Transcript Wireless Networking Understanding the departure from wired networks, Case study: IEEE 802.11 (WiFi)

Wireless Networking
Understanding the departure from wired networks,
Case study: IEEE 802.11 (WiFi)
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Many Motivations for Wireless
 Unrestricted mobility / deployability
 Unplugged from power outlet
 Significantly lower cost
 No cable layout, service provision
 Low maintenance
 Ease
 Direct communication with minimum infratructure
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From Links to Networks
 Variety of architectures
 Single hop networks
 Multi-hop networks
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The Wireless Future …
Internet
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No Free Lunch
 Numerous challenges






Channel fluctuation
Lower bandwidth
Limited Battery power
Disconnection due to mobility
Security
…
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Question Is …
Can’t we use the rich “wireline” knowledge ?
In solving the wireless challenges
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The Answer
Wireless channel: A dispersive medium
The PHY and MAC layer completely dissimilar
The whole game changes
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On Our Agenda
 Key Physical layer behavior
• From Wired to Wireless
 The principles of wireless medium access control
• Collision avoidance (CSMA/CA) not detection
 The emergence of 802.11 (WiFi)
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Medium Access Control
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The Channel Access Problem
 Multiple nodes share a channel
A
B
C
 Pairwise communication desired
 Simultaneous communication not possible
 MAC Protocols
 Suggests a scheme to schedule communication
• Maximize number of communications
• Ensure fairness among all transmitters
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The Trivial Solution
A
B
C
 Transmit and pray
 Plenty of collisions --> poor throughput at high load
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Don’t
transmit
The Simple Fix
A
B
C
 Transmit and pray
 Plenty of collisions --> poor throughput at high load
 Listen before you talk
 Carrier sense multiple access (CSMA)
 Defer transmission when signal on channel
Can collisions still occur?
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CSMA collisions
spatial layout of nodes
Collisions can still occur:
Propagation delay non-zero
between transmitters
When collision:
Entire packet transmission
time wasted
note:
Role of distance & propagation
delay in determining collision
probability
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CSMA/CD (Collision Detection)
 Keep listening to channel
 While transmitting
 If (Transmitted_Signal != Sensed_Signal)
 Sender knows it’s a Collision
 ABORT
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2 Observations on CSMA/CD
 Transmitter can send/listen concurrently
 If (Sensed - received = null)? Then success
 The signal is identical at Tx and Rx
 Non-dispersive
The transmitter can DETECT if and
when collision occurs
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Unfortunately …
Both observations do not hold for wireless
Leading to …
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Wireless Medium Access Control
A
C
B
D
Signal
power
SINR threhold
Distance
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Wireless Media Disperse Energy
A cannot send and listen in parallel
A
C
B
D
Signal
power
Signal not same at different locations
SINR threhold
Distance
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Collision Detection Difficult
 Signal reception based on SINR
 Transmitter can only hear itself
 Cannot determine signal quality at receiver
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Calculating SINR
B
A
C
SignalOfInterest( SoI)
SINR 
Interference( I )  Noise( N )
A
SoI 
A
B
Ptransmit
d AB
C
transmit
P
I  
d CB
C
B
A
Ptransmit

d
AB
SINRBA 
C
Ptransmit
N 
d CB
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Red < Blue = collision
Red signal >> Blue signal
X
A
C
B
D
Signal
power
SINR threhold
Distance
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Important: C has not heard A, but can interfere at receiver B
C is the hidden terminal to A
X
A
C
B
D
Signal
power
SINR threhold
Distance
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Important: X has heard A, but should not defer transmission to Y
Y
X is the exposed terminal to A
X
A
C
B
D
Signal
power
SINR threhold
Distance
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Hidden and Exposed Terminal Problems
Critical to wireless networks even today
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Idea!
X
A
C
B
D
Signal
power
SINR threhold
Sensitivity threshold
Distance
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Do not
transmit in
this region
Will this solve the wireless MAC problem?
Idea!
X
A
C
D
B
Signal
power
SINR threhold
T
Sensitivity threshold
Distance
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The Emergence of 802.11
 Wireless MAC proved to be non-trivial
 1992 - research by Karn (MACA)
 1994 - research by Bhargavan (MACAW)
 Led to IEEE 802.11 committee
 The standard was ratified in 1999
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IEEE 802.11 with Omni Antenna
RTS = Request
To Send
CTS = Clear
To Send
M
S
Y
RTS
D
CTS
K
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IEEE 802.11 with Omni Antenna
silenced
M
S
Data
Y
D
silenced
ACK
X
silenced
silenced
K
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But is that enough?
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RTS/CTS
 Does it solve hidden terminals ?
 Assuming carrier sensing zone = communication zone
E
RTS
F
CTS
A
B
C
D
E does not receive CTS successfully  Can later initiate transmission to D.
Hidden terminal problem remains.
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Hidden Terminal Problem
 How about increasing carrier sense range ??
 E will defer on sensing carrier  no collision !!!
E
RTS
F
CTS
A
B
C
Data
D
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Hidden Terminal Problem
 But what if barriers/obstructions ??
 E doesn’t hear C  Carrier sensing does not help
E
RTS
F
CTS
A
B
C
Data
D
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Exposed Terminal
 B should be able to transmit to A
 RTS prevents this
E
RTS
CTS
A
B
C
D
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Exposed Terminal
 B should be able to transmit to A
 Carrier sensing makes the situation worse
E
RTS
CTS
A
B
C
D
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Thoughts !
 802.11 does not solve HT/ET completely
 Only alleviates the problem through RTS/CTS and
recommends larger CS zone
 Large CS zone aggravates exposed terminals
 Spatial reuse reduces  A tradeoff
 RTS/CTS packets also consume bandwidth
 Moreover, backing off mechanism is also wasteful
The search for the best MAC protocol is still on.
However, 802.11 is being optimized too.
Thus, wireless MAC research still alive
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Questions?
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