Doc.: IEEE 802.11-01/144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 1 An E-DCF Proposal Using TCMA Mathilde Benveniste AT&T Labs,

March 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 1

doc.: IEEE 802.11-01/144

Submission

An E-DCF Proposal Using TCMA

Mathilde BenvenisteAT&T Labs, Research

Relevant submissions: IEEE 802.11-00/375 (.ppt and .doc); -00/456; -00/457; -01/002; -01/004; -01/019; -01/117

March 2001


Slide 2

doc.: IEEE 802.11-01/144

Submission

EDCF proposed features

Basic Contention Resolution: BackoffA backoff counter drawn from a random distribution [0, CWsize]

Class Differentiation

By Arbitration Time, UAT The waiting time to start countdown after a transmission (DIFS for legacy)

By Retrial Persistence Factor, CWPFactor The coefficient multiplying the contention window size on retransmission

By Transmit Lifetime Limit, TLT The maximum time allowed to transmit a packet

[When legacy stations are present, for priorities above those assigned to legacy]

By Contention Window size, CWSize

Adaptation to TrafficAP updates CWSize as neededEnable finer CWSize adjustment upon retrial

Elimination of stale packetsPackets can be discarded based on age limit that depends on class

March 2001


Slide 3

doc.: IEEE 802.11-01/144

Submission

EDCF Parameter Set element

CPM urgency class index for “contention” priority

TxOp Limit limit on transmission duration (μsec)

UCIi contains parameters defining Urgency Class i

Element ID

Length

CPM UCI0 UCI1 UCI2 UCI3TxOp Limit

Octets: 1 1 1 242

ASCi CWPFactori CWSizei TLTi

1 1 2 2Octets:

ASCi arbitration slot count

CWPFactori contention window persistence factor (1/16)

CWSizei contention window size

TLTi transmit lifetime (TU)

March 2001


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Submission

Priority to Urgency Class mapping

There are 4 urgency classes, 0, …, 3 [another number can be specified, if desired]

IEEE 802.11E allows ten values: the integers between and including 0 and 7 as well as the values allowed by IEEE 802.11.

IEEE 802.11 allows two values: “Contention” or “ContentionFree”.

CPM, the urgency class index for priority “Contention”, is specified in the

EDCF Parameter Set Element.

Priority Value Urgency Class

1 0

2 0

0 (Default) 1

3 1

4 2

5 2

6 3

7 3

Contention CPM

ContentionFree Not mapped since f rames withthis priority are not

transmitted during thecontention period.

March 2001


Slide 5

doc.: IEEE 802.11-01/144

Submission

ExampleThe backoff value m at T0 is equal to 1 for all nodes; m(A)= m(B)=m(C)=1

Transmit urgency ranking: (C, B, A); hence, UAT(A) >UAT(B)>UAT(C)

Differentiation by the Arbitration Time

TCMA Protocol Backbone

• Each urgency class is assigned a different urgency arbitration time (UAT) whose length decreases with increasing urgency.

• Arbitration Time = interval that the channel must be sensed idle by a node before decreasing its backoff counter.

For stations with classification i= 0,1,...

UATi = aSIFSTime + aASCi x aSlotTime

aASCi = arbitration slot count for class iTime

Node C

Node B

Node A transmission

time slot

UAT

End of lasttransmission

T0

March 2001


Slide 6

doc.: IEEE 802.11-01/144

Submission

Backward Compatibility

In the presence of legacy stations, there can be urgency classes above legacy.UATs are selected as follows:

For classes with urgency below legacy aASCi >=2 (UAT>=DIFS ), and X=0

For classes with urgency above legacy aASCi = 1 (UAT=PIFS), and X=1

Since all residual backoff values are 1 or greater, transmission waiting time >= PIFS+1 x aSlotTime = DIFS

no collisions with PCF or HCF

Multiple classes with urgency above legacy are obtained with different CWSize i values

Backoff Time = (Random() + X) aSlotTime

where X = 0 for all STAs and ESTAs with ASCi>1

X = 1 for ESTAs with ASCi =1

March 2001


Slide 7

doc.: IEEE 802.11-01/144

Submission

Slow Adaptation to Traffic (SAT)

Using CWSizei in the EDCF element, the AP may adjust the contention window in response to traffic conditions.

Upon joining a BSS or IBSS, or whenever they detect any change in the advertised values of CWSizei, ESTAs set their CWSizei to the value in the EDCF Element.

The new window is used when a new packet arrives or upon retrial of a failed transmission.

Retrial persistence factorThe new CW used for retrial of a failed transmission, is obtained by multiplication

with the persistence factor aCWPFactori .

new CWi = ((current CWi + 1) x (aCWPFactori /16) - 1

SAT, which chooses a window appropriate for current traffic, obviates the need for large retry adjustments.

High urgency classes benefit from smaller aCWPFactori values.

One can still use the aCWPFactori value effective now, 2 x 16.

March 2001


Slide 8

doc.: IEEE 802.11-01/144

Submission

Removal of Stale Packets

Time-sensitive packets are obsolete if delayed excessivelyDiscarding excessively aged packets relieves congestion without impact on QoSThe TLTi (Transmit Lifetime) limit, which is the maximum number of time units (TUs)

allowed to transmit an MSDU, is differentiated by urgency class i. The timer is started when the MSDU enters the MAC.TLTi = 0 indicates no restriction

March 2001


Slide 9

doc.: IEEE 802.11-01/144

Submission

Competing Traffic Streams at a Node

Packets generated by stations with multiple traffic types will not be disadvantaged Multiple backoff timers shall be maintained for each class at a node, each adhering

to backoff principles consistent with that class

In case of a tie, the higher urgency packet is selected. The packet not selected

starts a new backoff counter.

Packet Stream to Node A

Packet Stream to Node B

Access Buffer

Packet Stream to Node C

Contention for access

CHANNELTRANSMISSIONS

March 2001


Slide 10

doc.: IEEE 802.11-01/144

Submission

Simulation Study Overview

Purpose: Compare performance of E-DCF to DCF

Two traffic patterns were considered: – non-bursty (Poisson arrivals) and– mixed (fixed and ON/OFF)

Features included in simulations• Differentiation by Urgency Arbitration Time (UAT)• … by contention window (CWSize)• … by retrial persistence factor (CWPFactor)

Features not included in simulations• ‘Slow’ adaptation to traffic (SAT)• Differentiation by Transmit Lifetime limit (SAT)

March 2001


Slide 11

doc.: IEEE 802.11-01/144

Submission

Scenario I: Network Configuration

• Traffic• 30 stations generate 30 bi-

directional streams;• stream load split 1-to-2

between two directions

• WLAN Parameters• DS, 11 Mbps channel• buffer size=2.0 Mbits; no

fragmentation• RTS/CTS suppressed; max retry

limit=7

March 2001


Slide 12

doc.: IEEE 802.11-01/144

Submission

Scenario I: Traffic Poisson arrivals

Packet Size 1504 Bytes Load Load LoadStream Priority Start Traffic IA BPS BPS BPS

Class Time Multiplier Time Seg1 Seg1 Seg11 Top 0 1 0.06016 200000 200000 2000002 Top 0 2 0.03008 400000 400000 4000003 Top 0 1 0.06016 200000 200000 2000004 Top 0 2 0.03008 400000 400000 4000005 Top 0 1 0.06016 200000 200000 2000006 Top 0 2 0.03008 400000 400000 4000007 Medium 0 1 0.06016 200000 200000 2000008 Medium 0 2 0.03008 400000 400000 4000009 Medium 0 1 0.06016 200000 200000 20000010 Medium 0 2 0.03008 400000 400000 40000011 Medium 30 1 0.06016 200000 20000012 Medium 30 2 0.03008 400000 40000013 Low 30 1 0.06016 200000 20000014 Low 30 2 0.03008 400000 40000015 Low 30 1 0.06016 200000 20000016 Low 30 2 0.03008 400000 40000017 Low 30 1 0.06016 200000 20000018 Low 30 2 0.03008 400000 40000019 Low 30 1 0.06016 200000 20000020 Low 30 2 0.03008 400000 40000021 Top 60 1 0.06016 20000022 Top 60 2 0.03008 40000023 Top 60 1 0.06016 20000024 Top 60 2 0.03008 40000025 Medium 60 1 0.06016 20000026 Medium 60 2 0.03008 40000027 Medium 60 1 0.06016 20000028 Medium 60 2 0.03008 40000029 Low 60 1 0.06016 20000030 Low 60 2 0.03008 400000

3000000 6000000 9000000

Star

t at T

=0

Star

t at T

=30

Star

t at T

=60

6 Mbps

9 Mbps

3 Mbps

•Frames size -- 1504 bytes•Includes 192 microseconds of PHY overhead•Independent packet arrivals •Exponential inter-arrival times

March 2001


Slide 13

doc.: IEEE 802.11-01/144

Submission

Scenario I: Class Description

ClassI ndex

UAT I nitial BackoffRange

PersistenceFactor*

DCF-- All SI FS+2*TimeSlot [0, 31] 2TCMA-- 0 SI FS +TimeSlot [1, 15] 1.5-- 1 SI FS+2*TimeSlot [0,31] 2-- 2 SI FS+3*TimeSlot [0,31] 2

*Factor CWPFactor multiplies CWSize before retrial

March 2001


Slide 14

doc.: IEEE 802.11-01/144

Submission

DC

FTC

MA

Throughput(bits/sec)

Dropped Packets(bits/sec)

Delay(sec)

Scenario I: Simulation Results -- TCMA vs DCF

March 2001


Slide 15

doc.: IEEE 802.11-01/144

Submission

Scenario I: Simulation Results --TCMA CWPFactors

CW

PFa

ctor0

=2

CW

PFa

ctor0

=1.

5

Throughput(bits/sec)

Class 0 Delay(sec)

March 2001


Slide 16

doc.: IEEE 802.11-01/144

Submission

Scenario II: Traffic -- 1 Low Priority and 17 Voice calls

Call Duration0 20 40 60 80 100 120 140

1

3

5

7

9

11

13

15

17

Cal

l

Time (sec)TrafficVoice - 285 Kbps per callFixed arrivals Frame size* - 356 bytes

Low Priority - 3,318 KbpsFrame size* - 1,728 bytesFixed arrivals 12 ms ON/88 ms OFF

WLAN ParametersDS, 11 Mbps channelbuffer size=2.024 Mbits; no fragmentationRTS/CTS suppressed; max retry limit=7

* Includes 192 microsec PHY overhead

March 2001


Slide 17

doc.: IEEE 802.11-01/144

Submission

Scenario II: Offered LoadL

oa

d (

bit

s/s

ec)

Total

Voice

Video

Call Volume vs Time - No PHY Overhead

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120 140

Time (sec)

Lo

ad

(b

its

/se

c)

Total

Voice

Low

Call Volume vs Time - With PHY Overhead

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120 140

Time (sec)

Lo

ad

(b

its

/se

c)

March 2001


Slide 18

doc.: IEEE 802.11-01/144

Submission

Scenario II: Simulation Results -- TCMA vs DCF

Average End to End Delay - DCF

0

1

2

3

4

5

6

7

0 50 100 150

Time (sec)

En

d t

o E

nd

Del

ay (

sec)

Average End to End Delay - TCMA

0

1

2

3

4

5

6

7

8

9

0 50 100 150

Time (sec)

En

d t

o E

nd

Del

ay (

sec)

Voice Calls

Low Priority

Voice Calls

Low Priority

March 2001


Slide 19

doc.: IEEE 802.11-01/144

Submission

Conclusions

• TCMA differentiates effectively between classes of different priority– Delay is negligible for top priority traffic; and – remains small under overload conditions

• TCMA achieves greater total throughput – UATs prevent collisions by packets of different priorities in

congestion

• TCMA can coexist with legacy stations

Documents

Doc.: IEEE 802.11-01/144 Submission March 2001 Mathilde Benveniste, AT&T Labs - ResearchSlide 1 An E-DCF Proposal Using TCMA Mathilde Benveniste AT&T Labs,