1. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, QualcommSlide 1 Moderated Backoff Date: September 1, 2015 Authors: NameCompanyAddressPhoneEmail Menzo WentinkQualcommStraatweg 66, Breukelen, the Netherlands +31-65-183-6231mwentink@qti.qua lcomm.com

2. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Moderated Backoff Moderated backoff adapts CW based on an observed parameter, through a calibration curve –The calibration curve ensures that channel access is backwards compatible with EDCA exponential backoff –Examples of observed parameters with approximate calibration curves are Pc (collision rate) –target CW = 7.28*e^(4.29*Pc) IPT (interruptions per Tx) –target CW = 13.9 + 3.2*IPT -0.013*IPT2 Gap (time between TXOPs) –target CW = 8.34/(0.0223*gap-1) Slide 2

3. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Definition of IPT Slide 3 Tx Int 1 Int 2 Int 3 Int 4 Int 5 Tx Int 1 Int 2 Int 3 Tx Int 1 Int 8 IPT=8 IPT=5IPT=3 A backoff interruption is caused by a TXOP (or collision) by another node. Basically, a CCA idle-to-busy event after the CCA has been idle longer than SIFS. The IPT is the number of times a backoff is interrupted by another transmission on the channel. The average IPT in this example is (8+5+3)/3 = 5.3. When the node transmits, it determines how many times its backoff was interrupted.

4. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Definition of Gap Slide 4 TXOP STA1 TXOP STA6 TXOP STA3 AC_BE: 43+i*9 us Collisio n gap The gap is the duration of the backoff between subsequent channel accesses on the channel (TXOP or collision), including the AIFS. Basically, the duration of idle CCA times that are longer than SIFS.

5. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Convergence Convergence happens because the observed parameter will not be on the calibration curve when CW does not have the right value for the number of nodes –see illustration on next slide (for IPT, but gap and Pc work the same) Slide 5

6. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm 1.if CW does not have the right value for the number of nodes, the observed IPT is not on the red calibration curve 2.CW is adapted to the target value on the calibration curve 3.Repeat Equilibrium is reached when CW has the right value for the number of nodes The blue curve shows the observed IPT at a given CW, for a given number of nodes (10 in this example) The red curve is the calibration curve, which defines the target CW for a given measured IPT at the node Convergence for IPT Slide 6

7. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Advantages of moderated backoff Moderated backoff has much lower jitter than exponential backoff, because the CWs are more or less the same at each contending device –as opposed to exponential backoff, where the CWs vary wildly between devices Moderated backoff does not have to be based on collisions –can also be based on IPT or gap –packet errors can appear as collisions and cause unnecessary CW increase –gaps occur much more frequently than collisions at the node, so the CW can be tuned more often and diverge even less across nodes (does not apply for IPT or Pc) –gaps are visible to all nodes, while collisions are only visible to the colliding nodes (does not apply for IPT or Pc) Slide 7

8. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm 20 nodes Pc = 38%Pc = 30% 40 nodes *1000 TXOPs 20 new nodes start up at once Slide 8 Eponential EDCA

9. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm 20 nodes Pc = 38%Pc = 30% 40 nodes *1000 TXOPs 20 new nodes start up at once Slide 9 Moderated EDCA (IPT)

10. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm 20 nodes Pc = 38%Pc = 30% 40 nodes *1000 TXOPs 20 new nodes start up at once Slide 10 Moderated EDCA (Gap)

11. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Mixing moderated and exponential nodes Slide 11 20 nodes total The optimal success rate for 20 nodes is 1/20 = 0.05, but there is a loss due to collisions, which causes the practical success rate to be in the order of 0.035. Mixing in MB nodes does not materially affect the success rate at EB nodes, provided that the right calibration curve is used.

12. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Mixing moderated and exponential nodes Slide 12

13. doc.: IEEE 802.11-10/1152r0 Submission September 2015 Menzo Wentink, Qualcomm Mixing moderated and exponential nodes Slide 13