Lecture-06-intro 802.11

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Introduction to 802.11
Easy as a, b and g
Dr. Nguyen Tuan Nam
[email protected]
802.11 Modes
• Infrastructure (Access Point) mode:
– Communicate between wireless node
– Communicate with wired network
– All through Access Point
– Ex: Coffee shop, corporate WLAN
• Ad hoc:
– Wireless node communicate directly
– No need access point
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Infrastructure Mode
Dynamic IP
Node A
Static IP
Communication
Range?
INTERNET
Node B Access Point_ Provide connection
to wired network
_ Relay function _ Not mobile
Can node A talk directly to node B
without going through Access Point?
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When Is Such Infrastructure Not
Available?
• Single point of failure
• No single trusted AP
• Special area (jungle)
• Quick deployment (emergency, fire)
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Ad hoc Mode
Node A Communication
Range?
Routing?
Node B
Node C
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802.11 (Infrastructure Mode)
• 802.11 (1997):
– 1 to 2 Mbps
– 2.4 GHz
– 20m
• 802.11a (1999):
– 54 Mbps
– 5GHz
– 15m -> 30m
• 802.11b (1999):
– 11 Mbps
– 2.4 GHz
– 45m à 90m
– More interference
• 802.11g (2003):
– 54 Mbps
– 2.4 GHz
– 45m à 90m
• 802.11n (2009)
– 600 Mbps
– 2.4 GHz or 5GHz
– 91m à 182m
• 802.11y
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Radio Transmission
• FHSS (Frequency Hopping Spread Spectrum):
– Hop from frequency to frequency
– Wide band of frequencies
– FCC: 75+ frequencies, max 400ms/hop
• DSSS (Direct Sequence Spread Spectrum):
– Divides up the spectrum into a number of channels (802.11b)
– A data signal combined with chipping code
– Error can be recovered due to the redundancy of the transmission
• OFDM (Orthogonal Frequency Division Multiplexing):
– Split signal into smaller sub-signals
– Transmitted simultaneously at diff frequencies
• IR (infrared)
Differences between FHSS and DSSS?
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802.11
Frequency Hopping Spread Direct Sequence Spread Infrared (IR) PHY
Spectrum (FHSS) PHY Spectrum (DSSS) PHY 1,2 Mbps
1, 2 Mbps 1,2 Mbps
Orthogonal Frequency Division
Higher rate (DSSS) PHY High rate (DSSS) PHY Multiplexing (OFDM) PHY
54 Mbps 11, 5.5 Mbps 6,9,12,18,24,36,48,54 Mbps
802.11g 802.11b 802.11a
2.4 GHz 5.7 GHz
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802.11b
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802.11g
• Backward compatible with 802.11b
– Same channel assignments
– OFDM
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802.11a
• 5 GHz range
– Less crowded
– Bigger chunk of frequencies
– More room for data on each channel
– More channels
– More non-overlapping channels (8 vs. 3)
– Not as popular as 802.11g. Why?
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802.11 MAC
• CSMA/CA
• Interference
• Hidden nodes
• Power Saving
• Privacy and Access Methods
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CSMA/CD
• Carrier Sensing Media Access
– Listen for the carrier
– Carrier à Busy, not OK
– No carrier à Available, OK to transmit
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Collision Detection
• Detect 2 devices attempt to use data
channel simultaneously
• 32 bit CRC at end of transmission
• Monitoring its own transmission
– If current above what generated à collision
– Send 32 bit jam sequence instead of CRC
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Random Exponential Backoff
• Collision, wait
• Wait how long?
– R x slot_time
– Slot_time = constant
– R = random number between 1 and 2N
– N = min (10, trial attempt)
• Maximum trial = 16
• Why random?
• Why exponential?
• Why 10?
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Ethernet Capture
• If one NIC sends excessive number of
frames, it may dominate the LAN
• Some nodes may be locked out
• Why?
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CSMA/CD
• No longer use in the 10 Gigabit Ethernet
specification
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CSMA/CA
• Wireless:
– Avoid but not detect
– Why?
– DCF
R slots
DIFS
Busy Medium Next Frame
DIFS = DCF Interframe Space
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CSMA/CA with ACK
Source needs to send DATA to Dest
ACK return immediately
DIFS
No ACK: retransmit after backoff
Data
Source
SIFS
ACK
Dest
DIFS R slots
Next Frame
Others
SIFS = Short Interframe Space
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Problem with CSMA
• Exposed terminal problem
• Hidden terminal problem
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