January, 2001
doc.: IEEE 802.11-01/011r1
Comparing V-DCF with other EDCF
proposals
Wim Diepstraten
Menzo Wentink
Maarten Hoeben
Greg Chesson
Harold Teunissen
Submission
Slide 1
Agere Systems
Intersil
Intersil
Atheros
Lucent Technologies
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Agenda
• Discuss new Retry Backoff approach in V-DCF.
• Main Characteristics of the proposals
– where they are similar
– where they are different
• Comparison of V-DCF with P-DCF
– delay variance comparison
• Comparison of V-DCF with TCMA
• Conclusion
Submission
Slide 2
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
New Retry Backoff for V-DCF
• Based on simulation and further analyses we came to the
conclusion to change our Retry backoff approach, to better work in
a QoS environment.
• Target functionality is:
– We need a distributed mechanism that can stabilize the throughput for
high contention situations, preventing collision avelange effects when
the load and number of contenders increase.
– An other function of the Retry backoff mechanism is to circumvent
repeated interference situations at the receiver caused by hidden
stations contending or collisions with overlapping BSS traffic.
• Simulation analyses show that the pure exponential backoff
behavior is too aggressive for contention stabilization, while its
characteristics are ok for the hidden station case.
• Therefore the Retry Backoff procedure is slightly modified.
Submission
Slide 3
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Retry Backoff
• The new retry Backoff procedure:
– Start exponential backoff by CW doubling after the first retry, on all
Q’s, so per station.
• In an AP CW doubling per Q is more effective.
• This decreases the effect of exponential backoff on delay
performance.
– The probability of a double retry is the square of the individual
collision probability.
– So for situations where the collision probability is in the 5-10% range,
the probability for a double retry is in the .25-1% range, which is
significantly lower.
• If traffic in the next Q is likely going to the same destination then
the CW doubling should apply per station, else it is more effective
to apply CW doubling only for the Q experiencing the Retry (AP).
Submission
Slide 4
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Main characteristics
•
There are a lot of similarities among the proposals.
– They all are based on CSMA/CA using contention resolution mechanisms based
on random access procedures after a busy medium becomes available again.
– Main Priority differentiation mechanism is to force differentiation in average
access delay.
• And / or controlling the idle time after contention resolution is started.
– They all assume a “Differentiation Control” feedback mechanism that is
targeted to increase the delay differentiation between priority levels when the
load increases.
• However TCMA seems to favor a local rather then centralized feedback approach.
•
•
•
Roughly equivalent behavior can be achieved by the different mechanisms
by using a equivalent set of control parameters.
There seems to be agreement about the fairness criteria between the
proposals (Also TCMA??)
This paper intends to focus on the main differences in characteristics.
– Mainly between V-DCF and P-DCF
– Because TCMA is based on / combined with either of the two mechanisms.
Submission
Slide 5
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Probability Distribution
• Effect of different probability distributions:
– V-DCF uses a uniform distributed backoff mechanism
• resulting in a low backoff variance
• gives higher priority to “older” frames which have already counted
down part of their backoff.
• allows for immediate access if CCA>DIFS, which is advantageous
for high priority frames which Q’s are not backlogged.
– P-DCF uses a geometric access distribution mechanism
• resulting in high backoff variance (which is undesirable for QoS)
• is memoryless, so does not favor frames that are already backing
off for some time.
– frequent updates makes this worse
• Frames always go through backoff (no immediate access).
• Conclusion: Significant difference in backoff variance
Submission
Slide 6
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Stability Control / Retry Approach
• Stability control mechanisms.
– V-DCF uses an exponential backoff behavior after the first retry.
• Its stability control mechanism is fully distributed, and does not depend on
a centralized congestion control mechanism.
• It prevents for collision avalanches due to congestion.
• And is targeted to avoid effect of hidden nodes and overlap interference, by
avoiding overlap with the hidden message.
– Relates to interference situation at the receiver not visible to the transmitter
– P-DCF stability control mechanisms are not clear.
• Either an autonomous decrease of the PPC based on retry event
• Or a centralized control mechanism to control the PPC.
– If this is the main mechanism then it does fully depend on a centralized entity for
stability.
• Conclusion: A stability control mechanism is needed that is fully
distributed and does not depend on other stations.
Submission
Slide 7
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Complexity Considerations
• V-DCF:
– V-DCF is very similar to the legacy DCF approach.
• Can be implemented as n*DCF (parallel DCFs).
• Or a Scheduler function combined with a “Delta backoff DCF” as
presented during the Tampa meeting, and described in doc 00/399
– This scheduler is also effective to select candidate for CF-polled TxOp.
– Minimal complexity increase compared to plain DCF.
– Roughly the same number of computations per frame needed as in
legacy DCF.
• P-DCF:
– In its simplest form it requires a random number generation per slot
• This is a major computational increase for FW based solutions.
• Conclusion: V-DCF approach is less computation intensive.
Submission
Slide 8
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
P-DCF Motivation?
• What is the motivation that justifies the change to P-DCF?
– Properties that are claimed to be important:
• Better analyses possible (memoryless, no exponential backoff)
• Faster update from one parameter set to another allows better
optimization control
• No internal collisions
• Better performance due to faster control
• How different are they and how important are these
factors?
– It is our opinion that these factors are insignificant, as the
controllability of both approaches are equivalent.
– A disadvantage of P-DCF is the large variance of the geometric
distribution.
Submission
Slide 9
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Backoff Jitter Analyses
• The following figures show the difference in backoff jitter between the
uniform and geometric distribution.
– Clearly jitter is not desirable for QoS.
• Contention Offset (CO) and CW provide separation of functions
– CO provides for differentiation between priorities
• CO with geometric backoff will break the memoryless property, as will UAT
differentiation
– CW provides randomness to account for presence of other contenders
• CO allows the CW to be as small as necessary, which will reduce the delay
variance
• VDCF introduces randomness only when necessary
– Important for low jitter
– In general, less variance increases resource usage efficiency
Submission
Slide 10
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Backoff Jitter Simulations
• The backoff is calculated with different control parameters per priority
level (two TC system example).
– Top priority example CO=0, CW=15 equivalent to PP=2/17
• The high priorities have the same average backoff of 7.5 slots, but the geometric
process has a significantly higher backoff variance
– The next priority level use CO=10, CW=15 equivalent to PP=2/37
• Again same average backoff of 17.5 slots, but but the geometric backoff
variance has incresed even more
– The backoff variance is the same for both TC’s in V-DCF, because they use
the same CW
• In general, lower priorities will have higher backoff jitter in P-DCF
• The backoff variance will increase the frame delivery variance when both
processes have the same collision rate
Submission
Slide 11
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Geometric Backoff
PP=2/17, average backoff = 7.5
Uniform Backoff
CW=15, average backoff = 7.5
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Geometric Backoff
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Uniform Backoff with Contention Offset
CO=10, CW=15, average backoff = 17.5
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Wim Diepstraten, Agere Systems
5000
January, 2001
doc.: IEEE 802.11-01/011r1
V-DCF Throughput Optimisation
•
•
V-DCF throughput can be optimized by dynamically adapting CW based on
load measurements. This is similar to P-DCF.
Steady-state V-DCF analysis is straightforward when no exponential backoff
is considered (see references below)
–
–
•
The steady-state transmission probability is P(transmission)=2/(CW+1), when
queues are always backlogged (see ref. 1)
The optimization algorithm introduced by Jin-Meng Ho in paper 00/467, which
equalizes collision time and idle time, can be applied to V-DCF by taking
CW=2/PP-1 (or CWi=2/TCPP-1 when more queues are present)
Selection of Papers
1.
2.
3.
Submission
G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination
Function”, IEEE J. Select. Areas Comm., vol 18, March 2000, pp 535-547.
G. Bianchi, “IEEE 802.11 – Saturation Throughput Analysis”, IEEE Comm. Let.,
vol 2, pp. 318-320, Dec. 1998.
T.S. Ho and K.C. Chen, “Performance Evaluation and Enhancement of the
CSMA/CA Protocol for 802.11 Wireless LAN’s”, Proc. IEEE PIMRC, Taipei,
Taiwan, Oct. 1996, pp. 392-396.
Slide 13
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
TCMA comparison
• The only difference of TCMA as suggested is to use PIFS for high
priority and DIFS for other priorities.
• We think that this is NOT a good idea, as the PIFS can better be
reserved for the AP.
– Giving AP a slight EDCF access advantage by starting its backoff
downcount at PIFS rather then DIFS.
– While the PIFS access priority can also be used during Burst access as in
the V-DCF and HCF proposals.
• It is unclear how the TCMA approach which claims that it does NOT
like “Retry backoff increase” is assuring a stable throughput.
– Further a centralized monitoring feedback is assumed to perform better
then individual station monitoring:
– Because station does not see all traffic in BSS, and it can lead to unequal
priority differentiation within a BSS.
Submission
Slide 14
Wim Diepstraten, Agere Systems
January, 2001
doc.: IEEE 802.11-01/011r1
Conclusion
•
V-DCF has better QoS properties then P-DCF.
– V-DCF has lower backoff variance then P-DCF, particularly in combination with a
contention offset
– In V-DCF, frames that are in backoff for a while will have higher relative priorities
then new arrivals
– Stability of Retry Backoff is achieved in a distributed way without the need for a
centralized control entity for congestion control.
• The new “Retry Backoff” approach is better suited for QoS than exponential backoff
–
–
–
–
•
V-DCF has lower implementation complexity
V-DCF is very similar to the legacy DCF approach that everybody is familiar with.
There is no clear motivation to change to a P-DCF approach.
The PIFS tier can better be used to give an AP preferential access rather then for the
Top priority, to prevent breaking (HCF like) bursting assumptions.
Conclusion:
– We should adopt the V-DCF for inclusion in the baseline proposal.
Submission
Slide 15
Wim Diepstraten, Agere Systems