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)
March 2001
Jin-Meng Ho, et al., Texas InstrumentsSlide 2
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 3
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 4
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 5
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 6
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 7
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 8
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 9
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 10
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 11
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 12
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 13
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 14
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
Collisioninvolving frame 1
Access Delay 1
Channel busy Collisioninvolving frame 2
Access Delay 2 Access Delay Jitter
Backoff Time
Backoff Time
Backoff Jitter
Channel busy Transmissionof another frame
Transmissionof frame 1 only
Collisioninvolving frame 1
Access Delay 1
Channel busy Transmissionof another frame
Access Delay 2 Access Delay Jitter
Backoff Time
Backoff Time
Backoff Jitter
Transmissionof frame 2 only
Delay Jitter < Backoff Jitter
Delay Jitter > Backoff Jitter
Transmissionof another frame
Transmissionof frame 2 only
• 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
Jin-Meng Ho, et al., Texas InstrumentsSlide 15
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 16
doc.: IEEE 802.11-01/139
Submission
Aggregate data arrival rate = 6 Mbps
0
2
4
6
8
10
12
14
16
18
20
0 1 2 3 4 5 6 7 8 9 10
Simulation Time (sec)
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 17
doc.: IEEE 802.11-01/139
Submission
Aggregate data arrival rate = 6 Mbps
1
1.5
2
2.5
3
3.5
4
2 3 4 5 6 7 8 9 10
Simulation Time (sec)
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 18
doc.: IEEE 802.11-01/139
Submission
Aggregate data arrival rate = 6 Mbps
0
5
10
15
20
25
30
35
40
45
50
0 1 2 3 4 5 6 7 8 9 10
Simulation Time (sec)
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 19
doc.: IEEE 802.11-01/139
Submission
Average Delays for 4-Priority V-DCF
Aggregate data arrival rate = 6 Mbps
0
2
4
6
8
10
12
14
16
18
20
2 3 4 5 6 7 8 9 10
Simulation Time (sec)
Del
ay (
ms)
Legacy DCF
V-DCF: CWmin = 63
V-DCF: CWmin = 31
V-DCF: CWmin =15
V-DCF: CWmin = 7
March 2001
Jin-Meng Ho, et al., Texas InstrumentsSlide 20
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 21
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 22
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 23
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 24
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 25
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
Jin-Meng Ho, et al., Texas InstrumentsSlide 26
doc.: IEEE 802.11-01/139
Submission
Available Technology = Enhancement ?
Degraded DCF = Enhanced DCF ??
Stacked DCF = Simplicity = QoS ???
Questions
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