Peer Traffic Indication enhancements

1. Peer Traffic Indication enhancements Authors: Date: 2008-07-15 Jakub Majkowski (Nokia)

2. Abstract The presentation describes mechanisms to enhance the power save principles by using Peer Traffic Indication frame. The enhancements introduce capability to detect ‘out-dated’ PTI frame and define a simple activation mechanism for the Direct Link. Jakub Majkowski (Nokia)

3. AP Association Association STA2 STA1 Direct Link Power management modes in DLS AP operates always in active mode AP operates always in active mode • A STA defines a power management mode for every peer STA and to AP and obtains the power management mode used by its peer STA in their direct link • For instance STA2 operates in different Power Management mode for a direct link and for a AP • STA2 in power save for STA1 may trigger a service period over direct link at any time • The triggering will initialize a service period which is used for data transmission • STA1 may transmit frames directly to STA2 only during a service period • Service period handling over direct link is not studied in this presentation • STA 1 may use AP as a forwarder to deliver frames to STA2: • STA1 may transmit frames through the AP • or STA1 may transmit a Peer Traffic Indication (PTI) frame indicating that it has buffered frames and expects triggering over direct link from STA2 STA1 in Active mode to AP STA2 in active mode to AP STA1 in Active mode to STA2 STA2 in power save mode to STA1 Jakub Majkowski (Nokia)

4. AP 1. 3. STA2 Power save mode STA1 Active mode 2. Service period 4. Service period Problem of PTI frame use • STA1 transmits a PTI frame through AP to peer STA2 • STA2 triggers a service period with the STA1 before it gets the PTI indication from the AP. Service period between STA1 and STA2 is performed and terminated • The STA2 gets indication of buffered traffic in AP through the Beacon and fetches the buffered PTI frame • Because of the received “out-dated" PTI frame, the STA2 triggers a unnecessarily a service period with STA1, while the traffic is already transmitted in previous service period This is likely scenario, because Beacon transmission periodicity is usually ~100ms and not all Beacons are necessarily received by the STA in power save mode. Jakub Majkowski (Nokia)

5. Solution, detection of the out-dated PTIs • A PTI frame needs an information element which enables the receiver of the PTI frame to detect if the PTI frame is out-dated • Detection may be done by including a Sequence Number of the last transmitted frame in the Direct Link of the highest AC which backlogged field was set to non-zero value to PTI frame • If target STA detects that PTI frame is out-dated, it may ignore the frame • This eliminates unnecessary service periods and results in better power save Jakub Majkowski (Nokia)

6. Sending the Peer Traffic Indication (PTI) frame • PTI frame is used to indicate that a STA has buffered frames for direct link which have been buffered over a preconfigured time limit • This operation is good to have as recovery of the erroneous operation, but it may result to unnecessary delays in PTI frames usage • STA may use PTI frame to indicate to the peer STA that it is in awake state, has buffered frames and may be triggered by the peer STA • PTI transmitter remains in awake state and waits for a trigger frame • On demand created PTI frames enable data delivery before QoS gets even worse (larger delay and frame loss) Jakub Majkowski (Nokia)

7. AP Association Association STA2 Power save mode STA1 Active mode Direct Link Two STAs supporting Peer U-ASPD • According to current draft even though two station indicate support for Peer U-APSD only one peer STA at a time may operate in power save mode • The selection of the operation in power save mode is based on the competition: • Slower STA to signals operation in power save mode is forcedto operate in active mode • This creates a race condition: battery powered STA wants only to operate in power save mode • If it temporarily changes itself to active mode, it may be forced to operate in active mode, i.e. its peer STA is able to signal the operation in power save mode Jakub Majkowski (Nokia)

8. Peer U-ASPD, Peer STAs operating in power save mode • Both Peer STAs need to be allowed to operate in power save mode • To avoid unbalanced power consumption between STAs • To avoid race condition and optimal power management mode selection based on offered traffic • PTI frame may be used to setup a service period when both peer STAs operate in power save mode Jakub Majkowski (Nokia)

9. Use of PTI frame for wakeup schedule activation (Peer PSM mode) • In Peer Power Save mechanism wakeup schedule is deactivated, if no frames have been exchanged in consecutive wakeup periods • Stations using Peer PSM cannot communicate over the direct link when there is no defined schedule • To communicate with its peer STA over the direct link a STA may use a PTI frame in a similar way as it is used for buffered traffic indication in Peer U-APSD mechanism • Peer STA after receiving PTI frame triggers a service period with a PTI frame originator • In triggered service period wakeup schedule negotiation may be done according to Peer PSM rules Jakub Majkowski (Nokia)

10. PTI frame usage for buffered traffic indication in Peer PSM • To avoid losing data stations that are using Peer PSM mechanism could use PTI frame indication to trigger additional service period • PTI frame usage in Peer PSM stations is the same as for Peer U-APSD stations Jakub Majkowski (Nokia)

11. Jakub Majkowski (Nokia)