VDCF Presentation Greg Chesson, greg@atheros.com March 2001 VDCF Presentation Greg Chesson, greg@atheros.com Wim Diepstraten, wdiepstraten@agere.com Maarten Hoeben, Maarten.Hoeben@intersil.com Aman Singla, aman@atheros.com Harold Teunissen, hteunissen@lucent.com Menzo Wentink, menzo.wentink@intersil.com Greg Chesson, Atheros, et al

VDCF Overview VDCF is license-free, royalty-free Enhancement to DCF March 2001 VDCF Overview VDCF is license-free, royalty-free 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 Greg Chesson, Atheros, et al

March 2001 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 Greg Chesson, Atheros, et al

VDCF Components Prioritized output queues (queue[i]) March 2001 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 feedback Queue[i] DCF queue[i] CWmin[i] QIFS[i] PRI TRANSMIT Queue[k] DCF queue[k] CWmin[k] QIFS[k] Greg Chesson, Atheros, et al

Two Controls Contention Window (CW) Inter-Frame Space (IFS) March 2001 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] Greg Chesson, Atheros, et al

Why two controls? Both controls provide effective differentiation March 2001 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 Greg Chesson, Atheros, et al

March 2001 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) Greg Chesson, Atheros, et al

Bandwidth Differentiation March 2001 Bandwidth Differentiation Equal TxOPs Similar Differentiation DCF CWmin(15,31) QIFS(0,0) CWmin(15,15) QIFS(0,1) Greg Chesson, Atheros, et al

Latency Differentiation March 2001 Latency Differentiation Hi-pri latency under 20ms High-priority latency plot (per-frame as load increases) Lo-pri latencies Above 50 ms Lower latency Variation with QIFS For guaranteed latency Use HCF 50 ms DCF CWmin(15,31) QIFS(0,0) CWmin(15,15) QIFS(0,1) Greg Chesson, Atheros, et al

Robust under load changes March 2001 Robust under load changes 8 Mbit CBR (video) Remove Loads Background stations 3 Mbit CBR 4 100 Kbit CBR (phones) Greg Chesson, Atheros, et al

Latency Differentiation March 2001 Latency Differentiation Video latency 5 ms Phone latency Greg Chesson, Atheros, et al

March 2001 Glad you asked that Greg Chesson, Atheros, et al

DCF State Machine March 2001 Queue Empty? Yes No Ready CCA >= DIFS? IDLE TRANSMIT No Retry Retry Limit? Fail Abort Success BC=0 BACKOFF Faithful rendering of Clause 9. Immediate access + post-backoff. See document 01/131 for greater detail. If (CCA>=DIFS) decrement BC Greg Chesson, Atheros, et al

VDCF State Machine (for queue[i]) March 2001 VDCF State Machine (for queue[i]) Queue[i] Empty? Yes No Ready CCA >= QIFS[i]? Yes PRI OK? Yes IDLE TRANSMIT No No Retry Retry Limit[i]? Fail Abort Success VDCF adds priority test replaces DIFS by QIFS[i], Selects CW from [0,aCWmin[i]]. BC[i]=0 BACKOFF If (CCA>=QIFS[i] & !Transmit decrement BC[i] Greg Chesson, Atheros, et al

Simulations Simulations based on Berkeley NS2 See document 01/008 March 2001 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 Greg Chesson, Atheros, et al

VDCF Design Choices Distributed Stability Control vs Centralized March 2001 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 Greg Chesson, Atheros, et al

VDCF Design Choices QoS Parameter Setting vs fast adaptation March 2001 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) Greg Chesson, Atheros, et al

Implementation Factors March 2001 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. Greg Chesson, Atheros, et al

Implementation Factors March 2001 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 Greg Chesson, Atheros, et al

Summary Simple Safe Differentiated Service Robust March 2001 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 Greg Chesson, Atheros, et al

March 2001 Conclusion Simple is good Greg Chesson, Atheros, et al