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TXOP Truncation

TXOP Truncation

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TXOP Truncation

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  1. TXOP Truncation Authors: Date: 2012-11-12 YonghoSeok, LG Electronics

  2. Authors: YonghoSeok, LG Electronics

  3. Authors: YonghoSeok, LG Electronics

  4. Channel Access in Restricted Access Window • A Restricted Access Window (RAW) is divided in time slots. • STA wakes up at TBTT and it listens to a Beacon frame that indicates the slot duration for each Restricted Access Window (RAW). • STA determines its channel access slot assigned by AP. • STA may sleep before its channel access slot. • STA shall start to access the channel at the slot boundary of its channel access slot based on EDCA. • AP may protect the PS-Poll/Trigger frames by setting the NAV Beacon Interval Slot Boundary Slot Boundary Slot Boundary Slot Boundary Slot Boundary Slot Boundary Slot duration Restricted Access Window (RAW) Yongho Seok, LG Electronics

  5. Channel Access in Restricted Access Window • When a STA gains access to the channel using EDCA and empties its transmission queue, it may transmit a CF-End frame provided that the remaining duration is long enough to transmit this frame. By transmitting the CF-End frame, the STA is explicitly indicating the completion of its TXOP. • A STA shall interpret the reception of a CF-End frame as a NAV reset, i.e., it resets its NAV timer to 0 at the end of the PPDU containing this frame. After receiving a CF-End frame with a matching BSSID, an AP may respond by transmitting a CF-End frame after SIFS. YonghoSeok, LG Electronics

  6. Motivation • When CF-END inside RAW is used to truncate the TXOP of the granted STAs, it also makes to reset the NAV of 3rd party STAs. Restricted Access Window Slot 0 Slot 1 Beacon CTS ACK CTS TXOP Limit GrantedSTA1 (assigned slot1) RTS DATA DATA CF-END Reset NAV GrantedSTA2 (assigned slot 1) NAV Reset NAV 3rd party STA NAV RTS YonghoSeok, LG Electronics

  7. Proposal • Duration field value of CF-END frame is set to either zero or a truncated time. • If the Duration field value of the received CF-END frame is set to zero, then all STAs reset its NAV. • Else if the Duration field of the received CF-END frame is matched with the local NAV of a STA, then the STA resets its NAV. • The matching condition allows for a tolerance of +/- TBD microsecond. • Else the received CF-END frame is discarded. YonghoSeok, LG Electronics

  8. Proposal (example) • Overall procedure of TXOP truncation in RAW. • Depending on the Duration field value of CF-END, STAs have two different behaviors of resetting NAV. Restricted Access Window Slot 0 Slot 1 Beacon CTS ACK CTS TXOP Limit Duration GrantedSTA1 (assigned slot1) RTS DATA DATA CF-END Reset NAV GrantedSTA2 (assigned slot 1) NAV RTS Discard CF-END 3rd party STA NAV YonghoSeok, LG Electronics

  9. Proposal (example) • Overall procedure of TXOP truncation in RAW. • Depending on the Duration field value of CF-END, STAs have two different behaviors of resetting NAV. Restricted Access Window Duration ACK CTS ACK CF-END Duration Reset NAV GrantedSTA1 (assigned to last slot N) NAV DATA CF-END GrantedSTA2 (assigned to last slot N) Reset NAV Reset NAV NAV RTS DATA 3rd party STA Discard CF-END Reset NAV NAV RTS YonghoSeok, LG Electronics

  10. Proposal (example) YonghoSeok, LG Electronics

  11. Conclusion • In 802.11ah, we defined a restricted access window. • During the restricted access window, only granted STAs access the channel using EDCA. But, the non-granted STAs set the NAV for the duration of the restricted access window. • Motivation of this contribution is • Inside the RAW, CF-END frame used for truncating TXOP does not reset the NAV of the non-granted STAs • For this purpose, we proposed to define two different behaviors of resetting the NAV YonghoSeok, LG Electronics

  12. Straw Poll 1 • Do you support the following CF-END rule ? • Duration field value of CF-END frame is set to either zero or a truncated time. • If the Duration field value of the received CF-END frame is set to zero, then all STAs reset its NAV. • Else if the Duration field of the received CF-END frame is matched with the local NAV of a STA, then the STA resets its NAV. • The matching condition allows for a tolerance of +/- TBD microsecond. • Else the received CF-END frame is discarded. YonghoSeok, LG Electronics

  13. Backup • In OBSS environment, CF-END frame makes to reset the NAV set by other BSS. AP1 (BSS1) ACK CTS ACK STA1 (BSS1) NAV DATA DATA CF-END Reset NAV STA2 (BSS1) NAV RTS DATA DATA NAV AP2 (BSS2) CTS ACK Collision STA3 (BSS2) RTS DATA DATA DATA DATA YonghoSeok, LG Electronics

  14. Backup • But, if we use the proposed CF-END rule (defined in slide 7), we can solve the problem of resetting the NAV. AP1 (BSS1) ACK RTS DATA DATA Duration STA1 (BSS1) DATA DATA CF-END CTS Discard CF-END STA2 (BSS1) NAV NAV AP2 (BSS2) CTS ACK STA3 (BSS2) RTS DATA DATA DATA DATA YonghoSeok, LG Electronics