May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 1

doc.: IEEE 802.11-01/243r2

Submission

Proposed Changes to the 802.11e D1.0 Draft

Mathilde BenvenisteAT&T Labs, Research

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 2

doc.: IEEE 802.11-01/243r2

Submission

Inter-frame Spaces and Backoff Procedure

Figure 49 - Some IFS Relationships

AIFS[j]

AIFS[i]

DIFS/AIFSContention Window

Slot time

Busy Medium

Defer Access

Next Frame

Select Slot and Decrement Backoff as long

SIFS

PIFSDIFS/AIFS

Immediate access when

Medium is free >= DIFS/AIFS[i]

as medium is idle

Backoff-Window

Earliest Tx time for STAs, or ESTAs, with backoff=1 -- AIFS = DIFS

Earliest Tx time for top-priority ESTAs with backoff=1 -- AIFS = PIFS

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 3

doc.: IEEE 802.11-01/243r2

Submission

Priority higher than legacy AIFS=PIFS

Using AIFS[i]=PIFS is feasible because • The backoff time at the end of a busy period is 1 or greater for all STAs with

interrupted backoff procedure. • By requiring the random backoff drawn to be 1 or greater for ESTAs with

AIFS[i]=PIFS, transmission will occur the earliest at DIFS.

There are no collisions with PC or HC.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 4

doc.: IEEE 802.11-01/243r2

Submission

AIFS properties

Property 1 (Backward compatibility): There must exist one EDCF traffic category that describes the behavior of legacy STAs

Property 2: For legacy-equivalent ESTAs, AIFS has the role of DIFS.

According to Note in 9.2.10, which reads: “Note:the default value for aAIFS [TC] is 2 for each Traffic Class (TC). Therefore the

default settings for TxAIFS [TC ] are equivalent to DIFS for each TC.”

the following holds:

The default AIFS=DIFS, which shall correspond to the TC equivalent to legacy

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 5

doc.: IEEE 802.11-01/243r2

Submission

Timing Relationships

Clause 9.2.3.3 defines DIFS as follows:

“A STA using the DCF shall be allowed to transmit if its carrier sense mechanism (see 9.2.1)determines that the medium is idle at the TxDIFS slot boundary after a correctly-received frame,and the backoff time for that TC has expired.”

Clause 9.2.3.4 defines AIFS as follows:

“A STA using the EDCF shall be allowed a transmit opportunity (TxOP) for a particular Traffic Class (TC)if its carrier sense mechanism (see 9.2.1)determines that the medium is idle at the TxAIFS [TC ] slot boundary (see 9.2.10) after a correctly-received frame,and the backoff time for that TC has expired.”

Given the above definitions,

the timing relationships in Clause 9.2.10 (Figure 58.1) are incorrect by Property 2

[which requires that ESTAs with AIFS=DIFS and legacy STAs have identical backoff procedure].

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 6

doc.: IEEE 802.11-01/243r2

Submission

AIFS=DIFS Not legacy-like behavior!

Figure 58.1 EDCF Timing Relationships for the Example Case in Which AIFS = DIFS

PHYRXEND.indica te

Rx/Tx

Medium Busy

D1

M1 D2

CCAdel

M2

Rx/Tx

D2

CCAdel

M2

Rx/Tx

D2

CCAdel

M2

Rx/Tx

SIFS

PIFS

DIFS

Slot Time Slot Time Slot Time

PHYRXEND.indica te

Rx/Tx

Medium Busy

D1

M1 D2

CCAdel

M2

Rx/Tx

D2

CCAdel

M2

Rx/Tx

D2

CCAdel

M2

Rx/Tx

SIFS

PIFS

DIFS

Slot Time Slot Time Slot Time

TxSIFSSlot Boundary

TxPIFSSlot Boundary

TxDIFSSlot Boundary

Fir st BackoffSlot Boundary

Slot Time

D1 = aRxRFDelay + aRxPLCPDelay (referenced from the end of the last symbol of a frame on the medium)D2 = D1 + Air Propagation TimeRx/Tx = aRXTXTurnaroundTime (begins with a PHYTXSTART.request)M1 = M2 = aMACPrcDelayCCAdel = aCCATime - D1

MA

ced from the end of the last symbol of a frame on the medium)D2 = D1 + Air Propagation TimeRx/Tx = aRXTXTurnaroundTime (begins with a PHYTXSTART.request)M1 = M2 = aMACPrcDelayCCAdel = aCCATime - D1

MAC SlotBoundaries

First Backoff SlotFirst Backoff Slot Second Backoff Slot

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 7

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 9.2.10Remove Figure 58.1

Modify Figure 58 as follows:

Figure 58 -- DCF Timing Relationship and EDCF Timing Relationships for the Example Case in Which AIFS = DIFS

______________________

Changes to existing text in current draft are shown underlined and in red for additions and (italicized in blue and in parentheses) for deletions.

PHYRXEND.indicate

Rx/TxMedium Busy

D1

M1 D2CCAdel

M2

Rx/Tx

D2CCAdel

M2

Rx/Tx

D2CCAdel

M2

Rx/Tx

SIFSPIFS

DiFS/AIFS

Slot Time Slot Time Slot Time

First Backoff Slot

TxSIFSSlot Boundary

TxPIFSSlot Boundary

TxUATSlot Boundary

First Backoff Slot Boundary

Slot Time

D1 = aRxRFDelay + aRxPLCPDelay (referenced from the end of the last symbol of a frame on the medium)D2 = D1 + Air Propagation TimeRx/Tx = aRXTXTurnaroundTime (begins with a PHYTXSTART.request)M1 = M2 = aMACPrcDelayCCAdel = aCCA Time - D1

MAC SlotBoundaries

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 8

doc.: IEEE 802.11-01/243r2

Submission

Backoff Procedure

Clause 9.2.5.2 describes the backoff procedure as follows:

“To begin the backoff procedure, the STA shall set its Backoff Timer to a random backoff time using the equation in 9.2.4. All backoff slots occur following a DIFS period during which the medium is determined to be idle for the duration of the DIFS period, or following an EIFS period during which the medium is determined to be idle for the duration of the EIFS period following detection of a frame that was not received correctly, or for EDCF stations for each queue[i] during and following a AIFS[i] period during which the medium is determined to be idle for the duration of the AIFS[i] period, the first slot time occurring during the last slot interval of the AIFS[i] period.”

The above is inconsistent with Property 2 [which requires that ESTAs with AIFS=DIFS and legacy STAs have

identical backoff procedure].

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 9

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 9.2.5.2

Modify text as follows:

To begin the backoff procedure, the STA shall set its Backoff Timer to a random backoff time using the equation in 9.2.4. All backoff slots occur following a DIFS period during which the medium is determined to be idle for the duration of the DIFS period,or following an EIFS period during which the medium is determined to be idle for the duration of the EIFS period following detection of a frame that was not received correctly,or for EDCF stations for each queue [i ] (during and) following an AIFS [i ] period during which the medium is determined to be idle for the duration of the AIFS [i ] period (the first slot time occurring during the last slot interval of the AIFS[i] period).

______________________

Changes to existing text in current draft are shown underlined and in red for additions and (italicized in blue and in parentheses) for deletions.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 10

doc.: IEEE 802.11-01/243r2

Submission

Random Backoff Time

Clause 9.2.4 reads:

“Backoff Time [i ] =Random(i) x aSlotTime

Where:

Random(i)=Pseudo random integer drawn from a uniform distribution over the interval [1,CW[i]+1],

where CW [i ]is an integer within the range of values of the MIB attributes aCWmin [i ]and aCWmax (or optionally aCWmax [i] if available)),aCWmin [i ] <=CW [i ] <=aCWmax. “

The above is inconsistent with Property 1 [which requires backward compatibility]

As written, legacy STAs will perform better than ESTAs with traffic of equivalent priority to

legacy as the random backoff time of the ESTAs would be greater by 1.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 11

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 9.2.4

1 must be added only to the random backoff of ESTAs with AIFS[i]=PIFS (the top priority traffic categories). ESTAs with AIFSDIFS may draw random backoff times as low as 0

Modify text as follows:

Backoff Time [i ] =[Random(i)+X]x aSlotTime

Where:

Random(i)=Pseudo random integer drawn from a uniform distribution over the interval ([1,CW [i ][1,CW [i ]+1 ])+1 ]) [0, CW[i]],

where CW [i ]is an integer within the range of values of the MIB attributes aCWmin [i ]and aCWmax (or optionally aCWmax [i] if available)),aCWmin [i ] <=CW [i ] <=aCWmax. and

X = 0 for all legacy STAs and each ESTA traffic category with a value of AIFS>PIFS.

X = 1 for each ESTA traffic category with a value of AIFS=PIFS.

______________________

Changes to existing text in current draft are shown underlined and in red for additions and (italicized in blue and in parentheses) for deletions.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 12

doc.: IEEE 802.11-01/243r2

Submission

Simplify Calculation of Retry Window Size

Clause 9.2.4 Contention Window Persistence Factor (P 60, L 38-45)

Background

The new contention window size CW[i] used upon transmission retry, following a failure, is calculated so that computationally convenient values are always used. The new window size is calculated from the previous window size by multiplication with the persistence factor PF[i] (see Clause 7.3.2.14). This factor is used in EDCF because the AP is able to adapt CWmin to current traffic/contention conditions; doubling the backoff window upon each retransmission attempt [i.e., always setting PF=2] would cause the backoff window to grow too fast, with long delays and delay jitter as the result. Lower values of PF are more appropriate with traffic-adaptive window sizing. [See Doc 144r2.]

Problem

As written in the draft, the calculation is not as simple as possible.

Rationale for change

A simpler computation can achieve the same result.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 13

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 9.2.4

Modify text as follows:

(To compute the new CW[i] value, denoted CWnew[i], from the old CW[i] value, denoted CWold[i] ) In the event of a collision, an EDCF station shall choose a value of CW[i] that is set to the convenient resolution greater than or equal to CWnew[i], which is computed from the old CWnew[i], denoted CWold [i ] (CWnew [i] which meets) according to the following criterion:

CWnew[i] (>=) = ((CWold[i]+1)*PF)– 1

Where the persistence factor, PF, is computed using the following procedure:

The WPFactor[i] (Contention Window Persistence Factor)corresponding to each queue[i] is distributed in the EDCF parameter set element described in 7.3.2.14. Each CWPFactor[i] field is one octet in length and indicates a scaling factor in units of 1/16 ths. PF is CWPFactor[i] divided by 16 (and optionally rounded up to the nearest convenient fractional resolution).

______________________

Changes to existing text in current draft are shown underlined and in red for additions and (italicized in blue and in parentheses) for deletions.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 14

doc.: IEEE 802.11-01/243r2

Submission

Global setting of MSDULifetime[i] values

Clause 7.3.2.14 QoS Parameter Set Element (P 45, L 15)

Background

The MSDULifetime[TC] is used in 9.2.5.3 to discard aged MSDUs that are obsolete because of excessive delay.

It specifies the maximum time allowed to transmit an MSDU for a traffic category TC, once it enters the MAC.

This provides a quick and simple mechanism for reduction of contention for the medium by clearing the obsolete packets before transmission is attempted.

Problem

As written In the present draft, the MSDULifetime[TC] values are set in the MIB, independently of all other class-differentiating parameters, which can be updated by the AP. Furthermore, these values may alter the Tspec specified value of TxLifetime of a TC that has a Tspec.

Rationale for change

The change is necessary in order to enable the AP to provide a consistent specification of all the class-differentiating parameters, and, when a TC has a Tspec, to give precedence to the TxLifetime from the Tspec.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 15

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 7.3.2.14

Create a new element, known as the QoS Traffic Class Parameters element. This element is added to the allowable elements of (Re)Association Response and as the frame body of a new QoS Management Action Request, known as Set QoS Traffic Class Parameters Request.

Move the AIFS[TC] values field and CWPFactor[TC] values field from the QoS Parameter Set element into this new element and add a new field, known as the aMSDULifetime[TC] field, which can be used by the AP to update the MIB value of MSDULifetime[TC]. If a TC has a Tspec which specifies an alternate TxLifetime, the value from the TSpec takes precedence over aMSDULifetime.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 16

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 9.2.5.3

P 64, L 6 (Needed for completeness.)

Insert text:

In an EDCF STA an RTS is retried when unsuccessful until the associated MSDU timer exceeds the aMSDULifetime [i] for traffic category of the associated MSDU.

May 2001

Mathilde Benveniste, AT&T Labs - Research

Slide 17

doc.: IEEE 802.11-01/243r2

Submission

Corrections Needed in Clause 9.2.5.3 (Cont.)

P 64, L 21 (Needed for completeness.)

Insert text:

An EDCF STA, after transmitting a frame that requires acknowledgment, shall perform the ACK procedure, as defined in 9.2.8. All retransmission attempts for an MSDU or MMPDU that has failed the ACK procedure one or more times shall be made with the Retry field set to 1 in the Data or Management type frame. Retries for failed transmission attempts shall continue until the associated MSDU timer exceeds the aMSDULifetime [i] for its traffic category.