1. Moderated Backoff Date: September 1, 2015 Authors: Name Company
July 2007
doc.: IEEE yy/xxxxr0
Moderated Backoff
Date: September 1, 2015
Authors:
Name
Company
Address
Phone
Menzo Wentink
Qualcomm
Straatweg 66, Breukelen, the Netherlands
Menzo Wentink, Conexant

2. July 2007
doc.: IEEE yy/xxxxr0
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 = *IPT *IPT2
Gap (time between TXOPs)
target CW = 8.34/(0.0223*gap-1)
Slide 2
Page 2
Menzo Wentink, Conexant

3. Definition of IPT July 2007 doc.: IEEE 802.11-yy/xxxxr0
When the node transmits, it determines how many times its backoff was interrupted.
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.
Int 8 Tx
Int 1
Int 2
Int 3
Int 4
Int 5 Tx
Int 1
Int 2
Int 3 Tx
Int 1
IPT=8
IPT=5
IPT=3
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.
Menzo Wentink, Conexant

4. Definition of Gap July 2007 doc.: IEEE 802.11-yy/xxxxr0 gap gap gap
TXOP
STA1
TXOP
STA6
TXOP
STA3
Collision
AC_BE: 43+i*9 us
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.
Menzo Wentink, Conexant

5. July 2007
doc.: IEEE yy/xxxxr0
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
Page 5
Menzo Wentink, Conexant

6. Convergence for IPT
if CW does not have the right value for the number of nodes, the observed IPT is not on the red calibration curve
CW is adapted to the target value on the calibration curve
Repeat
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
Equilibrium is reached when CW has the right value for the number of nodes

7. Advantages of moderated backoff
July 2007
doc.: IEEE yy/xxxxr0
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
Page 7
Menzo Wentink, Conexant

8. 20 new nodes start up at once
Eponential EDCA
20 nodes
40 nodes
*1000
TXOPs
Pc = 30%
20 new nodes start up at once
Pc = 38%

9. 20 new nodes start up at once
Moderated EDCA (IPT)
20 nodes
40 nodes
*1000
TXOPs
Pc = 30%
20 new nodes start up at once
Pc = 38%

10. 20 new nodes start up at once
Moderated EDCA (Gap)
20 nodes
40 nodes
*1000
TXOPs
Pc = 30%
20 new nodes start up at once
Pc = 38%

11. Mixing moderated and exponential nodes
July 2007
doc.: IEEE yy/xxxxr0
Mixing moderated and exponential nodes
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 Mixing in MB nodes does not materially affect the success rate at EB nodes, provided that the right calibration curve is used.
Menzo Wentink, Conexant

12. Mixing moderated and exponential nodes

13. Mixing moderated and exponential nodes