1. VDCF Presentation Greg Chesson, greg@atheros.com
March 2001
VDCF Presentation
Greg Chesson,
Wim Diepstraten,
Maarten Hoeben,
Aman Singla,
Harold Teunissen,
Menzo Wentink,
Greg Chesson, Atheros, et al

2. 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

3. March 2001
VDCF Origins
Differentiated Service by traffic category rather than individual flows originates with the IETF Diffserv WG:
DQoS proposed by Jan Kruys at San Diego ad hoc meeting in September 2000, captured in later IEEE submissions:
Distributed QoS Model for (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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

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

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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

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