IEEE 802.11
Tarik Cicic
University of Oslo
December 2001
Overview
• Wireless LAN architecture:
– components
– operation
• Frequency Hopping vs. Direct Sequence
• Recent developments
2
Components
• Basic service sets, controlled by their base
station (Access Points, AS)
• base stations connected by a backbone --distribution system, usually Ethernet
3
1
Sample 802.11 LAN
Distribution system (Backbone)
Extended service set
AP1
AP2
BSS1
BSS2
4
IEEE 802.11 layers
• Architecture similar to other 802.x
technologies
• three physical layers supported:
– Frequency Hopping Spread Spectrum
– Direct Sequence Spread Spectrum
– Infra Red
• additional functionality in MAC:
– fragmentation
– packet retransmission
– acknowledgments
802.2
802.11 MAC
FH
DS
IR
Link Layer
PHY Layer
5
Spread spectrum
• IEEE 802.11 uses Spread Spectrum radio
transmission:
– Frequency hopping (FHSS)
– Direct Sequence (DSSS)
• diffused infrared transmission is the third
physical layer technology
6
2
Frequency hopping
• Pseudorandom sequence of frequencies is
chosen within a BSS
• all communicating parts follow the
sequence
• 2.4 GHz, 79.1 MHz bandwidth
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Frequency hopping (2)
2.40
2.41
2.42
2.43
2.44
2.45
2.46
2.47
2.48
GHz
(fictive frequencies)
Time
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Direct Sequence
1
0
Data
1
0
4-bit
random
1
0
Result
Data
1010 => 1111000011110000
4-bit random
1011101110111011
Result
0100101101001011
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3
Direct Sequence (2)
• Different senders use different frequencies
• dominant technology today:
– IEEE 802.11 b
– up to 11 Mb/s (~50 m distance)
10
Diffuse infrared ( in IEEE 802.11)
• Millimetre (and shorter) waves
• Close to the visible light --- cannot pass through
walls
• but is reflected by walls, no “line of sight” needed
• IR can be used on distances of ~10m, while radio
transmission can be used ~100m in buildings and
1km outside
11
CSMA/CA
• Carrier Sense Multiple Access with
Collision Avoidance
• sense the medium; transmit if free
• collision detection as in wired LAN is not
possible:
– full duplex needed
– no other stations heard means not free medium
12
4
CSMA: hidden nodes
• If A and C send in the same time to B, they cannot
notice the collision
A
B
C
A and C cannot
see each other
(hidden nodes)
13
CSMA: exposed nodes
• B transmits to A. C hears it, but does not transmit to D
• this is a mistake, since A cannot hear C and D cannot
hear B --- poor resource utilization
A
B
C
D
C is exposed
to B’s
transmission
14
Positive Acknowledgment
• If the medium is free for DIFS (Distributed Inter
Frame Space) period, transmit
• the receiver checks the CRC and, if OK,
sends an ACK
15
5
Collision Avoidance
• Virtual Carrier Sense decreases the collision
possibility
• sender station first sends a Request To Send
(RTS), a short packet with source and destination
ID and transaction duration
• the destination responds with “Clear To Send”
• all stations hearing this set their Network
Allocation Vector to the given duration
• stations delay the sending (+ sense the medium)
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Collision Avoidance
Src
RTS
DATA
SIFS
Dest
CTS
ACK
DIFS
Other
NAV(RTS)
NAV(CTS)
Next MPDU
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MAC Functionality
1. Acknowledgments (no ACKs in wired
LANs)
2. fragmentation & reassembly:
•
•
small packets at radio LANs, 20 ms FH and
2Mb/s give 5242 Bytes
1518 Bytes in Ethernet
3. exponential backoff procedure used
(modified compared to Ethernet)
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6
Joining a cell (BSS)
• Active / passive scanning (beacon message)
• the association process (capability
exchange)
• Roaming through re-association!
19
Security
• Authentication through knowledge of a
secret key
• eavesdropping hindered by use of pseudo
random number generator; packets are
scrambled with a random sequence
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Other features
• Power save
• Ad-hoc networks
21
7
Current Developments
• First 802.11 implementations were based on
FH, 3 Mb/s communication
• IEEE 802.11b standard opens for 11 Mb/s
communication using DS
• 802.11b is de-facto standard today (despite
shorter distances and worsened security)
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Discussion: Security in 802.11
• Why is WLAN technology often regarded
unsecure?
23
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