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V 2.2 Copyright SystemExperts 2001,2002,2003
117
Hidden Node Problem: Let’s try it again
n 802.11
n basically designed for indoor, relatively short
distances, active and long-lived connected clients,
and low noise level
n but 802.11 (“wi-fi”) is “winning” in the wireless arena
and being increasingly used as well in outdoor, long
distant, occasionally connected, potentially high
noise level environments
n So what does that mean to hidden nodes?
n The key is the need to minimize the amount of
overhead you introduce to manage them
V 2.2 Copyright SystemExperts 2001,2002,2003
118
Finding those Hidden Nodes
n Carrier Sense Multiple Access (CSMA)/
Collision Avoidance (CA)
n CSMA – the device listens to the media before
transmitting
n Request To Send (RTS)/ Clear To Send (CTS) –
media reservation mechanism
n Polling – adaptive mechanism
n Device can not start transmission before receiving a
special acknowledgement packet (a marker) from
the AP
n Defend against “sudden” chaos
- 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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