WLAN
What is 802.11?
• A family of wireless LAN (WLAN) specifications
developed by a working group at the Institute of
Electrical and Electronic Engineers (IEEE)
• Versions: 802.11a, 802.11b, 802.11g, 802.11e,
802.11f, 802.11i
WLANs – 802.11 Protocol Architecture
Real Time Traffic
Normal Data Traffic
(Asynchronous)
Point Coordination
Function (PCF)
MAC
Distributed Coordination Function (DCF)
Physical Layer (PHY)
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IEEE 802.11
- Physical Medium Specification
Three Physical Media:
• INFRARED
• Narrowband Microwave
• Spread Spectrum
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Infrared
• Infrared signals used to transmit data (similar to TV remotes!)
•
Higher data rates possible (than spread spectrum)
• Line of sight point-to-point configuration required (or reflection
surface will reflects signals)
•
Too sensitive to obstacles.
•
10 m maximum range with no sunlight or heat interfere
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Narrowband Microwave
• Typically used to link two WLANs together (for example, to link
WLANs in two buildings)
•
Microwave dishes required at both ends of link
•
Unlike spread spectrum which operates in the unlicensed ISM
band, narrowband microwave requires FCC licensing
• Exclusive license typically effective within a 17.5 mile radius
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Spread Spectrum
• Distributed signals over multiple frequencies (to avoid
eavesdropping or jamming)
• Frequency Hopping Spread Spectrum (FHSS)
– Sender transmits over a seemingly random series of frequencies
– Intended receiver aware of sequence of frequencies and hops accordingly
– Allows the coexistence of multiple networks in the same area by using different
hopping sequences
• Direct Sequence Spread Spectrum (DSSS)
– DS systems use a carrier that remains fixed to a specific frequency band. The data
signal is spread onto a much larger range of frequencies (at a much lower power
level) using a specific encoding scheme.
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Wireless LAN Classification
• Infrared (IR) LANs
– An individual cell of an IR LAN is limited to a single
room, since infrared light does not penetrate opaque
walls.
• Spread Spectrum LANs
– In most cases these LANs operate in the ISM
(industrial, scientific, and medical) bands, so no FCC
licensing is required for their use in the United States.
• Narrowband Microwave LANs
– These LANs operate at microwave frequencies but do
no use spread spectrum. Some of these products
operate at frequencies that require FCC licensing;
others use one of the unlicensed ISM bands.
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IEEE 802.11 Medium Access Control
MAC layer covers three functional areas:
Reliable data delivery
Access control
Security
Reliable Data Delivery
Loss of frames due to noise, interference, and
propagation effects
Frame exchange protocol
Source station transmits data
Destination responds with acknowledgment (ACK)
If source doesn‘t receive ACK, it retransmits frame
Wireless LAN MAC
• CSMA as Wireless MAC?
• Hidden and Exposed Terminal Problems make
the use of CSMA an inefficient technique
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Hidden Terminal Problem
Collision
A
•
•
•
•
•
B
C
A talks to B
C senses the channel
C does not hear A’s transmission (out of range)
C talks to B
Signals from A and B collide
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Exposed Terminal Problem
A
•
•
•
•
B
C
Not
possible
B talks to A
C wants to talk to D
C senses channel and finds it to be busy
C stays quiet (when it could have ideally
transmitted)
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D
Hidden and Exposed Terminal
Problems
• Hidden Terminal
– More collisions
– Wastage of resources
• Exposed Terminal
– Underutilization of channel
– Lower effective throughput
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CSMA - Collision Avoidance
• CSMA-CA uses short signaling packets for collision avoidance
– RTS (request to send): a sender request the right to send
from a receiver with a short RTS packet before it sends a
data packet
– CTS (clear to send): the receiver grants the right to send as
soon as it is ready to receive
• Signaling packets contain
– sender address
– receiver address
– packet size
• Variants of this method can be found in IEEE802.11 as DFWMAC
(Distributed Foundation Wireless MAC)
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Hidden Terminal Revisited …
A
•
•
•
•
•
RTS
CTS
DATA
CTS
B
A sends RTS
B sends CTS
C overheads CTS
C inhibits its own transmitter
A successfully sends DATA to B
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C
Hidden Terminal Revisited
• How does C know how long to wait before it
can attempt a transmission?
• A includes length of DATA that it wants to
send in the RTS packet
• B includes this information in the CTS packet
• C, when it overhears the CTS packet,
retrieves the length information and uses it to
set the inhibition time
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Exposed Terminal Revisited
A
RTS
CTS
•
•
•
•
•
B
RTS
C
Cannot hear CTS
D
Tx not
inhibited
B sends RTS to A (overheard by C)
A sends CTS to B
C cannot hear A’s CTS
C assumes A is either down or out of range
C does not inhibit its transmissions to D
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Collisions
• Still possible – RTS packets can collide!
• Binary exponential backoff performed by
stations that experience RTS collisions
• RTS collisions not as bad as data collisions in
CSMA (since RTS packets are typically much
smaller than DATA packets)
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Access Control
• To deal with these two problems 802.11
supports two modes of access control DCF
(Distributed Coordination Function) and PCF
(Point Coordination Function).
• All implementations must support DCF, but
PCF is optional.