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doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, QualcommSlide 1
Moderated BackoffDate: September 1, 2015
Authors:
Name Company Address Phone EmailMenzo Wentink Qualcomm Straatweg 66,
Breukelen, the Netherlands
+31-65-183-6231 [email protected]
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
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
Definition of IPT
Slide 3
TxInt 1
Int 2
Int 3
Int 4
Int 5
TxInt 1
Int 2
Int 3
TxInt 1
Int 8
IPT=8 IPT=5 IPT=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.
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
Definition of Gap
Slide 4
TXOPSTA1
TXOPSTA6
TXOPSTA3
AC_BE: 43+i*9 us
Collision
gapgap gap gap 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.
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
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
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
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
20 nodes
Pc = 38%Pc = 30%
40 nodes
*1000TXOPs20 new nodes
start up at once
Slide 8
Eponential EDCA
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
20 nodes
Pc = 38%Pc = 30%
40 nodes
*1000TXOPs20 new nodes
start up at once
Slide 9
Moderated EDCA (IPT)
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
20 nodes
Pc = 38%Pc = 30%
40 nodes
*1000TXOPs20 new nodes
start up at once
Slide 10
Moderated EDCA (Gap)
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.
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
Mixing moderated and exponential nodes
Slide 12
doc.: IEEE 802.11-10/1152r0
Submission
September 2015
Menzo Wentink, Qualcomm
Mixing moderated and exponential nodes
Slide 13
