US Robotics USR2450 Technické informace stáhnout pdf (Strana 58)

V 2.2 Copyright SystemExperts 2001,2002,2003

115

Anatomy of 802.11b: Hidden Node Problem

n AP P sees A, B, and C, but A and C can’t see

each other (see means, the packets don’t

reach)

n Optional feature of RTS/CTS added to 802.11b

n RTS packet includes target address

n A sends: “This is for B” (C doesn’t see this)

n CTS includes sender address

n B sends: “Please send A” (C DOES see this)

n This feature is significant overhead but a very

common condition that needs to be accounted

for

116

Hidden Node Problem

“Hidden node”: STA 3 Out of range STA 1, in range STA 2

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Practical Wireless IP: 1
Course Contents 2
Course Objectives 2
What is Wireless 3
Wireless Component Overview 4
Wireless Devices 4
General Purpose Devices 5
FH, DS, IR, CCK(b), OFDM(a) 6
Physical Layer 7
802.11b Frame Control 8
802.11b IBSS 8
802.11b BSS 9
802.11b ESS 9
Exposures 10
Technology Problems 10
Theft of hardware 11
Registry Permissions 12
Masquerading 13
Eavesdropping 14
MAC Layer 14
MAC address configuration 15
Where are We? 17
Section Contents 17
Key Factors in Technology 18
Cellular Basics 18
Major Cellular Systems 19
GSM Security 20
Today’s Data Systems 20
CDPD Security 21
Other Popular Systems 22
What is 3G? 22
In a box … 23
Observations 23
Mobile Data Services 24
Uses a micro-browser 25
Popular delivery standards 25
Web Clipping 26
WAP Protocol Layers 28
The Gap in WAP 28
Gap in WAP 29
What Really Matters? 30
Wireless LAN Technologies 32
Upcoming WLAN 32
802.11 Local Area Wireless 33
802.11b Components 33
Current 802.11 Security 34
More on WEP Keys 35
Access Points and WEP 35
WEP Encryption Steps 36
WEP Encryption 36
WEP Decryption Steps 37
WEP Decryption 37
128-bit Version (WEP2) 38
WEP Key Management 38
The Major WEP Problems 39
Problem: Keystream Reuse 40
-- as key length increases 41
Current Status of WEP 42
Current Status of WEP (cont) 43
Current 802.11 Authorization 44
Wireless EAP: Cisco’s Version 45
Let’s take a look…J 47
Access Points 101 49
Frequency Overlap 50
Access Point Usage 50
Extension Point 51
Access Point Placement 51
Placement (cont.) 52
Placement, 3 Dimensional 53
Capacity and Bandwidth 53
Bandwidth Aggregation 54
Anatomy of 802.11b 55
Anatomy of 802.11b: the bits 55
Anatomy of 802.11b: the wave 56
Congestion (cont.) 57
Hidden Node Problem 58
Finding those Hidden Nodes 59
Surprising Results 60
What Does This Mean? 60
Anatomy of 802.11b: Roaming 61
Roaming (cont.) 61
Roaming Activities 62
IEEE IAPP 63
Wi-FI Roaming (cont.) 64
Configuring an Access Point 65
Access Point Medicine 65
Wireless at Home 67
Wireless at a Conference 68
Industry Setup: Ariba 68
CyberCafe 69
Architectural Considerations 69
Linux Wireless RF Sniffer 71
Linux Sniffer: How-To 72
How-To (cont.) 72
Sniffer Observations 73
AirSnort 73
Home Spun Access Point 74
Building your own AP 74
My Problems 75
My Observations 76
How Does it Work? 77
IEEE 802.11a 79
1W5.725-5.825 80
802.11a Coverage 81
802.11a Coverage Graphic 81
802.11a Problems 82
802.11 Thoughts 82
Antennas: The Skinny 84
Antennas: Basics 85
Antennas: Basics (cont.) 85
Antennas: Dipole 86
Dipole Radiation 86
Antennas: Directional 87
Directional Radiation: Biquad 87
Antennas: PCMCIA Cards 88
Typical PCMCIA Radiation 88
Antennas: More Facts 89
Antennas: More Facts (cont.) 89
Thanks to … 92
References 93
References (cont.) 93
Wireless Stuff 95
Mailing Lists 95
Glossary 96
Glossary (cont.) 96
Glossary(cont.) 97
End Matter 98
Apple Airport 99
Building your own AP (cont.) 100
Directions (cont.) 101
Floppy based Wireless Gateway 103
WFG Internals 103
WFG Internals (cont.) 104
Consultant 106
Brad C. Johnson 106
Vice President 106
Philip Cox 106

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