CTS 166: 802.11ax HE Channel Access

There are multiple functions in which a client or AP can gain access to the channel. What we’re going to discuss is in addition to the HCF and DCF.



* TXOP duration-based RTS/CTS* Intra-BSS or inter-BSS frame determination* SRG PPDU determination* Two NAVs* MU-RTS/CTS* EDCA using MU EDCA parameters



GOOD NEWS, CTS IS NOT GOING ANYWHERE!



It’s all new ways of thinking about channel access but with added complexity. You’re dealing with multi-access, TXOP, BSS coloring which complicates the channel access process.







TXOP Duration-based RTS/CTS



HE AP can use TXOP duration-based RTS/CTS exchanges to mitigate interference for dense environments.



RTS/CTS is now under TXOP Duration RTS Threshold.



There is a TXOP Duration RTS Threshold subfield within the HE Operation element. To disable TXOP Duration RTS Threshold, the AP can set this subfield to a value of 1023. A client receiving a non-zero value from the AP will set its TXOP Duration RTS Threshold to the value of the TXOP Duration RTS Threshold subfield.



A client shall use an RTS/CTS exchange to initiate a TXOP if the feature is enabled and:



* The client intends to transmit unicast frames to the AP or to a TDLS peer client* The transmission opportunity duration is greater than or equal to 32 μs * RTS/CTS threshold. (Ex: RTS/CTS threshold = 300 & duration is 12345. Then 12345 μs > 32*300=9600)



The RTS and CTS frames contain a Duration field that defines the period of time that the medium is to be reserved to transmit the actual Data frame and the returning ACK frame.



The AP can also send a MU-RTS and simultaneous CTS responses by clients prior to the actual Data frames is another means of distribution of the medium reservation information.



Otherwise, the client follows the normal DCF rules used today by 802.11ac.







Intra-BSS and Inter-BSS Frame Determination



Client determines if received PPDU is an inter-BSS PPDU based on the following:



* BSS_COLOR is not 0 and is not the same BSS color which the client is a member. Pretty much is the client doesn’t have the same BSS color.* BSS_COLOR is not 0 and if HE client is associated with a non-HE AP. If the client is not associated to a 802.11ax AP



Client determines if received PPDU is an intra-BSS PPDU based on the following:



* BSS color is the same as the client * The frame carries an RA/TA/BSSID field value that is equal to the BSSID the client is associated with



Essentially, in order for a transmitter to determine whether it can obtain access to the shared medium, it must determine if a received frame is within its own BSS color or not.







SRG PPDU Identification



A client looks to the Spatial Reuse Parameter Set element to see if the SRG Information Present subfield has been set to 1. It’s used to identify BSSs that are members of the client’s SRG.



It will be used by the client to identify if the received inter-BSS frame is a SRG frame.



It is used during Spatial Reuse Parameter (SRP) OBSS (Overlapping Basic Service Set) PD operation. We will have a dedicated episode on Spatial Reuse.







Updating Two NAVs



HE clients must maintain two NAVs. A HE AP can maintain two NAVs. Those two NAVs are:



* intra-BSS NAV* Basic NAV