2.1. Gii thiu chung v mng LAN khng dy WLAN...5

1.2.2. Chun 802.11 6

1.2.2.1. Nhm lp vt l PHY bao gm cc chun:.7

1.2.2.2. Nhm lp lin kt d liu MAC bao gm cc chun:.8

1.2.3. Cc m hnh WLAN (chun 802.11).9

1.2.3.1. Trm thu pht STA.. 9

1.2.3.2. im truy cp AP 9

1.2.3.3. Mng 802.11 linh hot v thit k, gm 3... 10

ii


1.2.3.4. WEP Wired Equivalent Privacy ... 14

1.2.3.5. WEP key lengths ..14

1.2.3.6. WPA Wi- fi Protected Access ...15

1.2.3.7. WPA2 Wi- fi Protected Access 2 .... 15

1.3. Kin trc c bn ca mng WAN khng dy..16

1.3.1. Th h th 1 (1G) ... 17

1.3.2. Th h th 2 (2G) 17

1.3.3. Th h di ng th 3 (3G)... 18

1.4. Kin trc c bn ca Internet khng dy.22

1.4.1. Kin trc c bn ca Internet khng dy chun WAP.22

1.4.1.1. S b v WAP....22

1.4.1.2. Cc m hnh giao tip trn WAP ..24

1.4.1.3. u v nhc im ca WAP 28

1.4.1.4. Cc thnh phn ca WAP. 30

1.4.2. Kin trc c bn ca mng WPAN khng dy... 37

1.4.3. Kin trc c bn ca mng WMAN khng dy ..49

1.4.3.1. c im ni bt ca WiMAX di ng 40

1.4.3.2. M hnh ng dng WiMAX..40

1.4.4. Mng khng dy WRAN.42

1.5. Tng kt...42

CHNG 2. TNG QUAN V AN NINH MNG INTERNET KHNG DY ..44

iii


2.1. Mt s k thut tn cng Internet khng dy.......................................................44

2.1.1. Tn cng b ng Passive attacks44

2.1.1.1. nh ngha.44

2.1.1.2. Kiu tn cng b ng c th - Phng thc bt gi tin (Sniffing)....45

2.1.2. Tn cng ch ng Active attacks47

2.1.2.1. nh ngha47

2.1.2.2. Cc kiu tn cng ch ng c th..48

2.1.3. Tn cng kiu chn p - Jamming attacks 54

2.1.4. Tn cng theo kiu thu ht - Man in the middle attacks.. 55

2.1.5. Tn cng vo cc yu t con ngi ..55

2.1.6. Mt s kiu tn cng khc ...56

2.2. Gii php an ninh cho mng Internet khng dy (WAP)...57

2.2.1. Vn bo mt trn WAP..57

2.2.1.1. So snh cc m hnh bo mt57

2.2.1.2. WAP Gateway..63

2.2.1.3. TLS v WTLS...66

2.3. Tng kt ....68

CHNG 3: MNG INTERNET KHNG DY V TH NGHIM 70

3.1. Thit k m hinh mang Internet khng dy trong trng Vit c TN ..70

3.1.1. Nguyn tc thit k.70

3.1.2. M hinh logic v s ph sng vt ly tng th ti trng..71

3.1.2.1. M hnh thit k logic....71

3.1.1.2. S ph sng vt ly tng th ti trng..71

3.1.3. Thit k chi tit cua h thng..73

3.1.3.1. M hnh thit k chi tit h thng mng khng dy...73

3.1.3.2. Thit b s dng trong h thng mng khng dy..73

3.1.3.3. Phn b thit b s dng trong h thng.75

iv


3.2. Gii php bo mt trong mng khng dy ti CCN Vit c Thi Nguyn...75

3.2.1. Yu cu bao v thng tin...76

3.2.1.1. Bo v d liu:77

3.2.1.2. Bo v cc ti nguyn s dng trn mng:.77

3.2.1.3. Bo v danh ting c quan:78

3.2.2. Cc bc thc thi an ton bao mt cho h thng..78

3.2.2.1. Cc hot ng bo mt mc mt.78

3.2.2.2. Cc hot ng bo mt mc hai..79

3.3. Chng trinh thc t a xy dng...79

3.4. anh gia kt qua..80

3.5. Mt s hng dn bo v my tnh an ton khi dng Internet khng dy.80

3.5.1. Ti u ha Wi-Fi cho cc VoIP, Video Game.80

3.5.2. u tin ha ti gi d liu... 81

3.5.3. Tt Wi-Fi khi khng dng n 83

3.5.4. Theo di nhng ngi khng mi m n trn mng Wi-Fi ca bn..83

3.5.5. Loi b im kt ni khng dy an ton. 84

3.5.6. V hiu ha Peer-to-Peer Wi-Fi .85

3.6. Tn cng Website Cch x l....87

3.7. Tng kt.....88

KT LUN ...90

TI LIU THAM KHO..92

PH LC...94

v


DANH MC CC K HIU, CH CI VIT TT

AAA - Authentication Authorization Audit

ACL - Access control lists

ACS - Access Control Server

ACU - Aironet Client Utility

AES Advanced Encryption Standard

AP - Access point

APOP - Authentication POP

BSS - Basic Service Set

BSSID - Basic Service Set Identifier

CA - Certificate Authority

CCK - Complimentary Code Keying

CDMA - Code Division Multiple Access

CHAP - Challenge Handshake Authentication Protocol

CMSA/CD - Carrier Sense Multiple Access with Collision Detection

CRC - Cyclic redundancy check

CSMA/CA - Carrier Sense Multiple Access with Collision Avoidance

CTS - Clear To Send

DES - Data Encryption Standard

DFS - Dynamic Frequency Selection

DHCP - Dynamic Host Configuration Protocol

DMZ - Demilitarized Zone

DOS - Denial of service

DRDOS - Distributed Reflection DOS

DS - Distribution System

DSSS - Direct Sequence Spread Spectrum

vi


EAP - Extensible Authentication Protocol

EAPOL - EAP Over LAN

EAPOW - EAP Over Wireless

ESS - Extended Service Set

ETSI - European Telecommunications Standards Institute

FCC - Federal Communications Commissio

FHSS Frequency Hopping Spread Spectrum

GPS - Global Positioning System

HiperLAN - High Performance Radio LAN

HTML -HyperText Markup Language

HTTP - HyperText Transfer Protocol

IBSS - Independent Basic Service Set

ICMP -Internet Control Message Protocol

ICV Intergrity Check Value

IEEE - Institute of Electrical and Electronics Engineers

IETF - Internet Engineering Task Force

IR - Infrared Light

IKE - Internet Key Exchange

IP - Internet Protocol

IPSec - Internet Protocol Security

IrDA - Infrared Data Association

ISDN -Integrated Services Digital Network

ISM - Industrial Scientific and Medical

ISP - Internet Service Provider

ITU - International Telecommunication Union

IV - Initialization Vector

LAN - Local Area Network

vii


LCP Link Control Protocol

LEAP - Light Extensible Authentication Protocol

LLC - Logical Link Control

LOS - Light of Sight

MAC - Media Access Control

MAN - Metropolitan Area Network

MIC - Message Integrity Check

MSDU - Media Access Control Service Data Unit

OCB - Offset Code Book

OFDM - Orthogonal Frequency Division

OSI - Open Systems Interconnection

OTP - One-time password

PAN - Person Area Network

PBCC - Packet Binary Convolutional Coding

PCMCIA - Personal Computer Memory Card International Association

PDA - Personal Digital Assistant

PEAP - Protected EAP Protocol

PKI-Public Key Infrastructure

PRNG - Pseudo Random Number Generator

QoS - Quality of Service

RADIUS - Remote Access Dial-In User Service

RF - Radio frequency

RFC - Request For Comment

RTS - Request To Send

SIG - Special Interest Group

SSH - Secure Shell

SSID - Service Set ID

viii


SSL - Secure Sockets Layer

STA - Station

SWAP - Standard Wireless Access Protocol

TACACS - Terminal Access Controller Access Control System

TCP - Transmission Control Protocol

TFTP - Trivial File Transfer Protocol

TKPI - Temporal Key Integrity Protocol

TLS - Transport Layer Security

TPC - Transmission Power Control

UDP - User Datagram Protocol

UWB Ultra Wide Band

UNII - Unlicensed National Information Infrastructure

VLAN - Virtual LAN

WAN - Wide Area Network

WECA - Wireless Ethernet Compatibility

WEP - Wired Equivalent Protocol

Wi-Fi - Wireless fidelity

WLAN - Wireless LAN

WPAN - Wireless Personal Area Network

ix


DANH MC CC BNG

Bng 1.1. Technology Features Comparison 22

Bng 1.2. Pre-4G Technology Requirement Comparison .....22

Bng 2.1. So snh s khc nhau gia WTLS v TLS ...67

Bng 3.1. Cc c tnh k thut ca AP TP-Link 108Mbits 1 Port (TL-WA601G). 74

x


DANH MC CC HNH V

Hnh 1.1. M hnh mng AD HOC ....10

Hnh 1.2. M hnh mng c s ...11

Hnh 1.3. M hnh mng m rng.. 12

Hnh 1.4. M hnh mng khng dy kt ni vi mng c dy ...13

Hnh 1.5. M hnh 2 mng c dy kt ni vi nhau bng kt ni khng dy..14

Hnh 1.6. Con ng pht trin ca cc cng ngh mng...16

Hnh 1.7. WAP dng truy cp Internet....24

Hnh 1.8. WAP c dng truy cp trong Intranet .....25

Hnh 1.9. Wap Client...30

Hnh 1.10. Wap Stack..31

Hnh 1.11. Wap Stack..31

Hnh 1.12. Yu cu khng tin cy...34

Hnh 1.13. Yu cu tin cy. 34

Hnh 1.14. Yu cu tin cy vi thng ip kt qu.35

Hnh 1.15. M hnh lm vic ca Wap gateway.36

Hnh 1.16. M hnh ng dng Wimax....41

Hnh 2.1. Cc phng thc dng trong tn cng b ng ..45

Hnh 2.2. Phn mm bt gi tin Ethereal ...46

Hnh 2.3. Phn mm thu thp thng tin h thng mng khng dy NetStumbler ..47

Hnh 2.4. Tn cng ch ng .48

Hnh 2.5. M t qu trnh tn cng DOS tng lin kt d liu ...50

Hnh 2.6. M t qu trnh tn cng mng bng AP gi mo ..52

Hnh 2.7. M t qu trnh tn cng theo kiu chn p.... 54

Hnh 2.8. M t qu trnh tn cng theo kiu thu ht..55

Hnh 2.9. M hnh bo mt trn Internet ....57

Hnh 2.10. M hnh bo mt trn WAP.. 59

xi


Hnh 2.11. WAP 1.0.. 60

Hnh 2.12. WAP 2.0 ..61

Hnh 2.13. WAP 61

Hnh 2.14. S dng WAP proxy/gateway. 63

Hnh 2.15. Cc bc thc hin khi tin hnh mt phin giao dch WAP 64

Hnh 2.16. Qu trnh bin dch cc yu cu ti gateway chuyn i giao thc 65

Hnh 2.17. M t chc nng m ha/ gii m ca WAP gateway 65

Hnh 3.1. M hnh logic mng khng dy ti trng ...71

Hnh 3.2. M hnh ph sng ti trng CCN Vit c Thi Nguyn ...72

Hnh 3.3. Access Point (AP) TP-Link 108Mbits 1 Port (TL-WA601G).. 73

Hnh: 3.4. M phng kin trc hin ti h thng mng Internet khng dy. 80

Hnh 3.5. Cu hnh ca Router Linksys 81

Hnh 3.6. Ti u cho gi d liu gi nhn thng qua thit lp tren Rounter 82

Hnh 3.7. Cp quyn u tin ..82

Hnh 3.8. Tt Wi-Fi khi khng dng n ...83

Hnh 3.9. Thit lp theo di khch khng mi m n.. 84

Hnh 3.10. Loi b im kt ni khng dy an ton ......84

Hnh 3.11. V hiu ha Peer-to-Peer Wi-Fi ...85



Hnh PL1. Nokia Mobile Internet Toolkit.. 97

Hnh PL2. Nokia WAP Gateway Simulator....98

Hnh PL3. Nokia WAP Gateway.... 99

Hnh PL4. Nokia Browser Simulator.. 99

Hnh PL5. H thng Menu Nokia. 100

xii


- 1 -


M U

1. Nn tng v mc ch.

Mng Internet khng dy hin nay c p dng trong rt nhiu lnh vc bi

nhng u th ni tri ca n so vi mng Internet hu tuyn truyn thng: ngi

dng c th di chuyn trong phm vi cho php, c th trin khai mng Internet

khng dy nhng ni m mng Internet hu tuyn khng th trin khai c. Tuy

nhin, khc vi mng Internet hu tuyn truyn thng, mng Internet khng dy s

dng knh truyn sng in t, v do n t ra nhiu thch thc trong vic xy

dng c t v trin khai thc t mng ny. Mt trong nhng thch thc v cng

l vn nng hi hin nay l vn an ninh cho mng Internet khng dy.

c nhiu gii php an ninh ra i nhm p dng cho mng Internet khng

dy, trong chun WAP c c t vi tham vng mang li kh nng an ton cao

cho mng Internet khng dy. Tuy vy, vic h tr cc phn cng c cng vi vic

c t cho php cc nh sn xut phn cng c quyt nh mt s thnh phn khi

sn xut khin cho cc mng Internet khng dy khi trin khai khng nhng khng

ng nht m cn c nhng ri ro an ninh ring.

Do , mc ch ca lun vn ny l nghin cu, phn tch nhng c im ca

mng Internet khng dy, nhng k thut tn cng mng Internet khng dy t

a ra nhng gii php an ninh, bo mt cho mng Internet khng dy da trn

cc tiu ch: tnh bo mt, tnh ton vn, xc thc hai chiu v tnh sn sng. Trn

c s , xut xy dng mt m hnh an ninh, bo mt cho mng Internet khng

dy ti trng Cao ng Cng nghip Vit c Thi Nguyn.

2. Cu trc ca lun vn.

Ngoi phn m u v kt lun, ni dung ca lun vn c b cc nh sau:

Chng 1: Trnh by cc kin thc tng quan v mng Internet v c bit l

mng Internet khng dy. Kin trc c bn ca: mng LAN khng dy, mng WAN

khng dy, mng Internet khng dy (chun WAP v cc chun mi) t c

c ci nhn bao qut v cch thc hot ng ca mng Internet khng dy.

- 2 -


Chng 2: i su vo nghin cu cc k thut tn cng mng Internet khng

dy (cc tng trn WAP) t a ra cc gii php an ninh, bo mt cho mng

Internet khng dy da trn hai kha cnh: m bo an ton d liu v ton vn d

liu. Bn cnh vic cung cp tng qut v qu trnh pht trin cng nh ci tin cc

phng php, chng nay cng s ch ra nhng ri ro an ninh ph bin trong mng

Internet khng dy.

Chng 3: T nhng kin thc nghin cu hai chng trc, chng 3

gii thiu ng dng mng Internet khng dy vo xy dng m hnh an ninh, bo

mt cho mng Internet khng dy ti trng Cao ng Cng nghip Vit c Thi

Nguyn. Ngoi ra, cn gii thiu mt s k thut bo v an ton my tnh khi s

dng Internet khng dy, cch x l khi website b tn cng.

Cui cng l phn ph lc v ti liu tham kho.

- 3 -


CHNG 1. TNG QUAN V MNG INTERNET

1.1. Gii thiu cng ngh mng Internet khng dy v ng dng

1.1.1. Cng ngh mng Internet khng dy.

Mng Internet t lu tr thnh mt thnh phn khng th thiu i vi

nhiu lnh vc trong i sng x hi, t cc c nhn h gia nh, n v, doanh

nghip dng mng Internet phc v cho cng vic, hc tp, hot ng t chc kinh

doanh, qung b..v.vcho n h thng mng Internet ton cu m c x hi, c

th gii ang hng ngy hng gi s dng. Cc h thng mng hu tuyn v v

tuyn ang ngy cng pht trin, pht huy vai tr ca mnh trong mng Internet

khng dy ni ln nh mt phng thc truy nhp Inetrnet ph bin dn thay th

cho mng Internet c dy kh trin khai, lp t.

Mc d mng Internet khng dy xut hin t nhiu thp nin nhng cho n

nhng nm gn y, vi s bng n cc thit b di ng th nhu cu nghin cu v

pht trin cc h thng mng Internet khng dy ngy cng tr nn cp thit. Nhiu

cng ngh, phn cng, cc giao thc, chun ln lt ra i v ang c tip tc

nghin cu v pht trin.

Mng Internet khng dy c tnh linh hot, h tr cc thit b di ng nn

khng b rng buc c nh v phn b a l nh trong mng Internet hu tuyn.

Ngoi ra, ta cn c th d dng b sung hay thay th cc thit b tham gia mng

Internet m khng cn phi cu hnh li ton b toplogy ca mng. Tuy nhin, hn

ch ln nht ca mng Internet khng dy l kh nng b nhiu v mt gi tin so vi

mng Internet hu tuyn. Bn cnh , tc truyn cng l vn rt ng

chng ta quan tm.

Hin nay, nhng hn ch trn ang dn c khc phc. Nhng nghin cu v

mng Internet khng dy hin ang thu ht cc Vin nghin cu cng nh cc

Doanh nghip trn th gii. Vi s u t , hiu qu v cht lng ca h thng

mng Internet khng dy s ngy cng c nng cao, ha hn nhng bc pht

trin trong tng lai.

- 4 -


Trong cc h thng mng Internet hu tuyn, d liu nhn v truyn t cc

my ch ti h thng cc Website thng qua cc dy cp hoc thit b trung gian.

Cn i vi mng Internet khng dy, cc my ch truyn v nhn thng tin t

Internet thng qua sng in t, sng radio.

Tn hiu Internet c truyn trong khng kh trong mt khu vc gi l vng

ph sng Internet. Thit b nhn Internet ch cn nm trong vng ph sng Internet

ca thit b pht Internet th s nhn c tn hiu.

1.1.2. u v nhc im ca cng ngh mng Internet khng dy.

1.1.2.1. u im

- Tnh tin li, di ng: Cho php ngi dng truy xut ti nguyn trn mng

Internet bt k ni u trong khu vc c trin khai (cng vin, nh hay vn

phng), iu ny rt kh i vi mng Internet c dy v kh trin khai ngay lp

tc, khng c ng, kh i vi nhiu khu vc khng ko dy c, mt nhiu thi

gian, tin ca..v.v...Tnh di ng ny s tng nng xut v tnh kp thi tha mn

nhng nhu cu thng tin m mng Internet hu tuyn khng th c c.

- Tnh hiu qu: Ngi dng c th duy tr kt ni mng Internet khi h i t

ni ny n ni khc trong phm vi vng ph sng ca mng Internet khng dy

(trong mt ta nh, mt khu vc nht nh).

- Tit kim chi ph lu di: Vic thit lp h thng mng Internet khng dy

ban u ch cn 1 Accesspoint v Accesspoint ny c kt ni vi Internet thng qua

Switch hoc Modem. Nhng t 1 Accesspoint ny rt nhiu my tnh c th truy

cp Internet, tit kim chi ph rt nhiu so vi phi ko dy trong mng Internet hu

tuyn, chi ph di hn c li nht trong mi trng ng cn phi di chuyn v thay

i thng xuyn, cc chi ph v thi gian tn ti ca mng Internet hu tuyn c

th thp hn ng k so vi mng Internet khng dy.

- Kh nng m rng: Mng Internet khng dy c th p ng tc th khi gia

tng s lng ngi dng (iu khng th i vi mng Internet c dy v phi lp

t thm thit b,).

- 5 -


- Tnh linh hot: D dng b xung hay thay th cc thit b tham gia mng m

khng cn phi cu hnh li ton b topology mng.

1.1.2.2. Nhc im.

- Bo mt: Mi trng kt ni Internet khng dy l khng kh -> kh nng b

tn cng ca ngi dng l rt cao.

- Phm vi: Mt mng chun 802.11g vi cc thit b chun ch c th hot

ng tt trong phm vi vi chc mt, ngoi phm vi cc thit b truy cp Internet

khng th nhn c tn hiu hoc nhn c tn hiu th rt yu, ngt qung khng

m bo .

- Cht lng: V mng Internet khng dy s dng sng v tuyn truyn

thng nn vic b nhiu, tn hiu b gim do tc ng ca cc thit b khc ( l vi

sng....) l khng trnh khi.

- Tc : Tc ca mng Internet khng dy (1 125 Mbps) rt chm so vi

mng s dng cp (100 Mbps n hng Gbps).

1.2. Kin trc c bn ca mng LAN khng dy.

1.2.1. Gii thiu chung v mng LAN khng dy WLAN.

Wireless LAN (Wireless Local Area Network) s dng sng in t ( thng

l sng radio hay tia hng ngoi) lin lc gia cc thit b trong phm vi trung

bnh. So vi Bluetooth, Wireless LAN c kh nng kt ni phm vi rng hn vi

nhiu vng ph sng khc nhau, do cc thit b di ng c th t do di chuyn

ga cc vng vi nhau. Phm vi hot ng t 100m n 500m vi tc truyn d

liu trong khong 1Mbps 54 Mbps (100Mbps)

IEEE ( Institute of Electrical and Electronic Engineers ) l t chc i tin

phong trong lnh vc chun ha mng LAN vi n IEEE 802 ni ting bt u

trin khai t nm 1980 v kt qu l hng lot chun thuc h IEEE 802.x ra i,

to nn mt s hi t quan trng cho vic thit k v ci t cc mng LAN trong

thi gian qua.

- 6 -


802.11 l mt trong cc chun ca h IEEE 802.x bao gm h cc giao thc

truyn tin qua mng khng dy. Trc khi gii thiu 802.11 chng ta s cng im

qua mt s chun 802 khc:

- 802.1: cc Cu ni (Bridging), Qun l (Management) mng LAN, WAN

- 802.2: iu khin kt ni logic

- 802.3: cc phng thc hot ng ca mng Ethernet

- 802.4: mng Token Bus

- 802.5: mng Token Ring

- 802.6: mng MAN

- 802.7: mng LAN bng rng

- 802.8: mng quang

- 802.9: dch v lung d liu

- 802.10: an ninh gia cc mng LAN

- 802.11: mng LAN khng dy Wireless LAN

- 802.12: phng phc u tin truy cp theo yu cu

- 802.13: cha c

- 802.14: truyn hnh cp

- 802.15: mng PAN khng dy

- 802.16: mng khng dy bng rng

Chun 802.11 ch yu cho vic phn pht cc MSDU (n v d liu dch v

ca MAC ) gia cc kt ni LLC (iu khin lin kt logic ).

1.2.2. Chun 802.11

Chun 802.11 c chia lm hai nhm: nhm lp vt l PHY v nhm lp

lin kt d liu MAC.

- 7 -


1.2.2.1. Nhm lp vt l PHY bao gm cc chun:

a. Chun 802.11b

802.11b l chun p ng cho phn ln cc ng dng ca mng. Vi mt

gii php rt hon thin, 802.11b c nhiu c im thun li so vi cc chun

khng dy khc. Chun 802.11b s dng kiu tri ph trc tip DSSS, hot ng

di tn 2,4 GHz, tc truyn d liu ti a l 11 Mbps trn mt knh, tc thc

t l khong t 4-5 Mbps. Khong cch c th ln n 500 mt trong mi trng m

rng. Khi dng chun ny ti a c 32 ngi dng / im truy cp.

y l chun c chp nhn rng ri trn th gii v c trin khai rt

mnh hin nay do cng ngh ny s dng di tn khng phi ng k cp php phc

v cho cng nghip, dch v, y t.

Nhc im ca 802.11b l hot ng di tn 2,4 GHz trng vi di tn

ca nhiu thit b trong gia nh nh l vi sng , in thoi m con ... nn c th b

nhiu.

b. Chun 802.11a

Chun 802.11a l phin bn nng cp ca 802.11b, hot ng di tn 5

GHz , dng cng ngh tri ph OFDM. Tc ti a t 25 Mbps n 54 Mbps trn

mt knh, tc thc t xp x 27 Mbps, dng chun ny ti a c 64 ngi dng /

im truy cp. y cng l chun c chp nhn rng ri trn th gii.

c. Chun 802.11g

Cc thit b thuc chun ny hot ng cng tn s vi chun 802.11b l 2,4

Ghz. Tuy nhin chng h tr tc truyn d liu nhanh gp 5 ln so vi chun

802.11b vi cng mt phm vi ph sng, tc l tc truyn d liu ti a ln n

54 Mbps, cn tc thc t l khong 7-16 Mbps. Chun 802.11g s dng phng

php iu ch OFDM, CCK Complementary Code Keying v PBCC Packet

Binary Convolutional Coding. Cc thit b thuc chun 802.11b v 802.11g hon

ton tng thch vi nhau. Tuy nhin cn lu rng khi bn trn ln cc thit b ca

- 8 -


hai chun vi nhau th cc thit b s hot ng theo chun no c tc thp

hn. y l mt chun ha hn trong tng lai nhng hin nay vn cha c chp

thun rng ri trn th gii.

1.2.2.2. Nhm lp lin kt d liu MAC bao gm cc chun:

a. Chun 802.11d

Chun 802.11d b xung mt s tnh nng i vi lp MAC nhm ph bin

WLAN trn ton th gii. Mt s nc trn th gii c quy nh rt cht ch v tn

s v mc nng lng pht sng v vy 802.11d ra i nhm p ng nhu cu .

Tuy nhin, chun 802.11d vn ang trong qu trnh pht trin v cha c chp

nhn rng ri nh l chun ca th gii.

b. Chun 802.11e

y l chun c p dng cho c 802.11 a, b, g. Mc tiu ca chun ny

nhm cung cp cc chc nng v cht lng dch v - QoS cho WLAN. V mt k

thut, 802.11e cng b xung mt s tnh nng cho lp con MAC. Nh tnh nng

ny, WLAN 802.11 trong mt tng lai khng xa c th cung cp y cc dch

v nh voice, video, cc dch v i hi QoS rt cao. Chun 802.11e hin nay vn

ang trong qua trnh pht trin v cha chnh thc p dng trn ton th gii.

c. Chun 802.11f

y l mt b ti liu khuyn ngh ca cc nh sn xut cc Access Point

ca cc nh sn xut khc nhau c th lm vic vi nhau. iu ny l rt quan trng

khi quy m mng li t n mc ng k. Khi mi p ng c vic kt ni

mng khng dy lin c quan, lin x nghip c nhiu kh nng khng dng cng

mt chng loi thit b.

d. Chun 802.11h

Tiu chun ny b xung mt s tnh nng cho lp con MAC nhm p ng

cc quy nh chu u di tn 5GHz. Chu u quy nh rng cc sn phm dng

- 9 -


di tn 5 GHz phi c tnh nng kim sot mc nng lng truyn dn TPC -

Transmission Power Control v kh nng t ng la chn tn s DFS - Dynamic

Frequency Selection. La chn tn s Access Point gip lm gim n mc ti

thiu can nhiu n cc h thng radar c bit khc.

e. Chun 802.11i

y l chun b xung cho 802.11 a, b, g nhm ci thin v mt an ninh cho

mng khng dy. An ninh cho mng khng dy l mt giao thc c tn l WEP,

802.11i cung cp nhng phng thc m ha v nhng th tc xc nhn, chng

thc mi c tn l 802.1x. Chun ny vn ang trong giai on pht trin.

1.2.3. Cc m hnh WLAN (chun 802.11).

1.2.3.1. Trm thu pht - STA

STA Station, cc trm thu/pht sng. Thc cht ra l cc thit b khng dy

kt ni vo mng nh my vi tnh, my Palm, my PDA, in thoi di ng, vv...

vi vai tr nh phn t trong m hnh mng ngang hng Pear to Pear hoc Client

trong m hnh Client/Server. Trong phm vi lun vn ny ch cp n thit b

khng dy l my vi tnh (thng l my xch tay cng c th l my bn c

card mng kt ni khng dy). C trng hp trong lun vn ny gi thit b khng

dy l STA, c lc l Client, cng c lc gi trc tip l my tnh xch tay. Thc ra

l nh nhau nhng cch gi tn khc nhau cho ph hp vi tnh hung cp.

1.2.3.2. im truy cp AP

im truy cp Acces Point l thit b khng dy, l im tp trung giao tip

vi cc STA, ng vai tr c trong vic truyn v nhn d liu mng. AP cn c

chc nng kt ni mng khng dy thng qua chun cp Ethernet, l cu ni gia

mng khng dy vi mng c dy. AP c phm vi t 30m n 300m ph thuc vo

cng ngh v cu hnh.

- 10 -


1.2.3.3. Mng 802.11 linh hot v thit k, gm 3 m hnh mng sau:

- M hnh mng c lp mng Adhoc

- M hnh mng c s (BSSs).

- M hnh mng m rng (ESSs).

a. M hnh mng c lp Adhoc

Mi my tnh trong mng giao tip trc tip vi nhau thng qua cc thit b

card mng khng dy m khng dng n cc thit b nh tuyn hay thu pht

khng dy.

Hnh 1.1. M hnh mng AD HOC

b. M hnh mng c s (Basic Service (BSSs)).

Bao gm cc im truy nhp AP (Access Point) gn vi mng ng trc

hu tuyn v giao tip vi cc thit b di ng trong vng ph sng ca mt cell.

AP ng vai tr iu khin cell v iu khin lu lng ti mng. Cc thit b di

ng khng giao tip trc tip vi nhau m giao tip vi cc AP. Cc cell c th

chng ln ln nhau khong 10 15% cho php cc trm di ng c th di chuyn

m khng b mt kt ni v tuyn v cung cp vng ph sng vi chi ph thp nht.

Cc trm di ng s chn AP tt nht kt ni. Mt im truy nhp nm trung

tm c th iu khin v phn phi truy nhp cho cc nt tranh chp, cung cp truy

- 11 -


nhp ph hp vi mng ng trc, n nh cc a ch v cc mc u tin, gim

st lu lng mng, qun l chuyn i cc gi v duy tr theo di cu hnh mng.

Tuy nhin giao thc a truy nhp tp trung khng cho php cc nt di ng truyn

trc tip ti nt khc nm trong vng vi im truy nhp nh trong cu hnh mng

WLAN c lp. Trong trng hp ny, mi gi s phi c pht i 2 ln ( t nt

pht gc v sau l im truy nhp) trc khi n ti nt ch, qu trnh ny s lm

gim hiu qu truyn dn v tng tr truyn dn.

BSS c lp IBSS: Trong m hnh IBSS Independent BSS, l cc BSS c

lp, tc l khng c kt ni vi mng c dy bn ngoi. Trong IBSS, cc STA c

vai tr ngang nhau. IBSS thng c p dng cho m hnh Adhoc bi v n c th

c xy dng nhanh chng m khng phi cn nhiu k hoch.

H thng phn tn DS: Ngi ta gi DS Distribution System l mt tp

hp ca cc BSS. M cc BSS ny c th trao i thng tin vi nhau. Mt DS c

nhim v kt hp vi cc BSS mt cch thng sut v m bo gii quyt vn

a ch cho ton mng

Hnh 1.2. M hnh mng c s

- 12 -


c. M hnh mng m rng (Extended Service Set(ESSs))

Mng 802.00 m rng phm vi di ng ti mt phm vi bt k thng qua

ESS. Mt ESS l mt tp hp cc BSSs ni m cc Access Point giao tip vi nhau

chuyn lu lng t mt BSS ny n mt BSS khc lm cho vic giao tip

thng qua h thng phn phi. H thng phn phi lm mt lp mng trong mi

Access Point m n xc nh ch n cho mt lu lng c nhn t mt BSS.

H thng phn phi c tip sng tr li mt ch trong cng mt BSS, chuyn

tip trn h thng phn phi ti mt Access Point khc, hoc gi ti mt mng c

dy ti ch khng nm trong ESS. Cc thng tin nhn bi Access Point t h thng

phn phi c truyn ti BSS s c nhn bi trm ch.

Hnh 1.3. M hnh mng m rng

Nh rt nhiu ti liu nghin cu v bo mt trong mng Wireless th c th

bo mt ti thiu cn mt h thng c 2 thnh phn sau:

- Authentication - chng thc cho ngi dng, quyt nh cho ai c th s

dng mng WLAN.

- Encryption - m ho d liu: cung cp tnh bo mt d liu.

- Authentication + Encryption = Wireless Security.

- 13 -


Bi v mng Wireless truyn v nhn d liu da trn sng radio v v AP pht

sng lan truyn trong bn knh cho php nn bt c thit b no c h tr truy cp

Wireless u c th bt sng ny, sng Wireless c th truyn xuyn qua cc vt

liu nh btng, nha, st,... Cho nn ri ro thng tin b cc attacker nh cp hoc

nghe trm rt cao, v hin ti c rt nhiu cng c h tr cho vic nhn bit v phn

tch thng tin ca sng Wireless sau dng thng tin ny c th d kho WEP

(nh AirCrack, AirSnort,...)

d. Cc m hnh thc t

Trn thc t th c rt nhiu m hnh mng khng dy t mt vi my tnh

kt ni Adhoc n m hnh WLAN, WWAN, mng phc hp. Sau y l 2 loi m

hnh kt ni mng khng dy ph bin, t 2 m hnh ny c th kt hp to ra

nhiu m hnh phc tp, a dng khc.

d1. Mng khng dy kt ni vi mng c dy

WAN

Access

Point

Wireless Station

Wireless Station

`

Wireless

Network

Wireline

Network

Hnh 1.4. M hnh mng khng dy kt ni vi mng c dy

AP s lm nhim v tp trung cc kt ni khng dy, ng thi n kt ni

vo mng WAN (hoc LAN) thng qua giao din Ethernet RJ45, phm vi hp c

th coi AP lm nhim v nh mt router nh tuyn gia 2 mng ny

- 14 -


d2. Hai mng c dy kt ni vi nhau bng kt ni khng dy

Wireless NetworkWAN

Wireline Network

BridgeBuilding

WAN

Wireline Network

BridgeBuilding

Hnh 1.5. M hnh 2 mng c dy kt ni vi nhau bng kt ni khng dy

Kt ni khng dy gia 2 u ca mng 2 mng WAN s dng thit b Bridge

lm cu ni, c th kt hp s dng cho thu pht nh truyn sng viba. Khi

khong cch gia 2 u kt ni c th t vi trm mt n vi chc km ty vo loi

thit b cu ni khng dy.

1.2.3.4. WEP Wired Equivalent Privacy

WEP l mt h thng m ho dng cho vic bo mt d liu cho mng

Wireless, WEP l mt phn ca chun 802.11 gc v da trn thut ton m ho

RC4, m ho d liu 40bit ngn chn s truy cp tri php t bn ngoi. Thc t

WEP l mt thut ton c dng m ho v gii m d liu.

- c tnh k thut ca WEP:

+ iu khin vic truy cp, ngn chn s truy cp ca nhng Client khng c

kha ph hp.

+ S bo mt nhm bo v d liu trn mng bng cch m ho chng v ch

cho nhng Client no ng kho WEP gii m.

1.2.3.5. WEP key lengths

Mt kho WEP chun s dng kho 64 bits m ho theo thut ton RC4.

Trong 64 bits c 40 bits c n. Nhiu nh cung cp s dng nhiu tn khc nhau

cho kha WEP nh: standar WEP 802.11 compliant WEP, 40- bits WEP,

40 + 24 bits WEP hoc thm ch l 64 bits WEP. Nhng hin ti th 64 bits

WEP thng c nhc n hn ht. Nhng vi nhng thit b s dng 64 bits

- 15 -


WEP thng th tnh bo mt khng cao v d dng b tn cng. Hin nay c mt

chun tt hn l 128 bits WEP, hu ht cc doanh nghip, c nhn u dn

chuyn sang 128 bits WEP s dng thut ton RC4 m ho, tnh bo mt cao hn,

cc Attacker cng kh khn trong vic d thy kho WEP. Nhng v sau tnh bo

mt ca kho WEP 128 bits cng khng c kh khn na i vi cc Attacker nh

s h tr ca cc cng c d tm kho WEP, th lc Wifi Protected Access

WPA l mt chun bo mt cao cp hn WEP c ra i (chng ta s nghin cu

su hn v WPA trong phn sau).

1.2.3.6. WPA Wi- fi Protected Access

WPA c thit k nhm thay th cho WEP v c tnh bo mt cao hn.

Temporal Key Intergrity Protocol (**IP) cn c gi l WPA key hashing l mt

s ci tin da trn WEP, l v n t ng thay i kho, iu ny gy kh khn rt

nhiu cho cc Attacker d thy kho ca mng.

Mc khc WAP cng ci tin c phng thc chng thc v m ho. WPA

bo mt mng hn WEP rt nhiu. V WPA s dng h thng kim tra v bo m

tnh ton vn ca d liu tt hn WEP.

1.2.3.7. WPA2 Wi- fi Protected Access 2

WPA2 l mt chun ra i sau v c kim nh ln u tin vo ngy

1/9/2004 . WAP2 c National Institute of Standards and Technology (NIST)

khuyn co s dng, WPA2 s dng thut ton m ho Advance Encryption

Standar (AES).

WPA2 cng c cp bo mt rt cao tng t nh chun WPA, nhm bo v

cho ngi dng v ngi qun tr i vi ti khon v d liu.

Nhng trn thc t WPA2 cung cp h thng m ho mnh hn so vi WPA

v y cng l nhu cu ca cc tp on v doanh nghip c quy m ln. WPA2 s

dng rt nhiu thut ton m ho d liu nh **IP, RC4, AES v mt vi thut

ton khc. Nhng h thng s dng WPA2 u tng thch vi WPA.

- 16 -


1.3. Kin trc c bn ca mng WAN khng dy.

Mng v tuyn din rng: Nhm ny bao gm cc cng ngh mng thng tin

di ng nh UMTS/GSM/CDMA 2000.... Vng ph ca n cng tm vi km n

tm chc km.

Vi s ra i ca mng thng tin di ng t bo, chng ta chng kin s

tng vt v nhu cu dch v khng dy & di ng. Chng ta v ang chng kin

s pht trin n chng mt ca mng khng dy: Nm 2002 nh du thi im

lch s ca mng vin thng vi s thu bao di ng vt s thu bao c nh. Theo

ITU, thng 9 nm 2005, s thu bao di ng trn th gii vt con s 2 t. Theo

thng k ca GSA (Global mobile Supplies Association) gn y, con s ny

vt 3 t. Tuy nhin, lch s ca mng t bo cn rt ngn ngi. N mi tri qua 3

th h v nhiu quc gia n vn cn ang th h th 2.

Trong mng thng tin di ng t bo, mi mt thp k chng kin mt th h

mng mi. Th h u tin (1G) khi u t nhng nm 80s. l th h in thoi

di ng analog. Th h th 2 (2G) bt u ni ln t nhng nm ca thp nin 90.

Th h th 2G l cng ngh di ng k thut s, cung cp dch v voice v c data.

Th h th 3 (3G) bt u t nm 2001 Nht Bn, c trng bi dch v thoi d

liu v a phng tin vi tc cao. H thng tin 4G, nhng vin tng cho

th h th 4G, hy vng s c thng mi ho vo khong u nm 2010. Mt th

h 4G s ct cnh vo nhng nm 2012. Con ng pht trin ca cc cng ngh

mng t bo c th hin hnh di y.

Hnh 1.6. Con ng pht trin ca cc cng ngh mng

- 17 -


1.3.1. Th h th 1 (1G)

Mng di ng th h th nht khi mo Nht vo nm 1979. y l h

thng truyn tn hiu tng t (analog). Nhng cng ngh chnh thuc th h th

nht ny c th k n l AMPS (Advanced Mobile Phone System), TACS ( Total

Access Communication System), JTACS (Japan TACS) NMT (Nordic Mobile

Telephone). Tuy nhin cha hon ho v mt cng ngh k thut, th h thng tin

di ng 1G ny thc s l mt mc pht trin quan trng ca ngnh vin thng

(khi nim di ng (mobile)) bt u i vo phc v nhu cu lin lc ca con

ngi trong i sng hng ngy. Nhng im yu ni bt ca th h 1G lin quan

n cht lng truyn tin km, vn bo mt v vic s dng km hiu qu ti

nguyn tn s.

1.3.2. Th h th 2 (2G)

H thng mng 2G c c trng bi cng ngh chuyn mch k thut s

(digital circuit switched). k thut ny cho php s dng ti nguyn bng tn hiu

qu hn nhiu so vi 1G/. Hu ht cc thu bao di ng trn th gii hin ang

dng cng ngh 2G ny. Cng ngh 2G s cn tn ti thm mt thi na trc khi

3G thay th hon ton n. Nhng chun di ng 2G chnh bao gm GSM (Global

System for Mobile Communication )IS 136 v CdmaOne.

GSM s dng k thut a truy cp TDMA v song cng FDD. GSM tr

thnh cng ngh truyn thng c tc pht trin nhanh nht t trc n nay v l

mt chun di ng c trin khai rng ri trn th gii.

- IS 136 c bit n vi tn D AMPS (Digital - AMPS) s dng k

thut a truy cp TDMA v song cng TDD . Cng ngh ny c trin khai nhiu

Chu M, c bit l M v Canada. IS 136 c trin khai nh mt mng

overlay k thut s, ph trn nn h tng mng AMPS.IP 136 cho tc d liu

ln n 30 Kbps.

- CdmaOne l tn gi ca chun di ng ITU IS 95 s dng k thut a truy

cp CDMA. CDMA c chun ho nm 1993. Ngy nay, c 2 phin bn IS 95

gi l IS 95 B. IS 95A dng FDD vi rng knh l 1,25 MHz cho mi hng

- 18 -


ln v xung. Tc d d liu ti a ca IS 95 A l 14,4 Kbps. IS 95 B c th

cung ng tc d liu ln n 115 Kbps bng cch gp 8 knh li vi nhau. Vi

tc ny, IS 95B cn c phn loi nh l cng ngh 2,5 G.

Th h 2,5 G : Th h 2,5 G c trng bi dch v d liu tc ci tin.

Chun chnh ca th h ny l GPRS, EDGE v IS 95 B. GPRS l mt bc pht

trin tip theo cung cp dch v d liu tc cao cho ngi dng GMS v IS

136. L thuyt m ni th GPRS c th cung ng tc d liu ln n 172, 2

Kbps. GPRS l mt gii php chuyn mch gi. y cng l mt bc m trong

qu trnh chuyn t th h 2G ln 3G ca cc nh cung cp dch v GSM/ IS 136 .

Trn con ng di i n 3G, EDGE ra i ci tin tc d liu hn na (

tc ti a tm 384 Kbps). EDGE i khi cn c trch dn nh cng ngh 2,75

G.

1.3.3. Th h di ng th 3 (3G)

Mng 3G c trng bi tc d liu cao, capacity ca h thng ln tng

hiu qu s dng ph tn v nhiu ci tin khc. C mt lot cc chun cng ngh

di ng 3G, tt c u da trn CDMA bao gm: UMTS (dng c FDD ln TDD) ,

CDMA 2000 v TD SCDMA.

- UMTS (i khi cn c gi l 3GSM) s dng k thut a truy cp

WCDMA. UMTS c chun ho bi 3GSM - UMTS l cng ngh 3G c la

chn bi hu ht cc nh cung cp dch v GSM/GPRS i ln 3G. Tc d liu

ti a l 1920 Kbps ( gn 2Mbps). Nhng trong thc t tc ny ch tm 384

Kbps thi. ci tin tc d liu ca 3G hai k thut HSDPA v HSUPA

c ngh. Khi c 3 k thut ny c trin khai, ngi ta gi chung l HSPA.

HSPA thng c bit n nh l cng ngh 3,5G.

+ HSDPA: Tng tc downlink ( ng xung, t NodeB v ngi dng di

ng). Tc ti a l thuyt l 14,4 Mbps, nhng trong thc t n ch t tm 1,8

Mbps (hoc tt lm l 3,6 Mbps). Theo mt bo co ca GSA thng 7 nm 2008,

207 mng HSDPA v ang bt u trin khai, trong 207 thng mi ho

89 nc trn th gii.

- 19 -


+ HSUPA: Tng tc uplink (ng ln) v ci tin QoS. K thut ny cho

php ngi dng upload thng tin vi tc ln n 5,8 Mbps (l thuyt). Cng

trong cng bo co trn ca GSA, 51 nh cung cp dch v thng tin di ng

trin khai mng HSUPA 35 nc v 17 nh cung cp mng ln k hoch trin

khai mng HSUPA.

- CDMA 2000 l ngi ni gii ca 2G CdmaOne i din cho h cng

ngh bao gm CDMA 2000 1xRTT (Radio Transmission Technology), CDMA

2000 EV DO (Evolution Data Optimized) v CDMA 2000 EV DO (

Evolution Data and voice). CDMA 2000 c chun ho bi 3GPP2. L thng

tnh th CDMA 2000 l cng ngh 3G c la chn bi cc nh cung cp mng

CdmaOne.

+ CDMA 2000 1xRTT: Chnh thc c cng nhn nh l mt cng ngh

3G, tuy nhin nhiu ngi xem n nh l mt cng ngh 2,75 G ng hn l 3G.

Tc ca 1xRTT c th t n 307 Kbps, song hu ht cc mng trin khai

ch gii hn tc peak 144 Kbps.

+ CDMA 2000 EV- DO: S dng mt knh d liu 1,25 MHz chuyn bit v

c th cho tc d liu n 2,4 Mbps cho ng xung v 153 Kbps cho ng

ln. 1xEV DO Rev h tr truyn thng gi IP, tng tc ng xung n 3,1

Mbps v c bit c th y tc ng ln n 1,2 Mbps. Bn cnh , 1xEV-

DO Rev B cho php nh cung cp mng gp n 15 knh 1,25 MHz li truyn

d liu vi tc 73,5 Mbps. Theo mt bo co trn www.cdg.org site, 3G CDMA

2000 EV DO vt con s 83 triu thu bao vo thng 9 nm 2007.

+ CDMA 2000 EV- DV : Tch hp thoi v d liu trn cng mt knh

1,25MHz CDMA 2000 EV-DV cung cp tc d peak n 4,8 Mbps cho ng

xung v n 307 Kbps cho ng ln. Tuy nhin t nm 2005, Qualcomm dng

v thi hn vic pht trin ca 1xEV- DV v a phn cc nh cung cp mng

CDMA nh Verizon Wireless v Sprint chn EV DO.

+ TD- SCDMA l chun di ng c ngh bi China Communications

Standards v c ITU duyt vo nm 1999. y l chun 3G ca Trung Quc.

- 20 -


TD- SCDMA dng song cng TDD. TD SCDMA c th hot ng trn mt di

tn hp 1,6MHz (cho tc 2Mbps) hay 5MHz (cho tc 6Mbps). Ngy xut

hnh ca TD SCDMA b y li nhiu ln. Nhiu th nghim v cng ngh

ny din ra t u nm 2004.

+ H thng 3GPP LTE l bc tip theo cn hng ti ca h thng mng

khng dy 3G da trn cng ngh di ng GSM/UMTS v l mt trong nhng

cng ngh tim nng nht cho truyn thng 4G. Lin minh Vin thng Quc T

(ITU) nh ngha truyn thng di ng th h 4 l IMT Advanced v chia thnh

hai h thng dng cho di ng tc cao v di ng tc thp. 3GPPLTE l h

thng dng cho di ng tc cao. Ngoi ra, y cn l cng ngh h thng tch

hp u tin trn th gii ng dng c chun modm3GPP LTE v cc chun dch

v ng dng khc, do NSD c th d dng thc hin cuc gi hoc truyn d

liu gia cc mng LTE v cc mng GSM/GPRS hoc UMTS da trn WCDMA.

- 3GPP LTE c kh nng cp pht ph tn linh ng v h tr cc dch v a

phng tin vi tc trn 100Mb/s khi di chuyn tc 3 km/h v t 30 Mb/s

khi di chuyn tc cao 120 km/ h. Tc ny nhanh hn gp 7 l n so vi tc

truyn d liu cho cng ngh HSDPA (truy nhp gi d liu tc cao). Do

cng ngh ny cho php s dng cc dch v a phng tin tc cao trong khi di

chuyn bt k tc no nn n c th h tr s dng cc dch v ni dung c

dung lng ln vi phn gii cao in thoi di ng, my tnh b ti PDA,

in thoi thng minh...

u im ni bt:

Dung lng truyn trn knh ng xung c th t 100Mbps v trn knh

ng ln c th t 50 Mbps.

Tng tc truyn trn c ngi s dng v cc mt phng iu khin. S

khng cn chuyn mch knh. Tt c s da trn IP.VoIP s dng cho dch v thoi.

Kin trc mng s n gin hn so vi mng 3G hin thi. Tuy nhin mng

3G LTE vn c th tch hp mt cch d dng vi mng 3G v 2G hin ti. iu

- 21 -


ny ht sc quan trng cho nh cung cp mng trin khai 3GPP LTE v khng cn

thay i ton b c s h tng mng c.

OFDMA v MIMO c s dng trong 3G LTE thay v CDMA nh trong 3G.

Chun UMB

Chun UMB hin nay c pht trin bi 3GPP2 vi k hoch l s thng

mi ho trc 2009.

Mt s c im k thut nh sau:

Cc k thut Miltiple radio v antenna tin tin.

Multiple Input Multiple Output (MIMO), a truy nhp phn chia theo khng gian

(Spatial Division Multiple Access (SDMA)) v k thut beamforming antenna.

Cc k thut qun l nhiu tin tin (Improved interference management

techniques).

Tc d liu cao nht (peak data rates).

Ln ti 288 Mbps ng ln, 75 Mbps ng xung.

Ln ti 1000 ngi s dng VoIP ng thi ( vi s cp pht 20 MHz FDD).

Chun IEEE 802.x.

Chun ny bt ngun t mng WiFi, sau tin ln 802.16e ri 802.16m v

by gi l 802.20. Chun IEEE 802.20 cn c gi l truy nhp v tuyn bng

rng di ng WBMA (Mobile Broadband Wireless Access). N c th h tr ngay

c khi di chuyn vi tc ln ti 250 km/h.

Trong khi chuyn vng (roaming) ca WiMAX nhn chung b gii hn trong

mt phm vi nht nh, th chun IEEE 802.20 ging nh 3G c kh nng h tr

chuyn vng ton cu. Ngoi ra, cng ging nh WiMAX, IEEE 802.20 cng h tr

cc k thut QoS nhm cung cp nhng dch v c yu cu cao v tr, jitter...

Trong mng EEE 802.20 vic ng b gia ng ln v ng xung u c

thc hin hiu qu. D kin chun IEEE 802.20 tng lai s kt hp mt s tnh

nng ca IEEE 802.16e v cc mng d liu 3G, nhm cung cp v to ra mt

truyn thng a dng (rich communication).

- 22 -


Bng 1.1. Technology Features Comparison

Bng 1.2. Pre-4G Technology Requirement Comparison

1.4. Kin trc c bn ca Internet khng dy (Chun WAP v cc

chun mi (WPAN, WRAN, WMAN)).

1.4.1. Kin trc c bn ca Internet khng dy chun WAP

1.4.1.1. S b v WAP.

Nhu cu truy cp thng tin t cc thit b di ng m ng cho cc cng

ngh khng dy pht trin mnh m. Yu t quan trng nht trong s ra i ca

Internet khng dy l Digital Cellphone trong nhng nm gn y. Vic m rng

mng Digital Cellphone v dch v thng tin c nhn PCS (Personal

Communication Services).

Wireless Application Protocol (WAP) l mt dng c t theo chun cng

nghip m cho cc ng dng thc thi trn mi trng mng khng dy, ch trng

vo cc ng dng trn thit b di ng, c bit l in thoi di ng. Cc tiu chun

ny c a ra bi WAP Forum, nhm ny hnh thnh vo thnh 6 nm 1997 bi

- 23 -


Erison, Nokia, Motorola v Unwired Planet v hin ti c hng trm cng ty

khc tham gia, bao gm IBM, Hewlett Packard, Visa v Microsoft. Theo thng k

chnh thc ca WAP Forum, nhng thnh vin thuc WAP Forum l i din cho

trn 90% nh sn xut in thoi di ng trn ton th gii. WAP v s c h

tr trn nhiu loi thit b, t n gin nh in thoi di ng thng thng cho n

nhng thit b th h mi cc in thoi thng minh vi mn hnh rng c th

chy c nhiu ng dng, thm ch l nhng my tr l c nhn k thut s

(PDA), cc palmtop hay cc my tnh vi kch thc nh hn. Tt c cc thit b di

ng ri s c p dng cng ngh WAP, trc tip t nh sn xut hay t phin

bn nng cp no thuc nhm cc cng ty th ba (third party). Mi mt thit b

c mt cch hin th khc nhau v cc phng thc nhp liu khc nhau. Cng vic

ca cng ngh WAP l sp xp li m hn n v cung cp mt khung lm

vic (framework) chung cho php cc ng dng chy c trn tt h nn khc

nhau ny.

M hnh WAP cn chnh l m hnh WWW (World Wide Web) vi mt s tnh

nng nng cao. Trong , hai tnh nng quan trng nht l: y (Push) v h tr

thoi. Ni dung thng tin WAP c truyn ti nh mt tp cc giao thc truyn

thng tiu chun trong tp giao thc WAP. WAP nh ngha mt tp cc thnh phn

tiu chun cho php truyn thng gia thit b u cui v my ch mng gm:

M hnh tn tiu chun: Cc URL c s dng nhn dng ni dung

WAP trn cc my ch, URI c s dng nhn dng ti nguyn trong

mt thit b, v d nh chc nng iu khin cuc gi.

Kiu ni dung: c a ra trn kiu c trng ging nh WWW.

Cc khun dng ni dung tiu chun: da trn cng ngh WWW v bao gm

ngn ng nh du, thng tin lch, cc i tng, hnh nh v ngn ng kch

bn (Script).

- 24 -


Cc giao thc truyn thng tiu chun: Cho php truyn thng cc yu cu

u cui di ng ti my ch mng thng qua cng WAP. Cc tiu chun

ny ti u theo hng ca thit b u cui s dng.

to ra mt Website c kh nng thm nhp qua thit b khng dy tht s l

mt th thch v vy ch c mt phn nh trong hn 1 t Website cung cp thnh

phn Internet khng dy. WAP c thit k lm vic vi bt k dch v khng

dy no tn ti nh:

Dch v nhn tin ngn SMS (Short Message Service).

D liu chuyn mch tc cao CSD (High-speed Circuit-switched Data).

Dch v GPRS (General Packet Radio Service).

D liu dch v b sung khng cu trc USSD (Unstructured Supplementary

Services Data).

Cc giao thc WAP c thit k trn nn ca cc giao thc web. Mc ch

cu WAP l s dng li cu trc c s ca web, t nng cao qu trnh giao

tip gia nh cung cp v cc thit b di ng, gip qu trnh ny tr nn hiu qu

v tn t thi gian hn l s dng chnh cc giao thc web.

1.4.1.2. Cc m hnh giao tip trn WAP

Do kin trc ca WAP c thit k gn ging vi Web, nn n cng k tha

m hnh client server c dng trn Internet ca Web. im khc nhau chnh l

s c mt ca WAP Gateway dng cho vic chuyn i gia HTTP v WAP.

Hnh 1.7. WAP dng truy cp Internet

- 25 -


Hnh 1.8. WAP c dng truy cp trong Intranet

truy cp vo mt ng dng trn server, client khi to mt ni kt vi

WAP gateway v gi i yu cu ca mnh. Gateway s chuyn i nhng yu cu

ny sang nh dng c dng trn Internet (HTTP) v sau chuyn chng n

server cung cp dch v. Ni dung tr v c gi t server n gateway, ti y n

s c chuyn sang nh dng WAP, sau gi v cho thit b di ng. Nh

vy, gateway gip Internet c th giao tip vi mi trng mng khng dy.

Cc ngn xp ca giao thc WAP c chia thnh cc lp cho php d dng m

rng, thay i v pht trin (tng t m hnh OSI). Giao thc truy nhp ng dng

v tuyn WAP gm c 5 lp:

Lp truyn ti: Giao thc datagram v tuyn (WDP)

Lp bo mt: Giao thc lp truyn ti v tuyn (WTLS)

Lp giao vn: Giao thc giao vn v tuyn (WTP)

Lp phin: Giao thc phin v tuyn (WSP)

Lp ng dng: Mi trng ng dng v tuyn (WAE)

Tt c cc ngn xp giao thc WAP u c thit k ph hp vi cc iu

kin rng buc ca mng di ng. Mi mt lp cung cp mt tp cc chc nng

hoc cc dch v ti cc dch v v ng dng khc qua tp giao din tiu chun.

- 26 -


Kin trc WAP tch cc giao tip dch v t cc giao thc cung cp dch v cho

php m rng cc c tnh v t do la chn cc giao thc thch hp cho mt ni

dung c th. Rt nhiu cc dch v trong ngn xp c th c h tr bi mt hoc

nhiu giao thc. V d dch v truyn a phng tin c h tr bi 2 giao thc

HTTP v WSP.

Cc giao thc trn lp ny c thit k v chn la iu hnh trn nhiu

dch v mang khc nhau, bao gm nhn tin ngn SMS, d liu chuyn mch knh

v d liu gi. Cc knh mang a ra nhiu mc cht lng dch v khc nhau

tng ng vi thng lng, t l li, v tr. Cc giao thc lp mang thng tin

c to ra nhm khc phc cc im yu ca knh mang thng tin, ty bin theo

tng loi hnh dch v.

a. Lp dch v truyn ti

Lp ny cung cp s hi t gia cc dch v mang vi cc phn cn li ca

ngn xp WAP. Giao thc d liu v tuyn WDP (Wireless Datagram Protocol)

cha mt tp cc kt ni knh mang khc nhau v h tr cc k thut cc giao

thc chy trn n. Cc tp kt ni ny thay i theo h tng c s mng v cc dch

v truyn thng cn cung cp. WDP truyn v nhn cc d liu t cc thit b u

cui mng, WDP cng thc hin vic phn on gi tin v ng gi cc datagram

cho ph hp vi c tnh ca knh mang thng tin. Giao thc bn tin iu khin v

tuyn WSMP l mt phn m rng ca WDP l giao thc bo co li c c ch

tng t ICMP trong Internet, giao thc ny hu dng khi WAP khng s dng trn

knh mang IP hoc cho mc ch thu thp thng tin v chn on mng.

b. Lp bo mt

Mc tiu ca bo mt lp truyn ti v tuyn WTLS (Wireless Transport

Layer Security) l m bo tnh nng bo mt gia cc thit b u cui WAP v

cng/y quyn WAP. WTLS a ra khung lm vic cho cc kt ni an ton cho cc

- 27 -


ng dng truyn thng 2 chiu. WTLS s dng cc thnh phn t cc giao thc bo

mt c bn ca Internet nh lp socket an ton SSL (Socket Security Layer) v bo

mt lp truyn ti TLS (Transport Layer Security). Nguyn tc ca WTLS cho php

chng nhn cc d liu gc, xc nhn bn quyn ca bn tin. m bo tnh ring

t v tnh ton vn ca d liu, cc k thut m ho v cc m nhn thc bn tin

c s dng. thit lp cc u ni an ton, trong pha thit lp c to ra cc

tham s cn thit nh: t tham s, chuyn i kho v nhn thc. Ging nh cc

giao thc khc ca WAP, WTLS ti u cho cc knh truyn thng bng hp.

c. Lp giao vn

Giao thc giao vn v tuyn WTP (Wireless Transaction Protocol) c nhim

v p ng cc yu cu v tr li v phng tin truyn thng t ngi s dng ti

my ch ng dng v ngc li. WTP tng thch vi cc iu kin rng buc v

bng thng hp ca mi trng v tuyn, trong n ti thiu tiu giao thc qua

vic ti thiu s lng ln pht li. Cc c tnh ch cht ca WTP l cung cp cc

dch v giao vn cho cc hot ng trc tuyn nh duyt Web.

WTP c thit k tng s lng cc pha giao vn, gim cc th tc pht

li, xc nhn v th tc gii phng. Ngoi ra, WTP cn c th m rng chc nng

phn on v to li bn tin. T hp giao thc giao vn v tuyn WTP v giao thc

phin v tuyn WSP (Wireless Session Protocol) cung cp dch v truyn ti siu

vn bn (hypermedia) gia cc phn t mng qua truyn ti phi kt ni, trong khi

giao thc truyn ti siu vn bn HTTP cung cp dch v truyn ti siu vn bn

qua truyn ti c hng kt ni.

d. Lp phin

Giao thc lp phin v tuyn WSP h tr lp ng dng ca WAP m t trong

phin vi mt giao tip ca 2 dch v phin: Kt ni c hng m bo tin cy

- 28 -


v phi kt ni khng m bo tin cy. WTP cung cp cc phng tin truyn

thng nh:

H tr chc nng HTTP, gim ti cho WSP th s dng phin bn HTTP

1.1.

Ghp ni ngi dng vo thnh vin ca phin truyn thng d liu c thi

gian truyn ln.

Yu cu cho cc my ch y d liu ti ngi s dng.

To ra mt chui th tc cho php ng dng my ch xc nh ngi dng

c hoc khng h tr cc phng tin v cu hnh giao thc thch hp.

Kh nng ngng v ti to phin.

WSP h tr c ch cache tiu tng hiu qu knh truyn. Giao thc HTTP

truyn thng khng h tr cache tiu nn khong 90% cc yu cu cha cc tiu

c nh vn phi chuyn trn mng.

e. Lp ng dng

Mi trng ng dng v tuyn WAE (Wireless Application Enviroment) nm

trong lp ng dng cung cp mi trng cho php m rng min cc ng dng

c s dng trn cc thit b v tuyn bao gm c dch v tin nhn a phng tin

[3]. WAP c hai kiu tc nhn (agent) trong thit b v tuyn: tc nhn s dng

WML (Wireless Markup Language) v agent s dng WTA (Wireless Telephony

Application) h tr thoi.

1.4.1.3. u v nhc im ca WAP

WAP ng dng ngn ng WML trin khai v th hin cc trang Web tiu

chun cho ph hp vi cc thit b di ng. S dng khun dng tn hiu d liu ti

u, WAP c thit k duyt cc ni dung web ti thit b v tuyn thng qua

loi b cc thnh phn ha nhm hin th trn mn hnh nh v hn ch bng

thng. Thc t rt nhiu m WML c sa i t m HTML.

- 29 -


Mc d WAP h tr cho hu ht cc thit b di ng nhng n vn tn ti mt

s im hn ch trong giao thc ny:

- tr: WAP da trn giao thc TCP/IP v khng t xy dng h thng bo

mt ring cng nh kh nng t y d liu, iu ny s nh hng ti nhng ng

dng cn c chy ngay khi ngi dng ang truyn d liu trn ng dng khc.

Nu trin khai ng dng kiu ny s tng phc tp ca h thng ln rt ln v

nh hng trc tip ti phn cng v bng thng yu cu.

- Bo mt: WAP l h thng giao thc in hnh khng cha bo mt ring,

iu c ngha l d liu khng c m ho khi truyn. Cc phn mm bo mt

c th c h tr cho WAP nhng b gii hn v n nh, gi thnh v thi

gian thc hin. Gateway: Gii php WAP yu cu c gateway v tuyn, v vy n

s lm tng gi thnh ca h thng.

- Kt ni lin tc: Cc ng dng WAP c xy dng da trn kin thc yu

cu/ p ng v vy n s kt ni lin tc khng ging nh trn cc trnh duyt trn

cc my PC. Mt s ngi s dng thng di chuyn vt qua vng ph sng v

gy ra cc li kt ni. Vn ny c th gii quyt bng phng php lu v

chuyn tip, gii php thm vo ny cng lm tng gi thnh v phc tp ca h

thng. Trn thc t, vic thm vo kh thng yu cu phn cng km theo v tng

thm bng thng s dng.

- Trin khai dch v: WAP c to ra duyt ni dung cc trang web, cc

nh cung cp ni dung c yu cu qun l v duy tr cc bn sao cho mi

website. Cc bn sao nh vy thc s l khng hiu qu v n lm tng gi thnh

khi m rng v bo dng h thng.

- Tng tc thp: WAP rt kh tch hp vi cc ng dng c sn trn cc thit

b, y l gii hn thng thy ca cc gii php trn cc u cui c nng lc x l

v giao din mn hnh nh.

- Kh nng y v ko: Cc gii php WAP yu cu ngi s dng gi cc

thng tin trc khi h nhn chng. Nh vy, email, cnh bo khng th nhn ngay

tc khc. Thut ng ko lin quan ti kh nng ca thit b cnh bo ngi s

- 30 -


dng khi c d liu ca h n. Chc nng y l chc nng c sn ca WAP

nhng n yu cu thm mt lp kin trc v nh vy s lm tng nguy c xy ra li

v tr.

1.4.1.4. Cc thnh phn ca WAP

Cc c t WAP cho php nhng nh sn xut di ng c nhiu la chn cho

ring mnh. N khng bt buc thit b WAP phi trng nh th no hay s hin th

ni dung nhn c t Internet ra sao, m n gn lin vi giao din ngi dng vi

t chc bn trong ca chc nng in thoi.

Yu cu duy nht cho mt thit b h tr WAP l n phi cung cp mt tc

nhn ngi dng WAE (WAE User Agent) mt tc nhn ngi dng WTA (WTA

User Agent) mt tc nhn ngi dng WTA (WTA User Agent) v ngn xp WAP

(WAP Stack)

Hnh 1.9. Wap Client

a. WAE User Agent.

L mt loi trnh duyt nh (microbrowser) thc hin hon tr ni dung phc

v vic hin th. N nhn vo WML, WML Script c bin dch v cc hnh

nh t WAP gateway, sau x l hoc hin th chng ln mn hnh. WAE User

- 31 -


Agent cng qun l vic giao tip vi ngi dng, chng hn nh nhp liu vn

bn, thng bo li hay cc thng ip cnh bo khc.

b. WTA User Agent.

Nhn cc tp tin WTA c bin dch t WTA server v thc thi chng. WTA

User Agent bao gm vic truy cp vo giao din in thoi v cc chc nng mng

nh quay s, tr li cuc gi, t chc phonebook, qun l thng ip v cc dch v

nh v.

c. WAP Stack.

Cho php in thoi ni kt vi WAP gateway s dng cc giao thc WAP.

Hnh 1.10. Wap Stack

Hnh 1.11. Wap Stack

- 32 -


c.1. Wireless Sesion Layer WSP.

Wireless Session Protocol cho php cc dch v trao i d liu vi cc ng

dng theo mt cch c t chc. N bao gm hai giao thc khc nhau:

- Dch v phin hng kt ni (Connection orented session services) hot ng

nh vo Wireless Transaction Protocol ( WTP).

- Dch v phin phi kt ni (Connectionless session services) hot ng trc

tip trn Wireless Transport layer (WDP).

Cc dch v phin (session services) l nhng chc nng gip cho vic thit

lp kt ni gia v mt server. Dch v ny c phn phi thng qua vic dng cc

primitives m n cung cp.

Primitives l cc thng ip c nh ngha m mt client dng gi cho

server yu cu dch v. Chng hn nh trong WSP, mt trong nhng primitives l

SConnect, vi n chng ta c th yu cu vic to lp mt ni kt vi server.

c.1.1. Dch v phin hng kt ni (Connection oriented session service).

Cung cp kh nng qun l mt phin lm vic v vn chuyn d liu tin cy

gia client v server. Phin lm vic to ra c th c hon li v phc hi sau

nu nh vic truyn ti d liu khng th thc hin c. Trong k thut push, d

liu khng mong mun c th c gi i t server n client theo hai cch: c

xc nhn hoc l khng c xc nhn.

- Trng hp c xc nhn (confirmed push), client s thng bo cho server

khi nhn c d liu.

- Trng hp khng c xc nhn (confirmed push) server khng c thng

bo khi d liu push c nhn.

Phn ln cc chc nng c cung cp bi dch v phin hng kt ni

(connection oriented session service) u c xc nhn: client gi cc thng ip

yu cu (Request primitive) v nhn li thng ip xc nhn (confirm prtimitive),

server gi cc thng ip phn hi (Response primitive) v nhn li thng ip chi

dn (Indication primitive).

- 33 -


c.1.2. Dch v phin phi kt ni (Connectionless session service).

Ch cung cp cc dch v khng c xc nhn (non confirmed servicess).

Trong trng hp ny cc client c th ch s dng thng ip yu cu (Request

primitive) v cc server cng ch c th dng thng ip ch dn (Indication

primitive).

bt u mt phin lm vic mi, client yu cu mt WSP primitive cung

cp mt s tham s nh a ch server, a ch client v cc client header. Cc tham

s ny c th c lin kt vi cc tiu HTTP ca client v c th c server

dng nhn ra loi tc v ngi dng bn trong WAP client (c th l phin bn

v loi ca trnh duyt). iu ny c ch khi ta mun nh dng li phn u ra khc

i, tu thuc vo loi thit b pha client.

Chng hn nh mt in thoi c th c mt mn hnh hin th cha c 20

k t; nhng thit b khc th li ch hin th c 16 k t.

WSP v c bn chnh l mt dng nh phn ca HTTP. WSP cung cp tt

c cc phng thc c nh ngha bi HTTP/1.1 v cho php m phn nhm t

c s tng thch vi chun HTTP/1.1 ny.

c.2. Wireless Transaction Layer WTP.

Wireless Transaction Protocol cung cp cc dch v nhm thc hin cc giao

tc tin cy v khng tin cy, n lm vic trn tng WDP hay tng an ninh WTLS.

Cng nh tt c cc tng khc trong WAP, WTP c ti u cho ph hp vi bng

thng nh ca giao tip trn sng v tuyn, c gng gim s lng cc giao tc thc

hin li gia client v server.

C th, c ba lp khc nhau ca cc dch v giao tc cung cp cho cc tng

bn trn l:

- Cc yu cu khng tin cy Unreliable requests

- Cc yu cu c th tin cy Reliable requests

- Cc yu cu tin cy vi mt thng ip kt qu.

c.2.1. Yu cu khng tin cy - Unreliable requests.

- 34 -


Trnh khi u (Inititor) ( trong trng hp ny l mt server cha ni dung

content server) gi yu cu n trnh p ng (Responder) (tc nhn ngi dng) v

khng c mt thng ip xc nhn no c gi tr v. Giao tc ny khng c trng

thi v kt thc ngay thng ip yu cu c gi i.

Hnh 1.12. Yu cu khng tin cy

c.2.2. Yu cu c th tin cy - Reliable requests.

Trnh khi u gi mt yu cu n cho trnh p ng, trnh ny s tr li li

khi nhn c yu cu. Trnh p ng lu tr thng tin trng thi ca giao tc trong

mt thi gian n c th gi li thng ip xc nhn (acknowledgement message)

nu nh server c yu cu li ln na. Giao tc kt thc ti trnh khi u khi trnh

ny nhn c thng ip xc nhn:

Hnh 1.13. Yu cu tin cy

- 35 -


c.2.3. Yu cu tin cy v mt thng ip kt qu.

Trnh khi u gi yu cu n cho trnh p ng, khi nhn c yu cu trnh

ny s gi tr li mt thng ip kt qu. Trnh khi u nhn thng ip ny, duy

tr thng tin trng thi ca giao tc trong mt thi gian sau khi xc nhn c gi

i, phng trng hp thng bo gi i khng n c ch. Giao tc kt thc ti

trnh p ng khi n nhn c thng ip xc nhn.

Hnh 1.14. Yu cu tin cy vi thng ip kt qu

c.3. Wireless Transprot Layer Security WTL

WTLS c cung cp bi WAP Forum, y l mt gii php cho vn bo

mt trn WAP. WTLS l mt tng lp chn hot ng trn tng vn chuyn (WDP)

v c xy dng da trn hai giao thc Internet l TLS (Transport Layer

Security) v1.0, tng ny cng da trn mt tng khc l SSL (Secure Sockets

Layer) v3.0.

WTLS cng c cc c im c bn nh tt c cc tng trc y trong ngn

xp WAP: n l iu chnh ca mt giao thc Internet cho ph hp vi iu kin

tr cao, bng thng thp, cng vi b nh v kh nng x l gii hn ca cc thit

b WAP. WTLS cng c gng gim bt chi ph lin quan n vic thit lp mt kt

ni an ton gia hai ng dng. WTLS cung cp cng mt mc bo mt nh

SSL 3.0 nhng gim i khong thi gian giao tc. Cc dch v m n cung cp l:

- 36 -


- Tnh bo mt (Privacy) bo m d liu gi i gia server v client khng

th c truy cp t bt k ngi no khc. Khng ai c th gii m thng ip cho

h c th nhn thy cc thng ip ny dng c m ho.

- nh danh server m bo mt server tht s.

- nh danh client gip server gc gii hn kh nng truy cp n nhng ni

dung m n cung cp. Xc nh ch mt s client no mi c th truy cp vo

nhng trang no cho php m thi.

- Bo ton d liu s m bo ni dung d liu trn ng truyn gia server

v client s khng b chnh sa m khng c thng bo.

Hnh di y m t cch WAP gateway iu khin cc phin lm vic an

ton. Mt phin SSL chun c m ra gia web server v WAP gateway v mt

phin WTLS c khi to gia gateway v thit b di ng. Ni dung m ho c

gi i thng qua ni kt ny t server n gateway, gateway bin dch v gi n n

cho in thoi di ng.

Hnh 1.15. M hnh lm vic ca Wap gateway

Sau WTLS giao quyn li cho giao thc SSL lm vic trn Internet. Vic

chuyn i gia SSL v WTLS thc hin bn trong b nh ca WAP gateway .

iu quan trng l cc thng tin khng c m ho s khng c lu tr bn

trong gateway, v nh th s lm mt tc dng tt c cc phng php bo mt c

dng bo v d liu lu tr vi nhng ngi khng c nh danh.

- 37 -


Mc d cc WAP gateway c cung cp nhiu chc nng bo m cp

an ton cao nht, th nhng vn cn nhiu vn lin quan n gii php an ton

cho WAP.

WTLS l mt tng tu chn trong ngn xp WAP. iu ny c ngha l c ch

bo mt trong WAP ch c gi tr khi c yu cu v khng c xy dng nh l

mt chc nng trong kin trc WAP. Do , thng tin lu chuyn n v i qua

WAP gateway thng khng c m ho, tr phi chng ta dng cc kt ni SSL

giao tip gia cc server gc v gateway.

c.4. Wireless Datagram Protocol WDP

WDP l lp di cng trong ngn xp WAP v l mt trong nhng phn t

lm cho WAP tr thnh mt giao thc cc k di ng, c th thc thi trn nhiu loi

mng di ng khc nhau. WDP che ch cc tng bn trn nh vo cc dch v nn

m mng cung cp. Cc dch v nn bao gm: SMS, CSD, DECT v CDMA.

1.4.2. Kin trc c bn ca mng WPAN khng dy

K t khi Bluetooth c trin khai, c rt nhiu li bn lun v cc mng

vng c nhn khng dy. Hu ht cc mi quan tm i vi mng PAN u lin

quan n vic s dng n trong cc in thoi di ng thng minh, chng hn nh

ng b ho vi phn mm my tnh hoc s dng cc tai nghe khng dy.

N cng bt u c s dng cho cc thit b nh cc tai nghe c gn micro khng

dy, vi vic truyn m thanh s cung cp m thanh r nt. Vic trin khai cng

ngh Bluetooth hin nay c xu hng s dng n nh mt s thay th cp ngoi vi

cho mt s lng hn ch cc thit b hn l mt cng c nhm cho php mt s

lng ln cc thit b trong nh hoc vn phng c th giao tip trc tip.

Nhng vin cnh di hn th ln hn nhiu. Nhiu thit b gia nh c th

hng li t kt ni khng dy. Chng ta ni n cc bn iu khin tr chi vn c

th tr chuyn v tuyn vi cc router, cc hp truyn tn hiu s vn c th truyn

- 38 -


tn hiu TV s ti my tnh hoc ti nhiu mn hnh trong nh, cc my ch ng

truyn vn c th pht qung b v tuyn m nhc ti cc b tai nghe tu nm

trong phm vi truyn, cc my nh vn c th giao tip trc tip vi cc my in v

cc u in, cc u chi MP3 cm tay vn c th gi tp m nhc ti h thng m

thanh ti nh. y l cc loi ng dng lin thng m nhng ngi tiu dng hng

in t m. Nhng Bluetooth khng nhanh cho cc ng dng video v chc

chn l khng bao gi. Bluetooth hin nay ch c kh nng truyn vi tc 1 n 2

Mbits/s trong mt phm vi khong 10m vi mt cng sut u ra khong 100mW.

Nh vy l qu tt cho m thanh v cho my in, cc thit b nhp nh TV s i hi

mt tc ti thiu 7Mbits/s. Nu mun truyn tn hiu TV phn gii cao, phi

cn mt h thng c kh nng x l 20 24 Mbits/s. Cng ngh xut sc hin nay

cho cc mng vng c nhn l UWB, cn c bit n vi ci tn 802.15.3a ( mt

chun IEEE khc). y c coi l cng ngh PAN m tt c cc cng ngh PAN

khc phi chu khut phc. L do chng c quan tm n vy l v UWB c rt

nhiu tim nng. UWB truyn nhng on d liu cc ngn, t hn mt nan giy

qua mt di ph rng.

Trong nhng khong cch rt ngn, cng ngh UWB c kh nng truyn d

liu vi vn tc ln ti 1Gbits/s vi mt ngun cng sut thp (khong 1mW). Vi

di ph rng ca n, UWB t c kh nng b nh hng bi suy lun mo hn cc

cng ngh khng dy v bi v cng sut truyn thp nh vy, n gy ra rt t nhiu

trong cc thit b khc. Phm vi d tnh ca n ch khong 10m v v cc vn v

chun ca n, ngi ta d tnh rng cng ngh UWB s c mt v tr trong c phin

bn khng dy ca USB v trong s lp li tip theo ca cng ngh khng dy. D

bo ca Intel (06/ 2006) v nhng ngi ng h UWB khc l UWB s hot ng

nh mt loi lp vn chuyn a nng cho cc ng dng khng dy phm vi ngn.

Trong d bo ny, mt phin bn tng lai ca Bluetooth s dng UWB nh mt

lp kim sot truy nhp ng truyn v vn chuyn ca n, cng ging nh s

dng USB khng dy. Cc giao thc cp cao hn m trch vic trin khai c th

ng dng. UWB c xem l mt thnh phn ct li ca th gii c kt ni

- 39 -


khng dy, c iu khin bi cc chun m vn cho php tt c cc thit b giao

tip vi nhau. phm vi ngn cng ngh UWB c th c s dng trong WPAN

vi nhng vai tr:

- Thay cp IEEE1394 ni gia thit b in t a phng tin dn dng nh

my quay phim, my chp hnh s, thit b pht MP3.

- Thit lp tuyn bus chung khng dy tc cao ni gia PC vi thit b

ngoi vi, gm my in, my qut v thit b lu tr gn ngoi.

- Thay cp v Bluetooth trong cc thit b th h mi, nh in thoi di ng

3G, kt ni IP/ UPnP cho th h thit b di ng/ in t dn dng/ my tnh dng

IP.

- To kt ni khng dy tc cao cho thit b in t dn dng, my tnh v

in thoi di ng.

1.4.3. Kin trc c bn ca mng WMAN khng dy

WMAN hay cn gi l WiMAX. WiMAX l t vit tt ca Worldwide

Interoperability for Microwave Access c ngha l kh nng tng tc ton cu vi

truy nhp vi ba. Cng ngh WiMAX hay cn gi l chun 802.16 l cng ngh

khng dy bng thng rng ang pht trin rt nhanh vi kh nng trin khai trn

phm vi rng v c gi l c tim nng to ln tr thnh gii php dm cui

l tng nhm mang li kh nng kt ni Internet tc cao ti cc gia nh v

cng s.

Trong khi cng ngh quen thuc Wi- Fi(802.11 a, b, g) mang li kh nng kt

ni ti cc khu vc nh nh trong vn phng hay cc im truy cp cng cng

hotspot, cng ngh WiMAX c kh nng ph sng rng hn, bao ph c mt khu

vc thnh th hay mt khu vc nng thn nht nh. Cng ngh ny c th cung cp

vi tc truyn d liu n 75 Mbps ti mi trm pht sng vi tm ph sng t 2

n 10km. Vi bng thng nh vy, cng ngh ny c kh nng h tr cng

lc (thng qua mt trm pht sng n l) kh nng kt ni ca hn 60 doanh

- 40 -


nghip vi tc kt ni ca ng T1/E1 v hng trm gia nh vi tc kt ni

DSL.

1.4.3.1. c im ni bt ca WiMAX di ng

WiMAX di ng cng c nhng c im ging EV- DO hoc HSxPA nhm

tng tc truyn thng (data rate). Nhng c im bao gm: M ho v iu

ch thch nghi (Adaptive Modulation and Coding - AMC), k thut sa li bng d

lp (Hybrid Automatic Repeat Request HARQ), phn b nhanh (Fast

Scheduling) v chuyn giao mng (handover) nhanh v hiu qu.

Khng ging nh cng ngh 3G da trn CDMA c xy dng nhm vo

dch v thoi, WiMAX c thit k p ng dch v truyn d liu dung lng

ln (trong c c dch v thoi VoIP), WiMAX c s dng k thut tri ph

SOFDMA v h tng mng xy dng trn nn IP.

WiMAX cung cp kh nng kt ni Internet khng dy nhanh hn so vi

WiFi, tc uplink v downlink cao hn, s dng c nhiu ng dng hn v

quan trng l vng ph sng rng hn v khng b nh hng bi a hnh.

WiMAX c th thay i mt cch t ng phng thc iu ch c th tng

vng ph bng cch gim tc truyn v ngc li. tng vng ph, chun

WiMAX hoc s dng mng Mesh hoc s dng antenna thng minh hoc MIMO.

D liu truyn trong mng WiMAX c phn chia thnh 5 lp dch v vi nhng

u tin khc nhau nhm cung ng QoS. ngoi ra bo mt cng l mt c im vt

tri ca WiMAX vi WIFI.

1.4.3.2. M hnh ng dng WiMAX.

Tiu chun IEEE 802.16 xut 2 m hnh ng dng.

- M hnh ng dng c nh.

- M hnh ng dng di ng.

a. M hnh ng dng c nh (Fixed WiMAX)

M hnh c nh s dng cc thit b theo tiu chun IEEE.802.16 2004.

Tiu chun ny gi l khng dy c nh v thit b thng tin lm vic vi cc

- 41 -


anten t c nh ti nh cc thu bao. Anten t trn nc nh hoc trn ct thp

tng t nh cho thng tin v tinh.

Hnh 1.16. M hnh ng dng Wimax

Tiu chun IEEE 802.16 2004 cng cho php t anten trong nh nhng tt

nhin tn hiu thu khng khe bng anten ngoi tri. Bng tn cng tc (theo quy

nh v phn b ca quc gia ) trong bng 2,5 GHz hoc 3,5 GHz. rng bng

tng l 3,5 MHz. Trong mng c nh, WiMAX thc hin cch tip ni khng dy

n cc modem cp, n cc i dy thu bao ca mch xDSL hoc mng Tx/Ex

(truyn pht/chuyn mch) v mch OC x (truyn ti qua sng mch). WiMAX c

nh c th chuyn phc v cho cc loi ngi dng (user) nh: cc x nghip, cc

khu dn c nh l, mng cp truy nhp WLAN cng cng ni ti mng th, cc

trm gc BS ca mng thng tin di ng v cc mch iu khin trm BS. V cch

phn b theo a l, cc user c th phn tn ti cc a phng nh nng thn v

cc vng su vng xa kh a mng cp hu tuyn n .

S kt cu mng WiMAX c a ra trn Hnh 1.18. Trong m hnh ny

b phn v tuyn gm cc trm gc WiMAX BS ( lm vic vi anten t trn thp

cao) v cc trm ph SS (SubStation). Cc trm WiMAX BS ni vi mng th

MAN hoc mng PSTN.

- 42 -


b. M hnh ng dng WiMAX di ng.

M hnh WiMAX di ng s dng cc thit b ph hp vi tiu chun IEEE

802.16e. Tiu chun 802.16e b sung cho tiu chun 802.16 -2004 hng ti cc

user c nhn di ng, lm vic trong bng tn thp hn 6GHz. Mng li ny phi

hp cng MLAN, mng di ng cellular 3G c th to thnh mng di ng c vng

ph sng rng. Hy vng cc nh cung cp vin thng digital truy nhp khng dy

c phm vi ph sng rng tho mn c cc nhu cu a dng ca thu bao. Tiu

chun IEEE 802.16e c thng qua trong nm 2005.

1.4.4. Mng khng dy WRAN

Mng v tuyn khu vc. Nhm ny i din l cng ngh 802.22 ang c

nghin cu v pht trin bi IEEE. Vng ph c n s ln tm 40- 100km. Mc

ch l mang cng ngh truyn thng n cc vng xa xi ho lnh, kh trin khai

cc cng ngh khc. N s s dng bng tn m TV analog khng dng t c

vng ph rng.

1.5. Tng kt

Ni dung chng ny trnh by cc kin thc tng quan v cng ngh

mng Internet v c bit l gii thiu v cng ngh mng Internet khng dy, kin

trc c bn ca: mng LAN khng dy (chun 802.11), ca mng WAN khng dy

v ca Internet khng dy (chun WAP v cc chun mi). Tnh n nay, sau hn

10 nm k t khi ra i, vic p dng cng ngh mng Internet khng dy rng ri

trong nhiu lnh vc chng t c tnh u vit v hiu qu ca n so vi cng

ngh mng Internet c dy truyn thng.

Cng ging nh mi cng ngh mng Internet khc, vn an ninh trong

mng Internet khng dy cng c t ra v c bit trong hon cnh c s

dng rng ri nh hin nay th vn an ninh cho mng Internet khng dy tr nn

l mt vn nng hi, cp thit trong lnh vc in ton v cng ngh mng. Do

- 43 -


, ni dung chng tip theo s i gii thiu, nghin cu cc k thut tn cng

mng Internet khng dy t a ra nhng gii php an ninh cho mng Internet

khng dy, nghin cu chi tit phng php bo mt v m bo ton vn d liu

bn trong cc gii php .

- 44 -


CHNG 2. TNG QUAN V AN NINH MNG

INTERNET KHNG DY

2.1. Mt s k thut tn cng Internet khng dy.

Mng my tnh Internet khng dy cng mang nhng c trng c bn ca

mt mng my tnh Internet v th vic tn cng v cc bin php i ph cng da

theo cc nguyn l trnh by cc chng trc. Ngoi ra t nhng c th ring

ca mng Internet khng dy v khng gian truyn sng nn n chu nhng kiu tn

cng khc v c nhng bin php i ph khc. C nhiu cch phn loi an ninh

mng Internet, chng ny s phn tch da vo phn loi theo tnh cht tn cng.

2.1.1. Tn cng b ng Passive attacks

2.1.1.1. nh ngha

Tn cng b ng l kiu tn cng khng tc ng trc tip vo thit b no

trn mng, khng lm cho cc thit b trn mng bit c hot ng ca n, v th

kiu tn cng ny nguy him ch n rt kh pht hin. V d nh vic ly trm

thng tin trong khng gian truyn sng ca cc thit b s rt kh b pht hin d

thit b ly trm nm trong vng ph sng ca mng ch cha ni n vic n

c t khong cch xa v s dng anten c nh hng ti ni pht sng, khi

cho php k tn cng gi c khong cch thun li m khng b pht hin.

Cc phng thc thng dng trong tn cng b ng: nghe trm (Sniffing,

Eavesdropping), phn tch lung thng tin (Traffic analyst).

- 45 -


Passive Attacks

Eavesdropping Traffic Analysis

Hnh 2.1. Cc phng thc dng trong tn cng b ng

2.1.1.2. Kiu tn cng b ng c th - Phng thc bt gi tin (Sniffing)

a. Nguyn l thc hin

Bt gi tin Sniffing l khi nim c th ca khi nim tng qut Nghe

trm Eavesdropping s dng trong mng my tnh. C l l phng php n

gin nht, tuy nhin n vn c hiu qu i vi vic tn cng WLAN. Bt gi tin c

th hiu nh l mt phng thc ly trm thng tin khi t mt thit b thu nm

trong hoc nm gn vng ph sng. Tn cng kiu bt gi tin s kh b pht hin ra

s c mt ca thit b bt gi d thit b nm trong hoc nm gn vng ph sng

nu thit b khng thc s kt ni ti AP thu cc gi tin.

Vic bt gi tin mng c dy thng c thc hin da trn cc thit b

phn cng mng, v d nh vic s dng phn mm bt gi tin trn phn iu khin

thng tin ra vo ca mt card mng trn my tnh, c ngha l cng phi bit loi

thit b phn cng s dng, phi tm cch ci t phn mm bt gi ln , vv.. tc

l khng n gin. i vi mng khng dy, nguyn l trn vn ng nhng khng

nht thit phi s dng v c nhiu cch ly thng tin n gin, d dng hn nhiu.

Bi v i vi mng khng dy, thng tin c pht trn mi trng truyn sng v

ai cng c th thu c.

Nhng chng trnh bt gi tin c kh nng ly cc thng tin quan trng, mt

khu, .. t cc qu trnh trao i thng tin trn my bn vi cc site HTTP, email,

cc instant messenger, cc phin FTP, cc phin telnet nu nhng thng tin trao i

- 46 -


di dng vn bn khng m ha (clear text). C nhng chng trnh c th ly

c mt khu trn mng khng dy ca qu trnh trao i gia Client v Server khi

ang thc hin qu trnh nhp mt khu ng nhp. Cng t vic bt gi tin, c

th nm c thng tin, phn tch c lu lng ca mng (Traffic analysis), ph

nng lng trong khng gian ca cc vng. T m k tn cng c th bit ch

no sng truyn tt, ch no km, ch no tp trung nhiu my.

Nh bt gi tin ngoi vic trc tip gip cho qu trnh ph hoi, n cn gin

tip l tin cho cc phng thc ph hoi khc. Bt gi tin l c s ca cc

phng thc tn cng nh n trm thng tin, thu thp thng tin phn b mng

(wardriving), d m, b m (Key crack), vv ..

Hnh 2.2. Phn mm bt gi tin Ethereal

Wardriving: l mt thut ng ch thu thp thng tin v tnh hnh phn b

cc thit b, vng ph sng, cu hnh ca mng khng dy. Vi tng ban u

dng mt thit b d sng, bt gi tin, k tn cng ngi trn xe t v i khp cc

ni thu thp thng tin, chnh v th m c tn l wardriving. Ngy nay nhng k

tn cng cn c th s dng cc thit b hin i nh b thu pht v tinh GPS xy

dng thnh mt bn thng tin trn mt phm vi ln.

- 47 -


Hnh 2.3. Phn mm thu thp thng tin h thng mng khng dy NetStumbler

b. Bin php i ph

V bt gi tin l phng thc tn cng kiu b ng nn rt kh pht hin v

do c im truyn sng trong khng gian nn khng th phng nga vic nghe

trm ca k tn cng. Gii php ra y l nng cao kh nng m ha thng tin

sao cho k tn cng khng th gii m c, khi thng tin ly c s thnh v

gi tr i vi k tn cng.

2.1.2. Tn cng ch ng Active attacks

2.1.2.1. nh ngha

Tn cng ch ng l tn cng trc tip vo mt hoc nhiu thit b trn mng

v d nh vo AP, STA. Nhng k tn cng c th s dng phng php tn cng

ch ng thc hin cc chc nng trn mng. Cuc tn cng ch ng c th

c dng tm cch truy nhp ti mt server thm d, ly nhng d liu

quan trng, thm ch thc hin thay i cu hnh c s h tng mng. Kiu tn cng

ny d pht hin nhng kh nng ph hoi ca n rt nhanh v nhiu, khi pht hin

ra chng ta cha kp c phng php i ph th n thc hin xong qu trnh ph

hoi.

So vi kiu tn cng b ng th tn cng ch ng c nhiu phng thc a

dng hn, v d nh: Tn cng t chi dch v (DOS), Sa i thng tin (Message

- 48 -


Modification), ng gi, mo danh, che du (Masquerade), Lp li thng tin

(Replay), Bomb, spam mail, v v...

Active Attacks

Masquerade Replay

Message Modification Denied of service

Hnh 2.4. Tn cng ch ng

2.1.2.2. Cc kiu tn cng ch ng c th

a. Mo danh, truy cp tri php

a.1. Nguyn l thc hin

Vic mo danh, truy cp tri php l hnh ng tn cng ca k tn cng i

vi bt k mt loi hnh mng my tnh no, v i vi mng Internet khng dy

cng nh vy. Mt trong nhng cch ph bin l mt my tnh tn cng bn ngoi

gi mo l my bn trong mng, xin kt ni vo mng ri truy cp tri php

ngun ti nguyn trn mng. Vic gi mo ny c thc hin bng cch gi mo

a ch MAC, a ch IP ca thit b mng trn my tn cng thnh cc gi tr ca

my ang s dng trong mng, lm cho h thng hiu nhm v cho php thc hin

kt ni. V d vic thay i gi tr MAC ca card mng khng dy trn my tnh s

dng h iu hnh Windows hay UNIX u ht sc d dng, ch cn qua mt s

thao tc c bn ca ngi s dng. Cc thng tin v a ch MAC, a ch IP cn gi

mo c th ly t vic bt trm gi tin trn mng.

- 49 -


a.2. Bin php i ph

Vic gi gn bo mt my tnh mnh ang s dng, khng cho ai vo dng tri

php l mt nguyn l rt n gin nhng li khng tha ngn chn vic mo

danh ny. Vic mo danh c th xy ra cn do qu trnh chng thc gia cc bn

cn cha cht ch, v vy cn phi nng cao kh nng ny gia cc bn.

b. Tn cng t chi dch v - DOS

b.1. Nguyn l thc hin

Vi mng my tnh khng dy v mng c dy th khng c khc bit c bn

v cc kiu tn cng DOS ( Denied of Service ) cc tng ng dng v vn chuyn

nhng gia cc tng mng, lin kt d liu v vt l li c s khc bit ln. Chnh

iu ny lm tng nguy him ca kiu tn cng DOS trong mng my tnh khng

dy. Trc khi thc hin tn cng DOS, k tn cng c th s dng chng trnh

phn tch lu lng mng bit c ch no ang tp trung nhiu lu lng, s

lng x l nhiu, v k tn cng s tp trung tn cng DOS vo nhng v tr

nhanh t c hiu qu hn.

- Tn cng DOS tng vt l

Tn cng DOS tng vt l mng c dy mun thc hin c th yu cu k

tn cng phi gn cc my tnh trong mng. iu ny li khng ng trong mng

khng dy. Vi mng ny, bt k mi trng no cng d b tn cng v k tn

cng c th xm nhp vo tng vt l t mt khong cch rt xa, c th l t bn

ngoi thay v phi ng bn trong ta nh. Trong mng my tnh c dy khi b tn

cng th thng li cc du hiu d nhn bit nh l cp b hng, dch chuyn

cp, hnh nh c ghi li t camera, th vi mng khng dy li khng li bt k

mt du hiu no. 802.11 PHY a ra mt phm vi gii hn cc tn s trong giao

tip. Mt k tn cng c th to ra mt thit b lm bo ha di tn 802.11 vi

nhiu. Nh vy, nu thit b to ra nhiu tn s v tuyn th s lm gim tn

hiu / t l nhiu ti mc khng phn bit c dn n cc STA nm trong di tn

nhiu s b ngng hot ng. Cc thit b s khng th phn bit c tn hiu mng

- 50 -


mt cch chnh xc t tt c cc nhiu xy ra ngu nhin ang c to ra v do

s khng th giao tip c. Tn cng theo kiu ny khng phi l s e do

nghim trng, n kh c th thc hin ph bin do vn gi c ca thit b, n qu

t trong khi k tn cng ch tm thi v hiu ha c mng.

- Tn cng DOS tng lin kt d liu

Do tng lin kt d liu k tn cng cng c th truy cp bt k u nn li

mt ln na to ra nhiu c hi cho kiu tn cng DOS. Thm ch khi WEP c

bt, k tn cng c th thc hin mt s cuc tn cng DOS bng cch truy cp ti

thng tin lp lin kt. Khi khng c WEP, k tn cng truy cp ton b ti cc lin

kt gia cc STA v AP chm dt truy cp ti mng. Nu mt AP s dng

khng ng anten nh hng k tn cng c nhiu kh nng t chi truy cp t cc

client lin kt ti AP. Anten nh hng i khi cn c dng ph sng nhiu

khu vc hn vi mt AP bng cch dng cc anten. Nu anten nh hng khng

ph sng vi khong cch cc vng l nh nhau, k tn cng c th t chi dch v

ti cc trm lin kt bng cch li dng s sp t khng ng ny, iu c th

c minh ha hnh di y:

Hnh 2.5. M t qu trnh tn cng DOS tng lin kt d liu

Gi thit anten nh hng A v B c gn vo AP v chng c sp t

ph sng c hai bn bc tng mt cch c lp. Client A bn tri bc tng, v

vy AP s chn anten A cho vic gi v nhn cc khung. Client B bn tri bc

tng, v vy chn vic gi v nhn cc khung vi anten B. Client B c th loi

client A ra khi mng bng cch thay i a ch MAC ca Client B ging ht vi

- 51 -


Client A. Khi Client B phi chc chn rng tn hiu pht ra t anten B mnh hn

tn hiu m Client A nhn c t anten A bng vic dng mt b khuch i hoc

cc k thut khuch i khc nhau. Nh vy AP s gi v nhn cc khung ng vi

a ch M

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