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VDCF Presentation PowerPoint PPT Presentation

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VDCF Presentation. Greg Chesson, [email protected] Wim Diepstraten, [email protected] Maarten Hoeben, [email protected] Aman Singla, [email protected] Harold Teunissen, [email protected] Menzo Wentink, [email protected] VDCF Overview.

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VDCF Presentation

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VDCF Presentation

Greg Chesson, [email protected]

Wim Diepstraten, [email protected]

Maarten Hoeben, [email protected]

Aman Singla, [email protected]

Harold Teunissen, [email protected]

Menzo Wentink, [email protected]

Greg Chesson, Atheros, et al

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

    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 DCFaccess method:

    • EnhanceD-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])

    • 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]






    DCF queue[k]



    Greg Chesson, Atheros, et al

    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

      • 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

    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

    Equal TxOPsSimilar Differentiation

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

    Greg Chesson, Atheros, et al

    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

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

    Greg Chesson, Atheros, et al

    Robust under load changes

    8 Mbit CBR (video)





    3 Mbit CBR

    4 100 Kbit CBR


    Greg Chesson, Atheros, et al

    Latency Differentiation



    5 ms



    Greg Chesson, Atheros, et al

    Glad you asked that

    Greg Chesson, Atheros, et al

    DCF State Machine

    Queue Empty?





    CCA >= DIFS?





    Retry Limit?






    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])

    Queue[i] Empty?




    CCA >= QIFS[i]?


    PRI OK?







    Retry Limit[i]?




    VDCF adds priority test

    replaces DIFS by QIFS[i],

    Selects CW from [0,aCWmin[i]].



    If (CCA>=QIFS[i] & !Transmit decrement BC[i]

    Greg Chesson, Atheros, et al


    • 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

      • 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

      • 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

    • 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

    • 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


    • 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


    Simple is good

    Greg Chesson, Atheros, et al

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