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Moderated Backoff

Date: September 1, 2015. Moderated Backoff. Authors:. 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

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Moderated Backoff

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  1. Date: September 1, 2015 Moderated Backoff Authors:

  2. 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. Definition of IPT 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=5 IPT=3 IPT=8 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.

  4. Definition of Gap gap gap 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.

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

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

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

  11. 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 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. Mixing moderated and exponential nodes

  13. Mixing moderated and exponential nodes

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