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802.11n and 802.11ac TXOP Protection and Truncation

This article discusses the methods used in 802.11n and 802.11ac to protect and truncate the TXOP, including the use of non-HT duplicate frames and channel bandwidth negotiation.

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802.11n and 802.11ac TXOP Protection and Truncation

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  1. 80MHz/160MHz Protection Date: 2010-09-14 Authors: L. Chu et al

  2. HT TXOP Protection and Truncation • 802.11n introduces features that a legacy 802.11 STAs can not understand: 40MHz transmission, frame aggregation, short GI etc. • 40MHz non-HT duplicate mode is used to protect the 11n transmission: • Non-HT duplicate frames (e.g. RTS/CTS) are used to establish protection of its TXOP in two 20MHz channels, and • Non-HT duplicate CF-End frame are used to truncate the protect TXOP protected by non-HT duplicate frames. • To get a 40MHz EDCA based TXOP, a STA does EDCA backoff procedure in the primary channel, and detects the second channel being idle during PIFS interval before the TXOP. FC HT-STA1 RTS ch0 A-MPDU (HT frame) FC HT-AP1 ch0+ch1 RTS ch1 CTS ch0 BLKACK 11a-STA3 ch0+ch1 FC HT-AP1 11a-AP2 FC HT-STA1 11a-AP3 CTS ch1 11a-STA2 ch0 ch1 ch0+ch1 11a –STA2 NAV ch0 “ch0(primary channel)+ch1” is a 40MHz channel. ch0 is a 20MHz channel. 11a –STA3 NAV ch1 ch1 is a 20MHz channel.

  3. 802.11ac TXOP Protection and Truncation • 802.11ac uses 80/160 MHz channels. • 802.11ac introduces new features that legacy 802.11a STAs and 802.11n STAs can not understand: DL-MU MIMO, new 802.11ac preamble etc. • 802.11ac TXOP should be protected by frames which can be understood by 802.11a STAs and 802.11n STAs in each 20MHz channel. • 80MHz/160MHz non-HT duplicate frames are used for such TXOP protection/truncation: • 80MHz/160MHz non-HT duplicate frame duplicates 20 MHz non-HT transmission in each 20 MHz channel of a 80MHz/160MHz channel. • 80MHz/160MHz Non-HT duplicate frames (e.g. RTS/CTS) are used to establish protection of its TXOP in four 20MHz channels or in eight 20MHz channels, and • 80MHz/160MHz Non-HT duplicate CF-End frame is used to truncate the protect TXOP protected by non-HT duplicate frames in four 20MHz channels or in eight 20MHz channels.

  4. 802.11ac TXOP Protection and Truncation (Cont’d) • To get a 80MHz/160MHz EDCA based TXOP, a STA does EDCA backoff procedure in the primary channel, and detects the other 20MHz channels being idle during PIFS interval before the TXOP. CTS ch3 CTS RTS A-MPDU (VHT frame) ch2 ch3 CTS ch1 RTS ch2 ch0+ch1+ch2+ch3 CTS ch0 RTS ch1 VHT-AP1 RTS ch0 VHT-STA1 BLKACK FC HT-STA3 Ch0+ch1+ch2+ch3 VHT-STA1 VHT-AP1 11a-AP2 FC HT-AP3 11a-STA2 ch3 ch0+ch1 ch0+ch1+ch2+ch3 11a–STA2 NAV “ch0(primary channel)+ch1+ch2+ch3” is a 80MHz channel. ch3 “ch0(primary channel)+ch1” is a 40MHz channel. FC HT–STA3 NAV ch0 ch3 is a 20MHz channel.

  5. Negotiation of Used Channel Bandwidth (TBD) • The TXOP holder and the TXOP responder notify the peer STA of the usage of channels for the following frame transmission through non-HT duplicate frame handshake. We call the two frames the non-HT duplicate initiating frame, the non-HT duplicate responding frame. • Another option method is that the TXOP responder notify the available 20MHz operating channels to the TXOP holder, the TXOP holder decides the transmit channels in the following transmission. • The selection of non-HT duplicate frames will allow legacy 802.11a, 802.11n device to set the NAV correctly. • The legacy devices will not delete the received frames per their frame receiving rules. • The TXOP holder transmits CF-End frame to truncate the TXOP through the same channel set that it transmits the non-HT duplicate initiating frame. And the TXOP responder transmits CF-End frame to truncate the TXOP through the same channel set that it transmits the non-HT duplicate responding frame. • The TXOP responder may transmit the non-duplicate responding frame by the subset of the channels that the TXOP holder transmits the non-HT duplicate initiating frame for TXOP-protection.

  6. Updated RTS/CTS method (TBD) • A RTS includes the candidate subchannel set in the reserved bits of Service field or PHY padding. • The TXOP holder selects the candidate subchannel set from the idle 20MHz operating channels that it detects. • A legacy STA may just set its NAV timer when it receive the updated RTS frame. • A CTS includes final subchannel set in the reserved bits of Service field or PHY padding. • The final subchannel set must be the same as or subset of the candidate subchannel set. The final subchannel set must only include the idle 20MHz operating channels that the TXOP responder detects. • The final subchannel set is used for the following frame transmission in the TXOP. • If a VHT STA that receives updated RTS does not receive the following updated CTS, it will reset the NAV timer. • If a legacy STA does not receive the following CTS, it may not reset its NAV timer since it may not thinkthe NAV timer is not set by RTS frame. • If the TXOP holder does not receive the responding control frame, it transmits CF-End to reset the legacy STA’s NAV timer. This can guarantee the fairness.

  7. New Control Frame Method (TBD) • A new control frame that uses non-HT duplicate format to initiate the TXOP includes candidate subchannel set by control frame body. • The TXOP holder selects the candidate subchannel set from the idle 20MHz operating channels that it detects. • A legacy STA will just set its NAV timer when it receive the new control frame. • A new control frame that uses non-HT duplicate format to respond the initiating frame which includes final subchannel set in frame body. • The TXOP responder does not transmit respond frame if its NAV timer is not 0 or all the channels in the candidate subchannel set are busy. • The final subchannel set must be the same as or subset of the candidate subchannel set. The final subchannel set must only include the idle 20MHz operating channels that the TXOP responder detects. • The final subchannel set is used for the following frame transmission in the TXOP. • If a VHT STA can not receive the non-HT duplicate responding frame, it will reset the NAV timer set by the non-HT duplicate initiating frame. • If a legacy STA does not receive the following non-HT duplicate responding frame, it may not reset its NAV timer since it may not thinkthe NAV timer is not set by RTS frame. • If the TXOP holder does not receive the responding control frame, it transmits CF-End to reset the legacy STA’s NAV timer. This can guarantee the fairness.

  8. 80MHz/160MHz non-HT Duplicate Transmission • 80MHz/160MHz Non-HT duplicate transmission is used to transmit to 802.11a, 802.11n STAs that nay be present in a 20MHz/40MHz channel covered by 80MHz/160MHz channel. • The L-STF, L-LTF and L-SIG shall be transmitted in the same way as in the 802.11ac 80MHz/160MHz VHT transmission. • The VHT-SIG, VHT-STF, VHT-LTF are not transmitted.

  9. Straw Poll • Do you support adding the following item into the specification framework document, 11-09/0992? • R5.X: 80MHz/160MHz duplicate mode shall be included in the specification to protect 80MHz/160MHz transmission.

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