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S ha bi Trung tm Hc liu i hc Thi Nguyn
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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
-
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
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
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i
S ha bi Trung tm Hc liu i hc Thi Nguyn
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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 3.12. V hiu ha Peer-to-Peer Wi-Fi ...86
Hnh 3.13. V hiu ha Peer-to-Peer Wi-Fi ...86
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
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S ha bi Trung tm Hc liu i hc Thi Nguyn
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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.
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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.
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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.
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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,).
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- 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.
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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.
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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
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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
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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.
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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
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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
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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.
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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
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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
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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.
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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
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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
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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.
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+ 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.
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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
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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).
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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
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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).
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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
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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.
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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
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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
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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.
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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
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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
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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
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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).
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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.
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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
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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:
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- 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.
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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
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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
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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
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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
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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.
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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
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, 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 .
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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).
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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
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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.
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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
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S ha bi Trung tm Hc liu i hc Thi Nguyn
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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.
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S ha bi Trung tm Hc liu i hc Thi Nguyn
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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
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S ha bi Trung tm Hc liu i hc Thi Nguyn
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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
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S ha bi Trung tm Hc liu i hc Thi Nguyn
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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