Doc.: IEEE 802.11-01/132r1 Submission March 2001 Greg Chesson et al, Atheros Slide 1 VDCF...

transcript

March 2001

Greg Chesson et al, AtherosSlide 1

doc.: IEEE 802.11-01/132r1

Submission

VDCF Presentation

Greg Chesson, greg@atheros.comWim Diepstraten, wdiepstraten@agere.comMaarten Hoeben, Maarten.Hoeben@intersil.comAman Singla, aman@atheros.comHarold Teunissen, hteunissen@lucent.comMenzo Wentink, menzo.wentink@intersil.com

March 2001

doc.: IEEE 802.11-01/132r1

Submission

VDCF Overview• VDCF is license-free, royalty-free (to be determined)

– It is the belief of the proposers this is possible.– There is plentiful prior art. IP statements have been requested.

• Enhancement to DCF• Same state machine as DCF • Minimal change to MAC (see document 01/131)• Compatible with DCF, PCF

• Properties• Prioritized access to MAC services per Traffic Category (TC)• Controls relative bandwidth per TC• Controls relative latency and jitter per TC• Robust over light, medium, heavy loads• Simple

• Simulation– Extensive validation results (see documents 01/008, 01/133)– Public software: contact authors

March 2001

doc.: IEEE 802.11-01/132r1

Submission

VDCF Origins• Differentiated Service by traffic category rather than individual flows

originates with the IETF Diffserv WG:• http://www.ietf.org/html.charters/diffserv-charter.html

• DQoS proposed by Jan Kruys at San Diego ad hoc meeting in September 2000, captured in later IEEE submissions:

• Distributed QoS Model for 802.11 (00/267), by Jan Kruys and Harold Teunissen

• Virtualized DCF access method:• Enhance D-QoS through Virtual DCF (00/351), by Maarten Hoeben and Menzo Wentink• Baseline D-QoS Proposal (00/399), by Chesson, Diepstraten, Kitchin, Teunissen, Wentink

• Differentiated Inter-Frame Space, Contention Window, Retry Policy• DFWMAC (93/190), by Diepstraten, Ennis, Belanger• Priority in CSMA/CA to support distributed Time-Bounded Services (94/058), by Wim

Diepstraten

• Distributed vs Centralized Control• Review of Distributed Time Bounded Services (94/121), by Tim Phipps

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doc.: IEEE 802.11-01/132r1

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VDCF Components– Prioritized output queues (queue[i]) – Legacy DCF finite state machine per queue (queue[i])

• CWmin differentiated per TC (CWmin[i]), controllable by EAP• DIFS differentiated per TC (QIFS[i]), controllable by EAP• Queue state machines count backoff slots in parallel• Low-priority queues defer to higher-priority queues

DCF queue[i] CWmin[i] QIFS[i]

Queue[i]

TRANSMIT

Queue[k]

feedback

DCF queue[k] CWmin[k] QIFS[k]

March 2001

doc.: IEEE 802.11-01/132r1

Submission

Two Controls• Contention Window (CW)

– Lower-priority TCs select random backoff counters from CWs, on average receiving fewer TxOPS than higher-priority TCs picking from CWs.

– Imposes bandwidth and access delay differentiation between TCs– Contention windows expand/contract

• Local adaptation: binary exponential backoff in response to collision• Also controllable by EAP in Beacon• CWmin[i] in QoS Parameter Set Element updates aCWmin[i]

• Inter-Frame Space (IFS)– Different IFS per TC: TxQIFS[i] = SIFS + aQIFS[i] x aSlotTime– Imposes bandwidth and latency differentiation between TCs– Controllable by EAP

• QIFS[i] in QoS Parameter Set Element updates aQIFS[i]

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doc.: IEEE 802.11-01/132r1

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Why two controls?• Both controls provide effective differentiation

– CWmin• Affects TxOP probability, collision probability• average backoff delay

– QIFS• Low-priority traffic defers to high-priority traffic• Slower backoff counting rate for lower-priority traffic

• Complementary when used together– Use small values for QIFS: e.g. 0, 2, 5, 5

• Large QIFS values can exclude traffic – Use smaller range of CWmins; e.g. 15, 15, 31, 63; or 15, 31, 31, 31– Achieve differentiation with better latency/jitter

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doc.: IEEE 802.11-01/132r1

Submission

Small Examples• Load(2,4) => 2 high-priority stations, 4 low-priority stations

– Add a station every 3 seconds– Track bandwidth/latency for

• DCF only• CWmin(15,31) and QIFS(0,0)• CWmin(15,15) and QIFS(0,1)

• Load(4,2,10) => 2 high-priority (phone), 4 high-bw (video), 10 background stations

– Add a station every 3 seconds: phone, video, background– Then remove a station every 3 seconds– Observe good performance over the entire load range using

• CWmin(15,15,31) and QIFS(0,2,7)

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doc.: IEEE 802.11-01/132r1

Submission

Bandwidth Differentiation

DCF CWmin(15,31) QIFS(0,0) CWmin(15,15) QIFS(0,1)

Equal TxOPs Similar Differentiation

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doc.: IEEE 802.11-01/132r1

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Latency Differentiation

DCF CWmin(15,31) QIFS(0,0) CWmin(15,15) QIFS(0,1)

High-priority latency plot(per-frame as load increases)

Hi-pri latency under 20ms

Lo-pri latenciesAbove 50 ms Lower latency

Variation with QIFS

For guaranteed latencyUse HCF

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Submission

Robust under load changes

4 100 Kbit CBR(phones)

8 Mbit CBR (video)

3 Mbit CBR

Backgroundstations

RemoveLoads

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doc.: IEEE 802.11-01/132r1

Submission

Latency Differentiation

Phonelatency

Videolatency

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doc.: IEEE 802.11-01/132r1

Submission

Glad you asked that

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doc.: IEEE 802.11-01/132r1

Submission

DCF State Machine

IDLE TRANSMIT

Queue Empty?

CCA >= DIFS?

Retry Limit?

BC=0Success

Ready Yes

BACKOFF Faithful rendering of Clause 9.Immediate access + post-backoff.See document 01/131 for greater detail. If (CCA>=DIFS)

decrement BC

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doc.: IEEE 802.11-01/132r1

Submission

VDCF State Machine (for queue[i])

BACKOFF

IDLE TRANSMIT

Queue[i] Empty?

CCA >= QIFS[i]?

Retry Limit[i]?

BC[i]=0Success

Ready Yes

PRI OK?

VDCF adds priority test replaces DIFS by QIFS[i],Selects CW from [0,aCWmin[i]]. If (CCA>=QIFS[i] & !Transmit

decrement BC[i]

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doc.: IEEE 802.11-01/132r1

Submission

Simulations• Simulations based on Berkeley NS2

– Codes simulate full protocol stacks (ARP, UDP, TCP)– Expose protocol stack coupling through AP and other effects

• See document 01/008– Demonstrates that priority queues in AP deliver effective QoS in many

cases using only legacy DCF– Shows some effects of different retry policies – Shows application of CWmin[i]– Shows effectiveness of PIFS access as it might be used by HCF in the

presence of a heavy DCF overload• See document 01/133

– Catalog of scenarios with various CWmin[], QIFS[] settings– Incomplete exploration of full parameter space– Demonstrates utility of the controls– Provides starting point for determining default IBSS parameter settings

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VDCF Design Choices• Distributed Stability Control vs Centralized

– Robust: does not depend on EAP or reliable channel for stability– Distributed: self-adapting at station via binary exponential backoff– IBSS-ready: doesn’t need updates for stability

• Uniform distribution vs Geometric– Better latency variance, delay jitter (see document 01/008)– No “mini-capture effect” (see 01/008) causing backoff amplification

• Post-backoff/immediate access vs Pre-backoff– Lower latency under light load– Equivalent to Pre-backoff when backlogged queues– Same as legacy DCF

• Use both QIFS[i] and CWmin[i]– Complementary mechanisms

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doc.: IEEE 802.11-01/132r1

Submission

VDCF Design Choices• QoS Parameter Setting vs fast adaptation

– QoS Parameters • AP adjusts at STA Association time, or RSVP time• AP adjusts to observed load average – not time-critical• “slow” adaptation: unlikely to stimulate control oscillation

– Fast Adaptation• Unacknowledged broadcast not well-suited for wireless media• System adaptation rate (sample+decide+broadcast+adopt) slower than rate of

change of offered load in many cases: cause of oscillation, degradation.• Fast adaptation consumes bandwidth, TxOPs, MAC logic cycles

• Independent queue[i] state– Fairness across TCs and stations– Backoff counts (BC[i]) retain age ordering (i.e. ensure forward progress)

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doc.: IEEE 802.11-01/132r1

Submission

Implementation Factors• Retain power-of-2 CWmin intervals

– Simple arithmetic, no division/mod ops needed – Simple random number generation

• Random number generation rate– Once per TxOP per queue

• Must recognize QoS-DATA and (TBD) TCID tags– Otherwise no new frame exchange sequences

• One new information element to process: QoS Parameter Set– Appears in Beacon and Probe Response– Adjusts CWmin[] and QIFS[] values

• Sequence numbers– No change at sender, can assign sequence number at TxOP– Receive cache must include TC, i.e. triples instead of tuples.

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Submission

Implementation Factors• New MIB variables:

• aCWmin[0-7], aQIFS[0-7], aSSRC[0-7], aSLRC[0-7], aCWmax[0-7].

• State variables for each output queue:• Backoff Counter BC[i]• Short/long retry counters QSRC[i] and QLRC[i]• Contention window CW[i]

• Virtual collisions• Priority test applied when BC[i] reaches zero• Losing queue[i] goes into backoff state

• Backoff• Frame ordering is not preserved between TCs• All queues can be in backoff at the same time• A single counter (plus logic) can represent multiple backoff states

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doc.: IEEE 802.11-01/132r1

Submission

Summary

• Simple– minimal control mechanism

• Safe– builds on proven MAC

• Differentiated Service– bandwidth differentiation– latency differentiation and mitigation

• Robust– self-adaptive, but also controllable– differentiates over changing loads

March 2001

doc.: IEEE 802.11-01/132r1

Submission

Conclusion

Simple is goodSimple is good

Doc.: IEEE 802.11-01/132r1 Submission March 2001 Greg Chesson et al, Atheros Slide 1 VDCF...

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