Proposed Changes to the 802.11e D1.0 Draft

1. Proposed Changes to the 802.11e D1.0 Draft Mathilde Benveniste AT&T Labs, Research Mathilde Benveniste, AT&T Labs - Research

2. Inter-frame Spaces and Backoff Procedure Figure 49 - Some IFS Relationships AIFS[j] AIFS[i] Immediate access when Medium is free >= DIFS/AIFS[i] DIFS/AIFS Contention Window DIFS/AIFS PIFS SIFS Backoff-Window Next Frame Busy Medium Slot time Select Slot and Decrement Backoff as long Defer Access as medium is idle Earliest Tx time for top-priority ESTAs with backoff=1 -- AIFS = PIFS Earliest Tx time for STAs, or ESTAs, with backoff=1 -- AIFS = DIFS Mathilde Benveniste, AT&T Labs - Research

3. 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. Mathilde Benveniste, AT&T Labs - Research

4. 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 Mathilde Benveniste, AT&T Labs - Research

5. 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]. Mathilde Benveniste, AT&T Labs - Research

6. AIFS=DIFS Not legacy-like behavior! Figure 58.1 EDCF Timing Relationships for the Example Case in Which AIFS = DIFS Mathilde Benveniste, AT&T Labs - Research

7. DiFS/AIFS PIFS SIFS Slot Time First Backoff Slot Medium Busy D1 Rx/Tx M1 D2 D2 D2 CCAdel CCAdel CCAdel M2 M2 M2 PHYRXEND.indicate Rx/Tx Rx/Tx Rx/Tx Slot Time Time Slot Time Slot MAC Slot TxSIFS TxPIFS TxUAT First Backoff Boundaries Slot Boundary Slot Boundary Slot Boundary Slot Boundary D1 = aRxRFDelay + aRxPLCPDelay (referenced from the end of the last symbol of a frame on the medium) D2 = D1 + Air Propagation Time Rx/Tx = aRXTXTurnaroundTime (begins with a PHYTXSTART.request) M1 = M2 = aMACPrcDelay CCAdel = aCCA Time - D1 Corrections Needed in Clause 9.2.10 Remove 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. Mathilde Benveniste, AT&T Labs - Research

8. 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]. Mathilde Benveniste, AT&T Labs - Research

9. 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. Mathilde Benveniste, AT&T Labs - Research

10. 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. Mathilde Benveniste, AT&T Labs - Research

11. 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 AIFSDIFS 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 ])[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. Mathilde Benveniste, AT&T Labs - Research

12. 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. Mathilde Benveniste, AT&T Labs - Research

13. 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. Mathilde Benveniste, AT&T Labs - Research

14. 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. Mathilde Benveniste, AT&T Labs - Research

15. 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. Mathilde Benveniste, AT&T Labs - Research

16. 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. Mathilde Benveniste, AT&T Labs - Research

17. 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. Mathilde Benveniste, AT&T Labs - Research