idle sense
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Idle Sense:
An Optimal Access Method for High
Throughput and Fairness in Rate Diverse
Wireless LANs
Martin Heusse Franck Rosseau
Romaric Guillier Andrzej Duda
Presented by Nikki Benecke, October 10th, 2006 for CS577
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Objective:
Define an access methodoptimized for
throughput and fairness, able to
dynamically adapt to physical channelconditions, to operate near optimum for a
wide range of error rates, and to provide
equal time shares when hosts usedifferent bit rates.
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Access method
Way of deciding who can access the media
at a given time
In WLANs, CSMA/CA as implemented by
DCF (required) or PCF
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Optimized for throughput and
fairness Throughput
Not goodput
Expect to maintain, not increase, with IS
Fairness inIdle Sense:
Jain Index
Time Fairness
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Dynamic channel adaptation
Physical conditions vary wildly with time
Frames received in error -> senders bitrate
lowered to reduce error rate
Idle Sense tries to intelligently decide when
lowering the bitrate is worthwhile
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Supporting a wide range of error
rates Sort of superfluoushandled by dynamic
channel adaptation
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Equal time shares
Step away from min-max fairness
Keep slow senders from unnecessarily
restricting fast senders
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Motivation
802.11 currently requires DCF as the access
method
Also allows PCF
Idle Sense addresses some key problems
with DCF
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DCF: Operation
[dcf in a nutshell]
Martin Heusses presentation at SIGCOMM 05
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DCF: Backoff
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DCF: Bad Day Problem
Bad transmission conditions -> host will lose many
frames
High error rate -> frequent backoffs
CW is increased -> transmission attempt probability is
lower
So the host will try to send less often and may eventually
starve!
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DCF: Physical Layer Capture
[physical layer capture]
Martin Heusses presentation at SIGCOMM 05
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Two Modifications to DCF
1. No exponential backoff
2. All senders have equal CW
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Channel contention
For a transmission to occur, one host must try to
send and all others must be idle, so:
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Channel contention
- probability that no hosts are sending
- probability of a collision
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Channel contention
- average number of consecutive idle
slots
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Channel contention
Since all CW are equal:
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Channel contention
Throughput as a function of :
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Channel contention
Goal: maximize the throughput by
minimizing the time spent in collisions and
contention
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Channel contention
Maximizing is equivalent to
minimizing defined as:
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Channel contention
Define as
Take the first derivative of the cost function toget:
is the unique solution for this equation
that is in [0,1]
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Formulas
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value based on variant
This is because it is based on the ts/tc ratio,
which varies by flavor of 802.11
= 0.1622 for 802.11b
(important for later use)
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Idle Sense: Principles
Each host estimates and uses it to compute
its CW
By adjusting CW, a host makes
converge to (common across hosts)
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vs
requires knowing N (# of hosts), which
we would like to avoid
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vs
quickly approaches , though,
so we can use this value as
with little penalty
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Channel adaptation
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Channel adaptation
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Adaptation example
802.11b (nitarget = 5.68)
N = 5
CW = 60
= 1.2
= 0.001
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Adaptation example
1. CW = 60 -> = 0.033, Pi = 0.847,
= 5.53
Ni < 5.68, so increase CW
CW = = 1.2 x 60 = 72
(Multiplicative decrease)
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Adaptation example
2. This increase leads to = 6.71
6.71 > 5.68, so decrease CW
CW =
CW = 69(Additive Increase)
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Adaptation example
3. Ni is now 6.41, still greater than 5.68.
So decrease again, CW gets value 67.
As this continues, we will oscillate around
(Additive Increase)
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Time-fairness
Argument: using min-max fairness restricts
fast hosts by making them send as slowly as
slow hosts
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Time-fairness
Using time-fairness eliminates two
problematic situations:
(i) Slower hosts limit the throughput of fasterhosts
(ii) Slow hosts suffer starvation because an
access point will not switch to a slowerrate
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Time-fairness
They say TF is better for both slow hosts and
fast hosts.
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Time-fairness
InIdle Sense, accomplish TF by controlling
the access probability for the hosts
Slow host transmitting at bit rate rcurr
receives a modified CW, CW =
CW x , so scales down.
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Idle Sense: Performance
Developed discrete-event simulator thatimplements 802.11 DCF andIdle Sense
No source code provided
Three variants of 802.11
802.11b, 802.11g, theoretical 100Mb/s 802.11
Idle Senseparameters:
= 68.17 for 802.11b= 31.0 for 802.11g
= 19.3 for 100Mb/s 802.11
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Performance
Simulations run for 106transmissions
Experimentally determined parameters
= 0.001
= 1.2
Ntrans= 5 (see figure 6)
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Throughput
Compare 802.11b DCF, Slow Decrease,
Asymptotically Optimal Backoff (AOB)
andIdle Sensefor an increasing number ofhosts
Throughput is the average of the throughput
for all hosts active in the network
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Throughput
[figure 7]
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Throughput
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Throughput: Conclusion
Authors expected throughput to stay around
that of DCF
Throughput actually improves slightly
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Delay
K = # of intertransmissions between other
hosts between two transmissions of a given
hostFor larger K, hosts experience greater delays
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Delay
[table 5]
Hosts should experience significantly less
delay!
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Collision Overhead
[table 3]
of DCF
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Convergence Speed
Start out with 5 greedy hosts, add another 5 at 2000, drop 5 at 3000
Stabilizes quickly after every change
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Time fairness
One 1Mb/s host and N-1 11Mb/s hosts
By TF, fast hosts should get 11 x the
throughput of the slow host in Idle Sense
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Time fairness
In DCF, all hosts get the slow hosts
throughputtput much lower than in IS!
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Possible weaknesses?
No info provided on actual simulation
tool/we cant recreate experiments
Some experimentally derived parameters
How is this actually implemented?
What happens if some senders have IS and
others dont?
Is time-fairness really fair?
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Questions/Comments
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Acknowledgements
Most figures taken directly from the paper
Slide 10 is from the 1999 801.11 technical
specifications (Section 9.2, DCF)
Slides 9 & 12 and the Bad Day and
physical capture ideas are from Martin
Heusses presentation ofIdle SenseatSIGCOMM 05