march 2001 jin-meng ho, et al., texas instrumentsslide 1 doc.: ieee 802.11-01/139 submission...

March 2001

Jin-Meng Ho, et al., Texas InstrumentsSlide 1

doc.: IEEE 802.11-01/139

Submission

Presentation for Proposed p-DCF Contention Access Enhancement

Jin-Meng Ho, Sid Schrum, Khaled Turki

Donald P. Shaver and Matthew B. Shoemake

Texas Instruments Incorporated

12500 TI Blvd.

Dallas, Texas 75243

(214) 480-1994 (Ho)

[email protected]

March 2001


doc.: IEEE 802.11-01/139

Submission

• P-DCF uses one backoff timer per station, just like legacy DCF.– It does not stack multiple DCFs within each station.

– It does not have the issue of checking, and resolving, simultaneous expiration of multiple backoff timers at any given station.

• P-DCF separates external behavior (access to medium) from internal behavior (selection from queues).

– Each ESTA performs its external contention just as a legacy DCF station.

– Multi-priority service per station appears as an internal enhancement to the legacy DCF MAC.

• P-DCF obeys DIFS usage as specified for legacy DCF.– No extra tiers of contention are required.

– No new backoff countdown rules are specified.

Outline

March 2001


doc.: IEEE 802.11-01/139

Submission

Performance

• P-DCF achieves differentiated service for prioritized traffic.– Higher-priority data encounters smaller access delay than lower-priority data.

– Lower-priority traffic is not starved nor suffers excessive access delay.

• P-DCF improves access delay over legacy/stacked DCF.– Higher-priority frames under P-DCF experience less delay than best-effort

frames under legacy DCF.• Not a case with stacked DCF.

– Lower-priority frames under P-DCF experience the same delay as best-effort frames under legacy DCF.

• Lower-priority frames are starved under stacked DCF.

• P-DCF increases channel throughput compared to legacy/stacked DCF.– More bits per second can be transmitted per channel.

– More stations and streams can be served per BSS.

March 2001


doc.: IEEE 802.11-01/139

Submission

Element ID(12)

Length(8)

TCPP Values (TCPP0, …, TCPP7)(8 octets)

ECA (Enhanced ContentionAccess) Parameter Set

• Traffic Category Permission Probabilities (TCPPs)– Each traffic category (TC) is assigned a TCPP.

– A frame from TCi is transmitted with a probability = TCPPi (conceptually).

– A frame from a WSTA is sent with a permission probability (PP) equal to the sum of the latest TCPPs for active local TCs (conceptually).

• Infrastructure Network – The hybrid coordinator (HC) regularly updates TCPPs for TCs of eight

priorities and broadcasts them via an ECA Parameter Set element in beacons.

– Update intervals of 20 - 50 ms long are found to provide good performance.

• IBSS and Backup Contention Access– TCPPs are adjusted in a way similar to binary exponential backoff for DCF.

Coordinated Contention

March 2001


doc.: IEEE 802.11-01/139

Submission

• IBSS and Backup Contention Access– If an active WSTA is located in an IBSS, or if it has not received TCPP

values for 50 TUs from the HC, it performs its contention for a frame transmission using the TCPP values calculated on its own:

• Any active local TC that has a non-zero TCPP value continues to have the same TCPP value until it has a frame transmitted.

• A local TC of priority i that has just successfully sent a frame has a TCPP value equal to TCPPi, max, where TCPP0, max = 1/33, and TCPPi, max = 2/17, i = 1, 2, …7, if the channel is busy, and has a TCPP value equal to TCPPi,idle, where TCPP0, idle =

1/4, and TCPPi, idle = 1/2, i = 1, 2, …7, if the channel is idle. • A local TC of priority i that has a retried frame to send after a collision changes its

TCPP value from TCPPi to max [TCPPi,min, 2 TCPPi / (4 – TCPPi)], where TCPPi,

min = 2/1025, i = 0, 1, 2, …7.

• Once the WSTA receives new TCPP values from the HC, it reverts to the HC-coordinated contention by immediately adopting the new TCPP values.

Distributed Contention

March 2001


doc.: IEEE 802.11-01/139

Submission

X P1

BC = 0

Idle Slot1

X > P2

Idle Slot1

X > P2

BC = 2

Idle Slot2

X > P2

BC = 3

Idle Slot3

X > P2

BC = 4

X > P3

BC = 1

Idle Slot1

X P3

BC = 1

X > P2

BC = 1

Idle Slot1

X > P2

BC = 2

Idle Slot2

X P2

BC = 2

X > P2

BC = 1

X P1 BC = 0 X > P2 BC = 1

X > P2 BC = 2

X > P2 BC = 3

X > P2 BC = 4

X > P2 BC = 5

X > P2 BC = 6

X > P2 BC = 7

X P2 BC = 7

X > P3 BC = 1

X P3 BC = 1

X > P2 BC = 1

X > P2 BC = 2

X P2 BC = 2

PP = P1 PP = P2 PP = P3

BC: Backoff Counter(BC 255)

Each X is a new pseudorandom numberuniformly distributed between 0 and 1

Tran

smits

Tran

smits

Tran

smits

ConceptualPersistent

Contention

EquivalentBackoffSetting

Backoff Timer• A WSTA uses its latest PP to (re)set its backoff timer.

– Conceptually, it generates a new pseudorandom number, X, at each idle slot (or after a busy channel becomes idle) to decide whether to transmit or not.

• If X PP, the ESTA sends a frame at the beginning of the next slot.

– Operationally, it repeats the above steps within a slot time to search for the equivalent backoff time and hence sets the backoff timer.

Over expanded interval

Over contracted interval

Unexpired backoff timer

Reset

March 2001


doc.: IEEE 802.11-01/139

Submission

0 1 0 0 0 0 1 1 0 0 0 1 0 0 0 1

16-Stage Maximum-Length Linear-Feedback Shift Register (LFSR)

Maximal-Length Linear-Feedback Shift Register

• Pseudorandom integer generation– Each shift of an m-stage maximum-length shift register produces an m-bit

binary pseudorandom integer represented by the bits stored in the register.

– The pseudorandom integers so generated are uniformly distributed over (0, 2m] and have a period of 2m – 1.

• Pseudorandom number generation– Such pseudorandom integers divided by 2m become pseudorandom numbers

uniformly distributed over (0, 1].

– Pseudorandom numbers can be generated in this way as fast as the clock frequency.

– Maximum-length shift registers are also widely used in generating CRC (FCS) parity check symbols.

March 2001


doc.: IEEE 802.11-01/139

Submission

Frame Transmission• Backoff Timer Setting and Countdown (Review of External Access)

– A WSTA (re)sets its backoff timer by repetitive search for a pseudorandom number, X, such that X PP.

– A WSTA decrements its backoff timer just as a DCF STA does, and hence uses the same station machine as for DCF.

– A WSTA transmits a frame when its backoff timer expires.

• Local Selection (Internal Access)– A WSTA selects a frame for transmission from a local TC of priority k such

that sum (TCPP0, …, k – 1) < X sum (TCPP0, …, k).• sum (TCPP0, …, k) = TCPP0 + TCPP1 + … + TCPPk, sum (TCPPk – 1) = 0 for k = 0,

and TCPPj = 0 if TC of priority j is locally inactive.

• Retry

– MIB attributes of aMaxTransmitMSDULifetime, dot11ShortRetryLimit, and

dot11LongRetryLimit apply to frame transmissions from individual TCs.

0 1PP

TCPP0 TCPP1 TCPPk TCPP7

X

... ...

March 2001


doc.: IEEE 802.11-01/139

Submission

Internal Selection

0 1PP

TCPP0 TCPP1 TCPP2 TCPP3

X

0 < X

T

CPP

0

FA

LSE

TC

PP0 < X

TC

PP0 +

T

CPP

1

FA

LSE

TC

PP0 +

TC

PP1

< X

TC

PP0 +

TC

PP1 +

T

CPP

2

TR

UE

TC

PP0 +

TC

PP1 +

TC

PP2

< X

TC

PP0 +

TC

PP1 +

TC

PP2 +

T

CPP

3

FA

LSE

• Example

Queue 0 Queue 1 Queue 2 Queue 3

March 2001


doc.: IEEE 802.11-01/139

Submission

TCPP Update and Load Control• Control Criterion

– Channel is optimally loaded /utilized when time on idles = time on collisions.• Time on an idle = a slot time.

• Time on a collision = longest transmission time of colliding stations + Ack transmission time + SIFS + DIFS.

• Control Mechanism– HC decreases TCPPs if channel is overloaded and vice versa.

– Contending WSTAs immediately respond to a change in TCPPs.

• Control Procedure– Compute the normalized difference, D = (TI - TC ) / T, between the time on

idles, TI, and the time on collisions, TC, over the time, T, allocated to contention in the CP since the last time when a TCPP update was broadcast.

– When D D0 or when a beacon is to be sent, update TCPPs as follows:• TCPP0 TCPP0 + G D, and TCPPk = Ck TCPP7, k =1, 2, …, 7, where D0 and

Ck are preset numbers, and G is positive.

• Algorithm self-stabilizing: TCPPk , TC , D , TCPPk , and vice versa.

March 2001


doc.: IEEE 802.11-01/139

Submission

• Fairness– All TCs of equal priority transmit with the same TCPP.

• Anytime -- before or after collision.

• Anywhere -- at the same WSTA or at different WSTAs.

• Differentiation– Relative differentiation: Higher priority TCs contend with larger TCPPs.

• TC Access probability ~ TCPP.

• Lower priority TCs not starved.

– Absolute differentiation: Some TCs may be stayed from contention.• TCPPs for stayed TCs set to 0.

• Higher priority TCs not impacted by lower priority TCs.

• Minimum bandwidth guaranteed for selected TCs.

• Maximum bandwidth imposed on certain TCs.

• Collision avoidance enhanced.

• Collision resolution accelerated.

• Some stayed TCs served by contention-free access for better QoS support.

Fairness and Differentiation

March 2001


doc.: IEEE 802.11-01/139

Submission

• P-DCF (CSMA with Adaptive Contention)– No new IFS rules.

– Single backoff timer per WSTA.

– No internal conflicts.

• V-DCF (Stacked DCF)– New IFS rules.

– Multiple backoff timers per WSTA.

– Internal conflicts.• Extra logic is required to detect and resolve internal collision.

Implementation Complexity

March 2001


doc.: IEEE 802.11-01/139

Submission

Backoff, Collision, and Delay

Collision

Access Delay

Backoff Time

Success

Access Delay

Backoff Time

Success

• Backoff of Smaller Variation

• Backoff of Larger Variation

• Reducing Collision Reducing Access Delay/Jitter

March 2001


doc.: IEEE 802.11-01/139

Submission

Channel busy Transmissionof another frame

Transmissionof another frame

Collisionof other frames

Transmissionof frame 1 only

Collisioninvolving frame 1


Access Delay 1

Channel busy Collisioninvolving frame 2

Access Delay 2 Access Delay Jitter

Backoff Time

Backoff Time

Backoff Jitter




Access Delay 1


Access Delay 2 Access Delay Jitter

Backoff Time

Backoff Time

Backoff Jitter


Delay Jitter < Backoff Jitter

Delay Jitter > Backoff Jitter

Transmissionof another frame


• Backoff of Smaller Variations

• Backoff of Larger Variations

Backoff Delay/Variation versus Access Delay/Variation

Access delay, and hence access delay variation, of a framedepends not only on backoff time and backoff variationbut also on many other factors. Reducing collision (whichcauses long delay) is more effective in minimizing accessdelay and variation than reducing backoff delay or variation.

A smaller backoff variation does not lead to a smaller accessvariation. A CSMA protocol with constant backoff (and hence zero backoff variation) is most likely to yield largeraccess delay and variation than a CSMA protocol withuniform backoff (and hence nonzero backoff variation).

March 2001


doc.: IEEE 802.11-01/139

Submission

Aggregate data arrival rate = 6 Mbps

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25 30 35 40 45 50

Simulation Time (sec)

Del

ay (

ms)

Legacy DCF (uniform backoff)

p-DCF (geometrical backoff)

p-DCF: PP offset by +/- 20%

Operation “recovered” from congestion as frames exceeding retry counts were discarded.

Access Delay and Variation (Simulation Result)

March 2001


doc.: IEEE 802.11-01/139

Submission


0

2

4

6

8

10

12

14

16

18

20

0 1 2 3 4 5 6 7 8 9 10


Del

ay (

ms)

Legacy DCF

P-DCF: TC0

P-DCF: TC1

P-DCF: TC2

P-DCF: TC3

TCPP update per 20 ms interval

Instantaneous Delays for 4-Priority P-DCF

March 2001


doc.: IEEE 802.11-01/139

Submission


1

1.5

2

2.5

3

3.5

4

2 3 4 5 6 7 8 9 10


Del

ay (

ms)

Legacy DCF

P-DCF: TC0

P-DCF: TC1

P-DCF: TC2

P-DCF: TC3

TCPP update per 20 ms interval

Average Delays for 4-Priority P-DCF

March 2001


doc.: IEEE 802.11-01/139

Submission


0

5

10

15

20

25

30

35

40

45

50

0 1 2 3 4 5 6 7 8 9 10


Del

ay (

ms)

Legacy DCF

V-DCF: CWmin = 63

V-DCF: CWmin = 31

V-DCF: CWmin = 15

V-DCF: CWmin = 7

Instantaneous Delays for 4-Priority V-DCF

March 2001


doc.: IEEE 802.11-01/139

Submission

Average Delays for 4-Priority V-DCF


0

2

4

6

8

10

12

14

16

18

20

2 3 4 5 6 7 8 9 10


Del

ay (

ms)

Legacy DCF

V-DCF: CWmin = 63

V-DCF: CWmin = 31

V-DCF: CWmin =15

V-DCF: CWmin = 7

March 2001


doc.: IEEE 802.11-01/139

Submission

• IEEE journals report adaptive contention (P-DCF) to achieve more than 30% throughput than binary exponential backoff.

– PP. 146-149 in F. Cali, et al., “Dynamic Tuning of the IEEE 802.11 Protocol to Achieve a Theoretical Throughput Limit,” IEEE INFOCOM’98.

– P. 1783 in . F. Cali, et al., “IEEE 802.11 Protocol: Design and Performance Evaluation of an Adaptive Backoff Mechanism,” IEEE J. Select. Areas Commun., vol. 8, Setp. 2000.

• Our own simulations also show P-DCF to have significant throughput, delay, and jitter improvement over binary exponential backoff.

– Adaptive contention is robust to PP miscalculations.

– 20 - 50 ms is adequate for TCPP updates.

• Performance improvement becomes even more substantial in high population areas such as in enterprise environments.

– Binary exponential backoff begins to fail as user population increases.

P-DCF Performance Improvement

March 2001


doc.: IEEE 802.11-01/139

Submission

• CWmin = 31 slots for 802.11b– This appears to be a choice of the right tradeoff between minimizing idles

and collisions for a CSMA based on binary exponential backoff rules.

– Using smaller CWmin results in increased collisions while using larger CWmin leads to excessive idles for typical channel loads.

• Changing CWmin values without changing other CWs leads to worse throughput and delay performance for all priorities than legacy DCF.

• CWmin = 15 slots for 802.11a– Choosing CWmin of 7 and 3 for higher-priority TCs leads to intensive

collision and diminished throughput.

– Using CWmin of 31 and 63 for lower-priority TCs yields QoS service much worse than simply following legacy DCF.

• There is no much room for changing CWmin values under binary exponential backoff rules.

Differentiation by CWmin?

March 2001


doc.: IEEE 802.11-01/139

Submission

…

…

…

…

…

…

…

…

…

…

Doubling CW for colliding TCs at low load unnecessarily delays frame transmission and decreases channel throughput.

The leftover backoff times of various TCs are inestimable to EAP/HC at the time of setting new CWmins.

Doubling CW for colliding TCs does not necessarily alleviate congestion state --collisions formed from past backoffs will still occur.

Randomizing backoff times without drastically increasing CW at low load is adequate and improves delay and throughput performance.

Setting CWmins without knowing leftover backoff times causes new frames to collide with backoff frames.

Each collision costs much channel time, aggravates congestion state, and results in more collisions in the future.

Doubling CW for colliding TCs substantially downgrades the access priority for both retried and new frames of those TCs, compared to frames of TCs not undergoing collision resolution.

Resetting CWmins cannot stop collisions developed in the past and bound to occur in the future.

CWmin update

fornew

frame arrivals

only

Contention-basedand

contention-free

transmissions

Doubling CW for colliding TCs over-penalizes the colliding TCs, even more so for those that had collided before.

Having multiple backoff timers per station makes delay and throughput ever more sensitive to CWmin values.

Adaptation for CWmin?

March 2001


doc.: IEEE 802.11-01/139

Submission

• Just not desirable.– Lower-priority traffic suffers excessive delays.

– Additional slots in expanded IFS wastes channel bandwidth.

• Especially so with HCF.– HC can do a much better job with tier access and absolute priority.

– WSTAs using differing IFSs for prioritized access complicate their operation as well as HC’s.

Differentiation by IFS?

March 2001


doc.: IEEE 802.11-01/139

Submission

Concluding Remarks• Binary exponential backoff has been considerably criticized for its poor

throughput and delay performance inside and outside 802.• Adaptive contention has been investigated by several experts:

– L. Kleinrock, inventor of the Internet technology.– F. Tobagi, author of CSMA and its various variants.– R. Gallager, communications and networking authority.

• Performance improvement from CSMA with adaptive contention (P-DCF) has been shown to be significant in the IEEE literature.

– Increasing bandwidth demand warrants such an improvement.– QoS is better supported by an efficient protocol.

• Lessons have been learned from Ethernet and are worth learning.– CSMA/CD degrades rapidly in performance as node population increases.– CSMA without collision detection costs much more collision bandwidth, and

hence performs even worse, than CSMA/CD.– Wireline Ethernet needs to become faster and faster.– Wireless LANs have scarce spectrum resources.– IC technology is much more advanced and affordable than 1970’s when

Ethernet was first developed.

March 2001


doc.: IEEE 802.11-01/139

Submission

Sample References

Books:

1. D. Bertsekas and R. Gallager, Data Networks, 2nd ed., Prentice Hall, NJ, 1992, Chapter 4.

2. A. S. Tanenbaum, Computer Networks, 3rd ed., Prentice Hall, NJ, 1996, Chapter 4.

Papers:

1. L. Kleinrock and/or F. Tobagi's CSMA and CSMA/CD papers published between 1975-1985.

2. F. Cali, et al., “IEEE 802.11 Protocol: Design and Performance Evaluation of an Adaptive Backoff Mechanism,” IEEE J. Select. Areas Commun., vol. 8, Setp. 2000, pp. 1774-1786.

3. F. Cali, et al., “Dynamic Tuning of the IEEE 802.11 Protocol to Achieve a Theoretical Throughput Limit,” IEEE INFOCOM’98, pp. 142-149.

March 2001


doc.: IEEE 802.11-01/139

Submission

Available Technology = Enhancement ?

Degraded DCF = Enhanced DCF ??

Stacked DCF = Simplicity = QoS ???

Questions

march 2001 jin-meng ho, et al., texas instrumentsslide 1 doc.: ieee 802.11-01/139 submission...

Documents

x p2 bc

tcpp i

x p3 bc

tcpp values tcpp

submission x p1 bc

p2 idle slot

backoff counter bc

submission pdcf