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Date: July, 2012. Slot-based Power Save without PS-Poll. Date: 2012-07-12. Authors:. Slot-Based Power Save in [1]. The time slot duration T July be chosen to be large enough to complete PS-Poll/trigger+SIFS+DATA+SIFS+ACK (e.g. 20 ~ 40 mS) .

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slot based power save without ps poll

Date: July, 2012

Slot-based Power Save without PS-Poll

Date: 2012-07-12

Authors:

Liwen Chu Etc.

slot based power save in 1
Slot-Based Power Save in [1]
    • The time slot duration T July be chosen to be large enough to complete PS-Poll/trigger+SIFS+DATA+SIFS+ACK (e.g. 20 ~ 40 mS).
  • A STA randomly chooses a time slot to start its uplink channel access. And, an AP can assign a time slot for a STA.
      • Both randomly selecting time slot and deterministic time slot need to synchronize to slot edge.
      • But deterministic time slot has higher efficiency.
    • Each STA starts to access the channel at the assigned time slot following EDCA.
    • Issues:
      • If a STA in the BSS can not receives the Beacon frames, It July still create hidden node problem.
      • OBSS STAs can collide with the slot transmission.
      • PS-Poll creates overhead.

TIM bitmap

Total N time slots, each time slot duration = T

1

1

0

1

0

0

1

1

STA1

STA10

STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

wake up bi of ps poll sta
Wake up BI of PS-Poll STA
  • STAs operating in PS modes shall periodically listen for Beacon frames, as determined by the STA\'s ListenInterval and the ReceiveDTIMs parameter in the MLME-POWERMGT.request primitive.
    • The STA shall wake up early enough to receive the first Beacon scheduled for transmission at the time corresponding to the last TBTT plus the ListenInterval.
      • A STA doesn’t need to wake up at each TBTT, but the AP knows when a STA shall wake up.
  • A STA in PS modes July not receive DTIM Beacon if ReceiveDTIMs is 0.
  • A STA July not receive DTIM Beacon When dot11MgmtOptionFMSActivated is false.
  • A WNM sleep STA needs not wake up every DTIM interval.
  • A WNM sleep STA wakes up every SleepInterval to check the TIM.
slot based ps poll listeninterval
Slot-based PS-Poll ListenInterval
  • STAs operating in PS modes shall periodically listen for Beacon frames, as determined by the STA\'s ListenInterval/SleepInterval and the ReceiveDTIMs parameter.
  • The STA sends ListenInterval/SleepInterval to the AP through Association Response or other method.
    • The AP knows when a STA shall wake up.
  • GroupInterval (in BI) of a slot-based PS-Poll STA is the interval between two adjacent TBTT that the AP sends the TIM of the STA’s group (Group TBTT).

TIM for group n will be transmitted in the beacon.

STAj July wake up.

Slot for STAj will not be allocated.

Group n

TBTT

Group n

TBTT

Group n

TBTT

Group n

TBTT

Group n

TBTT

GroupInterval (in BI)

TBTT

TBTT

STAj ListenInterval

STAj in Group n associates with AP

With ListenInterval=3*GroupInterval

STAj shall wake up.

TIM for STAj shall be transmitted in the beacon.

Slot July be allocated to STAj.

slot based ps poll listeninterval cont d
Slot-based PS-Poll ListenInterval (Cont’d)
  • A slot is allocated to a STA in the BI whose TBTT is the last TBTT plus the ListenInterval.
    • During other BIs corresponding to a group’s TBTT, slot is not allocated to the STA. Normal PS Poll can be used.
  • The STA shall wake up early enough to receive the first Beacon scheduled for transmission at the time corresponding to the last TBTT plus the ListenInterval/SleepInterval.
    • The last TBTT is the TBTT for the group (Group TBTT) that the STA belongs to after its association plus N* ListenInterval/SleepInterval (e.g. N=1, 2, 3…).
      • The ListenInterval/SleepInterval is GroupInterval*M (e.g. M=1, 2, 3…)

TIM for group n will be transmitted in the beacon.

STAj July wake up.

Slot for STAj will not be allocated.

Group n

TBTT

Group n

TBTT

Group n

TBTT

Group n

TBTT

Group n

TBTT

GroupInterval (in BI)

TBTT

TBTT

STAj ListenInterval

STAj in Group n associates with AP

With ListenInterval=3*GroupInterval

STAj shall wake up.

TIM for STAj shall be transmitted in the beacon.

Slot July be allocated to STAj.

slot based power save without ps poll1
Slot-Based Power Save Without PS-Poll
  • When the AP knows which slot (or which slots) that a STA uses, the AP knows that the STAs will wake up at the corresponding slot (or slots).
    • The AP can transmit the buffer frame to the STA without using PS-Poll.
  • With this improvement:
    • An AP can select to send a CTS-to-Self with the Duration value covering whole Slots. Only one backoff before CTS-to-Self is required.
      • An OBSS STA will set its NAV after receiving the CTS-to-Self. Collision from OBSS can be avoided.
      • A STA in the BSS which does not receive the beacon will set its NAV after receiving the CTS-to-Self.
    • When no CTS-to-Self is used, the AP starts to access the channel at the each time slot following EDCA.
    • The AP can select one backoff before the first time slot.
    • All slots are shorter, because the backoff and PS-Poll overhead is eliminated.

1

1

1

1

1

1

1

1

Total N time slots, each time slot duration = T

TIM bitmap

0

1

1

0

STA1

STA12

STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

slot based power save without ps poll cont d
Slot-Based Power Save Without PS-Poll (Cont’d)
  • When some STAs in power save mode do not receive frames correctly, then:
    • After finishing the total time slot access, the AP July transmit another TIM frame to start another slot based access.

Beacon

TIM frame

1

1

1

1

1

1

1

1

TIM bitmap

0

1

1

0

STA1

STA12

STA 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

STA 1, 3, 6, 8

The AP does not receives ACKs from STA 1, 3, 6, 8.

Total N=10 time slots, each time slot duration = T

reference
Reference

[1] 802.11-12/606r0, Uplink Channel Access.

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