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March 2001
Greg Chesson et al, AtherosSlide 1
doc.: IEEE 802.11-01/132r1
Submission
VDCF Presentation
Greg Chesson, [email protected] Diepstraten, [email protected] Hoeben, [email protected] Singla, [email protected] Teunissen, [email protected] Wentink, [email protected]
March 2001
Greg Chesson et al, AtherosSlide 2
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
Greg Chesson et al, AtherosSlide 3
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
March 2001
Greg Chesson et al, AtherosSlide 4
doc.: IEEE 802.11-01/132r1
Submission
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]
PRI
feedback
DCF queue[k] CWmin[k] QIFS[k]
March 2001
Greg Chesson et al, AtherosSlide 5
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]
March 2001
Greg Chesson et al, AtherosSlide 6
doc.: IEEE 802.11-01/132r1
Submission
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
March 2001
Greg Chesson et al, AtherosSlide 7
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)
March 2001
Greg Chesson et al, AtherosSlide 8
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
March 2001
Greg Chesson et al, AtherosSlide 9
doc.: IEEE 802.11-01/132r1
Submission
Latency Differentiation
DCF CWmin(15,31) QIFS(0,0) CWmin(15,15) QIFS(0,1)
High-priority latency plot(per-frame as load increases)
50 ms
Hi-pri latency under 20ms
Lo-pri latenciesAbove 50 ms Lower latency
Variation with QIFS
For guaranteed latencyUse HCF
March 2001
Greg Chesson et al, AtherosSlide 10
doc.: IEEE 802.11-01/132r1
Submission
Robust under load changes
4 100 Kbit CBR(phones)
8 Mbit CBR (video)
3 Mbit CBR
Backgroundstations
RemoveLoads
March 2001
Greg Chesson et al, AtherosSlide 11
doc.: IEEE 802.11-01/132r1
Submission
Latency Differentiation
Phonelatency
Videolatency
5 ms
March 2001
Greg Chesson et al, AtherosSlide 12
doc.: IEEE 802.11-01/132r1
Submission
Glad you asked that
March 2001
Greg Chesson et al, AtherosSlide 13
doc.: IEEE 802.11-01/132r1
Submission
DCF State Machine
IDLE TRANSMIT
Queue Empty?
CCA >= DIFS?
Retry Limit?
BC=0Success
Fail
Abort
Retry
Ready Yes
No
Yes
No
BACKOFF Faithful rendering of Clause 9.Immediate access + post-backoff.See document 01/131 for greater detail. If (CCA>=DIFS)
decrement BC
March 2001
Greg Chesson et al, AtherosSlide 14
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
Fail
Abort
Retry
Ready Yes
No
Yes
No
PRI OK?
Yes
No
VDCF adds priority test replaces DIFS by QIFS[i],Selects CW from [0,aCWmin[i]]. If (CCA>=QIFS[i] & !Transmit
decrement BC[i]
March 2001
Greg Chesson et al, AtherosSlide 15
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
March 2001
Greg Chesson et al, AtherosSlide 16
doc.: IEEE 802.11-01/132r1
Submission
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
March 2001
Greg Chesson et al, AtherosSlide 17
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)
March 2001
Greg Chesson et al, AtherosSlide 18
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.
March 2001
Greg Chesson et al, AtherosSlide 19
doc.: IEEE 802.11-01/132r1
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
March 2001
Greg Chesson et al, AtherosSlide 20
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
Greg Chesson et al, AtherosSlide 21
doc.: IEEE 802.11-01/132r1
Submission
Conclusion
Simple is goodSimple is good
