HCF Channel Access And Inter-BSS Channel Sharing Jin-Meng Ho, Sid Schrum, Khaled Turki, and Donald P. Shaver Texas Instruments Incorporated 12500 TI Blvd. Dallas, Texas 75243 (214) 480-1994 (Ho) jinmengho@ti.com

PIFS/DIFS Usage Issues PIFS separation between intra-BSS frames may lead to inter-BSS collision. Inter-BSS ESTAs deferring for a PIFS idle interval may transmit into the same channel at the same time. Concurrent transmissions may result in the undesirable loss of channel access to another BSS. PIFS deferral for contention-free access may cause repeated inter-HC collision. Adjacent HCs accessing a shared channel after waiting for a PIFS idle duration may cause collision. Colliding HCs re-accessing the channel after waiting for another PIFS idle time may collide again. DIFS offset for contention-based transmission may yield HC-STA collision. HCs seeking to re-access the channel after deferring for an idle duration of at least a DIFS interval may loose the channel to, or collide with, contending intra- or inter-BSS stations that transmit after a DIFS idle.

Retaining/Regaining CF Access SIFS separation between intra-BSS frames shall be used. This applies to frames transmitted under the continual control of an HC. This helps an HC retain the continual control of channel access. Randomized deferral for contention-free access shall be employed. This applies to HCs seeking to regain the control of channel access. This reduces and resolves collision between contending HCs. DIFS+ resumption for EDCF contention shall be applied. Non-HC ESTAs based on EDCF for data frame transmissions resume contention after the channel is idle for the max HC deferral plus a slot time (i.e., for DIFS + 3  SlotTime). HCs accessing the channel by deferring for between PIFS and PIFS + 3  SlotTime preempt non-HC ESTAs from concurrent contention. CC/RR shall be the primary contention mechanism for non-HC ESTAs. An HC broadcasts CC frames to solicit RR frames from non-HC ESTAs having buffered traffic and grants TXOPs to those ESTAs for CF access. Non-HC ESTAs transmit their data and management frames mostly using granted TXOPs but not (E)DCF-based contention.

Inter-BSS Channel Sharing An HC shall have continual control of channel access for not more than 5 ms or aMediumOccupancyLimit stored in the MIB. When an HC has the control of channel access, it grants itself and other ESTAs TXOPs, which include CCOPs granted via CC frames. An HC loses the control of channel access when it does not receive an expected ack or when it senses a larger than SIFS idle over a granted TXOP. An HC discontinues the control of channel access when it is ready, or is required, to release the control of channel access (to another HC or DCF). An HC shall defer for a randomized idle duration in regaining control of channel access. When an HC have no control of channel access, it transmits into the channel to regain the control after it senses the channel to be idle--based on both NAV and CCA indication--for PIFS + i  aSlotTime, with i chosen randomly from 0, 1, 2, or 3. The HC then considers itself to have regained the access control until it does not receive an expected ack or it senses a larger than SIFS idle over a TXOP. The HC re-chooses a randomized idle deferral time independently each time it seeks to regain the control of channel access.

HC Access Phase (HAP) HAP is the temporal representation of a continual access control. HAPs are started by HCs transmitting after the channel idle (virtual and physical) for PIFS, DIFS, DIFS + aSlotTime, or DIFS + 2  aSlotTime. Frames transmitted within a HAP are separated by a SIFS. NAVs set, and TXOPs granted, by an HC shall not exceed HAP length limit. Broadcast/multicast frames and directed frames requiring no immediate (SIFS-separated) response may, but should generally not, start a HAP. aSlotTime (Non-Ack = 1) QoS Data QoS CF-Poll Transmissions From HC Of QBSS 1 + CF-Poll QoS Data + CF-Poll QoS Data + CF-Poll + CF-Ack QoS Data RTS RTS Beacon SIFS PIFS DIFS Transmissions From non-HC ESTAs of QBSS 1 Collision (Non-Ack = 0, + CF-Ack QoS Data NF = 1) (Non-Ack = 1, + CF-Ack QoS Data NF = 1) (Non-Ack = 1, QoS Data NF = 0) transmission Intended DIFS + Slot CTS DIFS + 2 Slots HAP HAP HAP HAP HAP HAP (#. of CCOPs = 3) CC transmission No QoS CF-Poll QoS CF-Poll QoS CF-Ack Transmissions From HC Of QBSS 2 Collision reception Incorrect Collision RTS (From DCF STA) Data Transmissions From non-HC ESTAs of QBSS 2 (Non-Ack = 0, NF = 1) QoS Data transmission Intended (Non-Ack = 0, NF = 1) QoS Data (Non-Ack = 0, NF = 0) QoS Data RRs RR

NAV and TXOP An HC sets its Duration field to cover the TXOPs to be granted (for itself and non-HC ESTAs) within the limit of its HAP. A non-HC ESTA sets the Duration field in an outgoing frame to the value equal to the Duration field value contained in the SIFS-separated preceding frame minus one SIFS and the time required to transmit that outgoing frame. The HC may set/adjust the Duration field to any value within the HAP limit. An HC grants a TXOP to an ESTA within the limit of its HAP. A non-HC ESTA sends one or more frames within the granted TXOP limit, considering the time needed for the returned acks, if required, and the intervening SIFSs to be part of the TXOP. A non-HC ESTA loses its TXOP if it does not receive an expected ack within the ack timeout, resulting in the HC losing its channel access control. NAVs (Non-Ack = 1) QoS Data (Non-Ack = 0) Data QoS CF-Poll QoS CF-Ack QoS CF-Ack #(. of CCOPs = 3) CC + CF-Poll QoS Data + CF-Poll QoS Data + CF-Poll + CF-Ack QoS Data RTS RTS Beacon (Non-Ack = 0, + CF-Ack NF = 1) QoS Data (Non-Ack = 1, + CF-Ack NF = 1) QoS Data (Non-Ack = 1, NF = 0) QoS Data (Non-Ack = 0, NF = 1) QoS Data (Non-Ack = 0, QoS Data = 0) To different STA CF-Ack CTS RRs RR TXOPs HAP HAP HAP

Non-Ack and NF Bits A frame with the Non-Ack bit set to 0 requires an ack to be returned within a SIFS time. The frame transmission is considered to have failed if no ack is correctly received within the expected time. A frame with the Non-Ack bit set to 1 indicates no ack to be returned. The frame transmission is considered to have succeeded regardless of the actual reception status. An HC-transmitted frame with the NF bit set to 0 or 1 indicates the frame to be, or not to be, the final frame in this HAP, respectively. A non-HC ESTA transmitted frame with the NF bit set to 0 or 1 indicates the frame to be, or not to be, the final frame in this TXOP, respectively. Setting Non-Ack = 1 and NF = 0 ends the transmission from the non-HC ESTA into the TXOP and indicates that the HC can transmit after a SIFS, provided the current HAP has not reached its maximum limit. Setting Non-Ack = 0 and NF = 0 ends the transmission from the non-HC ESTA into TXOP and indicates that the HC can transmit another frame a SIFS after returning an Ack frame, provided the current HAP has not reached its maximum limit.

Embedded TXOP A non-HC ESTA grants an embedded TXOP to another non-HC ESTA within the limit of its TXOP. The granting ESTA sends a QoS CF-Poll, QoS Data + CF-Poll, or QoS Data + CF-Ack + CF-Poll frame to the granted ESTA using rules for these frames. The granted ESTA uses the embedded TXOP (as specified in the TXOP limit field of the received {+ }CF-Poll frame) by the regular TXOP usage rules. The granted ESTA loses its embedded TXOP if it does not receive an expected ack within the ack timeout, resulting in the granting non-HC ESTA losing its TXOP and the HC losing its channel access control. Embedded TXOP is useful for direct ESTA-ESTA communications and for extending the QBSS coverage. NAVs embedded TXOP Non-HC ESTA x grants embedded TXOP transmits into Non-HC ESTA y acks granted ESTA non-HC ESTA Granting embedded TXOP finishes using Non-HC ESTA granted ESTA non-HC ESTA lastly acks Granting (Non-Ack = 1) QoS Data QoS CF-Poll QoS CF-Ack QoS CF-Ack + CF-Poll QoS Data + CF-Poll QoS Data + CF-Poll + CF-Ack QoS Data Beacon RTS RTS (Non-Ack = 0, + CF-Ack NF = 1) QoS Data (Non-Ack = 1, + CF-Ack NF = 1) QoS Data (Non-Ack = 0, NF = 1) + CF-Poll QoS Data (Non-Ack = 0, + CF-Ack NF = 1) QoS Data QoS CF-Ack NF = 1) (Non-Ack = 0, NF = 0) QoS Data QoS CF-Ack NF = 0) (Non-Ack = 0, NF = 1) QoS Data (Non-Ack = 0, NF = 0) QoS Data CTS TXOPs Embedded TXOP HAP HAP HAP

CC/RR An HC sends a CC frame such that the ensuing CCI stays within the limit of its HAP. The entire CCI is considered to be part of the current HAP, even if no transmission is sensed in some CCOPs over its length. Adjacent CCOPs within the CCI are separated by a SIFS interval. An HC may send another CC frame, or any other frame, a SIFS after the end of the last CCOP within the limit of the HAP. An HC may, but should generally not, start a HAP by sending a CC frame. A wireless ESTA sets the Duration field in the RR frame equal to The Duration field value contained in the preceding CC frame minus n times the sum of SIFS and CCOP length if the RR frame is to be sent into the nth CCOP over the CCI, where n counts from 1. NAVs (#. of CCOPs = 3) CC (Non-Ack = 1) QoS Data QoS CF-Poll (# of CCOPs = 2) CC QoS CF-Ack QoS CF-Ack + CF-Poll QoS Data + CF-Poll QoS Data RTS RTS Beacon (Non-Ack = 1, + CF-Ack NF = 0) QoS Data (Non-Ack = 1, + CF-Ack NF = 1) QoS Data (Non-Ack = 1, NF = 0) QoS Data (Non-Ack = 0, NF = 1) QoS Data (Non-Ack = 0, NF = 0) QoS Data CTS RR RRs RR TXOPs HAP HAP