Enhancing R-TWT with Multi-AP Coordination in IEEE 802.11 Networks

 
M. Kumail Haider (Meta)
 
Slide 1
 
R-TWT Multi-AP Coordination
 
Date:
 2023-05-05
 
Authors:
 
Problem Statement
 
UHR is aiming for 
improving tail latency and jitter
compared to 802.11be including scenarios of
overlapping Basic Service Sets (BSSs)
R-TWT is a suitable candidate from 802.11be for
providing enhanced medium protection and
predictable latency for latency sensitive applications
Coordination among neighboring BSSs can further
enhance R-TWT operation and medium protection
This document explores candidate solutions for multi-
AP coordination for R-TWT
 
Slide 2
 
M. Kumail Haider (Meta)
 
Recap: R-TWT Operation in 802.11be
 
Uses broadcast TWT (802.11ax) signaling as baseline, with enhancements
AP advertises R-TWT schedules via TWT element in Management frames in the BSS
(e.g., Beacon, Probe response)
Indication of R-TWT UL/DL TIDs during membership setup to identify latency sensitive
traffic
 
Provides enhanced medium access protection and provisions for predictable
latency
R-TWT supporting STAs must end any TXOP at SP start boundary
Optional usage of overlapping quiet intervals to limit interference from legacy (non-EHT)
devices
Prioritization of traffic belonging to R-TWT UL/DL TIDs per membership
Spec recommends membership negotiated as 
Trigger-enabled
 for efficient medium access
 
 
Slide 3
 
M. Kumail Haider (Meta)
 
Next: Extending R-TWT for Multi-AP
deployments in UHR
 
Several contributions in UHR have discussed R-TWT enhancements with multi-
AP coordination
11-23/0226r0: 
Coordination of R-TWT for Multi-AP Deployment
, Abdel K. Ajami
(Qualcomm) et.al.
11-23/0250r0: 
AP coordination with R-TWT
, Liwen Chu (NXP) et.al.
11-23/0970r0: 
rTWT for Multi-AP
, Laurent Cariou (Intel) et.al.
 
This contribution shares our view and introduces further discussion on this
topic
 
 
Slide 4
 
M. Kumail Haider (Meta)
 
Extending R-TWT with Multi-AP Coordination
Solution
 
R-TWT in 11be focuses on building the basic signaling and solution for single BSS
case. It’s a first step. To scale up its effectiveness in wider deployments, we should
consider tackling the problem further in multiple BSS environments
R-TWT schedules outside of multiple or co-hosted BSSID sets are currently not advertised
inside an AP’s BSS
Interference from OBSS could result in delay in medium acquisition by member R-
TWT STAs from SP start time, if OBSS traffic continues into the SP, delaying
delivery of latency sensitive traffic
The peer-to-peer(s) link, esp. when none of the STAs are associated with the infra-
AP, also falls into this multi-BSS/OBSS scenario, and would benefit from a solution
in this direction
R-TWT SPs in neighboring BSSs could overlap in time, resulting in reduced
medium access efficiency. Coordination in usage of SPs will help mitigate
contention and improve predictable delivery of low latency traffic
 
Slide 5
 
M. Kumail Haider (Meta)
 
R-TWT Schedule Coordination
 
Neighboring APs may exchange R-TWT schedule information and
advertise OBSS schedules in their respective BSS
 
R-TWT supporting STAs in neighboring BSSs follow the R-TWT SP start
time protection rule, suppressing OBSS interference
 
An AP may consider OBSS schedules when devising schedules in its own
BSS (implementation dependent or further rules TBD)
 
AP coordination for allocation of resources within SPs
 
Slide 6
 
M. Kumail Haider (Meta)
 
Modes of Schedule Coordination
 
Two types of schedule coordination:
1.
OBSS schedules: Announcing schedules from OBSS for SP start time protection
R-TWT supporting STAs end any TXOP at SP start
Non-interfering transmissions may still happen within the BSS (spatial reuse)
 
Slide 7
 
M. Kumail Haider (Meta)
 
AP1
 
STA
11
 
AP2
 
STA2
 
STA
12
DATA
BA
Trigger
 
AP1 announces AP2’s R-TWT schedules in its BSS in beacons
 
R-TWT SP start
 
STA
12
 ensures its TXOP ends at R-TWT SP start boundary
 
STA
12 
doesn’t initiate further TXOP as it
defers to transmissions in BSS2
DATA
BA+DATA
BA
 
beacon
DATA
BA
 
STA
11 
initiates TXOP as it is not
suppressed by AP2’s TX
 
AP1 may respond to STA
11
 if its
inter-BSS NAV is not set
 
*AP1 and AP2 may exchange schedule info on the
same or different channel, or on a backhaul link
 
*Assuming all APs and STAs support R-TWT
R-TWT SP
 
AP1
 
AP2
 
STA
11
 
STA
2
 
STA
12
 
Modes of Schedule Coordination
 
Two types of schedule coordination:
1.
OBSS schedules: Announcing schedules from OBSS for SP start time protection
R-TWT supporting STAs end any TXOP at SP start
Non-interfering transmissions may still happen within the BSS (spatial reuse)
2.
Multi BSS schedules: APs may devise and announce coordinated schedules
Reduces number of schedules being advertised/devised
Schedule has the same TWT parameters (e.g., start time, duration, period) and
advertised in respective BSSs
STAs from multiple BSS may be members of this schedule (negotiated with respective
APs)
TXOP-level coordination for efficient medium access
 
Slide 8
 
M. Kumail Haider (Meta)
 
Modes of Schedule Coordination
 
Further thoughts on “Multi-BSS” schedules
Having a joint schedule, which serves STAs in multiple BSS, may improve medium efficiency
compared to advertising separate “OBSS schedules” where R-TWT supporting STAs have to
end TXOP at start boundary of each schedule
APs may coordinate using other features (e.g., enhanced spatial reuse) to allow multiple R-
TWT scheduled STAs to be served concurrently in their respective BSSs to enhance efficiency
(beyond the scope of R-TWT operation)
AP coordination and triggered access will be key to prioritizing latency sensitive traffic
delivery and reducing contention
It may also be helpful with mobility/roaming. E.g., STA switching between APs in an ESS
may remain member of the same schedule while “handed over” between APs. Any associated
QoS Char IE etc. may also be retained (further signaling needed between APs)
TXOP-level allocation within SPs can further improve STA’s power saving and devise
resource allocations that closely fulfil STA’s requirements for latency sensitive traffic delivery
 
Slide 9
 
M. Kumail Haider (Meta)
 
Signaling for Schedule Information
 
APs may acquire schedule information in other BSSs:
1.
Passively: By listening to beacons from neighboring BSSs
2.
Actively: By sharing schedule information via a new individually addressed frame
o
Frame could carry additional information about schedules helping with coordination
o
Schedules may be shared and coordinated on before announcing in respective BSSs
o
Could also be shared via a central management entity in ESSs
TWT element already provides basis for schedule announcement
 
Slide 10
 
M. Kumail Haider (Meta)
 
 
 
Restricted TWT Schedule Info field provides further info
about advertised schedules
 
o
Value 3 could be expanded to include OBSS/Multi-
         BSS schedules
o
For such schedules, 
TWT element 
may carry an identifier for
the originating BSS, or to identify between “OBSS” and
“Multi-BSS” schedules
 
Further thoughts on gaps in signaling
 
Several signaling gaps were discussed in 802.11be and deferred to next
round
TID indication in Trigger frames and BSR Control field
Basic Trigger frame can only indicate a Preferred AC, and not directly indicate a TID for soliciting UL
For enhanced SP coordination within BSS and OBSS to solicit UL traffic, TID indication in Trigger frames
will be useful
Similar issue with AC level buffer reporting in BSR Control
Explicit signaling for SP management
 
becomes necessary
R-TWT schedules may need to be extended in case R-TWT TID traffic is not delivered
within the SP
When coordinating and managing schedules in multiple BSS, this problem becomes even
more relevant and critical
Explicit signaling helps with better schedule management (SP termination and extension)
Enhanced support for p2p traffic
Member STA’s p2p traffic could be provisioned via AP e.g., via enhanced TXOP sharing
for more efficient medium utilization and coordination between infra and p2p link
Signaling enhancements are needed to indicate requirements for p2p traffic and its delivery
 
Slide 11
 
M. Kumail Haider (Meta)
 
Conclusion
 
We introduced concepts and solutions for R-TWT schedule
coordination between multiple APs in neighboring BSS to enhance R-
TWT operation in UHR
 
Two modes/levels of schedule coordination are identified
 
We discuss existing and new signaling for advertisement of such
schedules between APs and between AP and their associated STAs
 
Extending R-TWT operation for multi-AP coordination will facilitate
in mitigating OBSS interference during SPs and providing predictable
latency for low latency traffic
 
Slide 12
 
M. Kumail Haider (Meta)
Slide Note

doc.: IEEE 802.11-yy/xxxxr0

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Delve into the world of IEEE 802.11 wireless networks as this document explores solutions for multi-AP coordination for R-TWT, focusing on improving tail latency, jitter, and medium protection. Enhancements in R-TWT operation, coordination scenarios, and solutions for multi-AP deployments are discussed to optimize latency-sensitive applications across overlapping BSS environments.


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  1. May 2023 doc.: IEEE 802.11-23/0291r0 R-TWT Multi-AP Coordination Date: 2023-05-05 Authors: Name Affiliations Address Phone email M. Kumail Haider Meta Platforms, Inc. haiderkumail@meta.com Chunyu Hu Submission Slide 1 M. Kumail Haider (Meta)

  2. May 2023 doc.: IEEE 802.11-23/0291r0 Problem Statement UHR is aiming for improving tail latency and jitter compared to 802.11be including scenarios of overlapping Basic Service Sets (BSSs) R-TWT is a suitable candidate from 802.11be for providing enhanced medium protection and predictable latency for latency sensitive applications Coordination among neighboring BSSs can further enhance R-TWT operation and medium protection This document explores candidate solutions for multi- AP coordination for R-TWT Submission Slide 2 M. Kumail Haider (Meta)

  3. May 2023 doc.: IEEE 802.11-23/0291r0 Recap: R-TWT Operation in 802.11be Uses broadcast TWT (802.11ax) signaling as baseline, with enhancements AP advertises R-TWT schedules via TWT element in Management frames in the BSS (e.g., Beacon, Probe response) Indication of R-TWT UL/DL TIDs during membership setup to identify latency sensitive traffic Provides enhanced medium access protection and provisions for predictable latency R-TWT supporting STAs must end any TXOP at SP start boundary Optional usage of overlapping quiet intervals to limit interference from legacy (non-EHT) devices Prioritization of traffic belonging to R-TWT UL/DL TIDs per membership Spec recommends membership negotiated as Trigger-enabled for efficient medium access Submission Slide 3 M. Kumail Haider (Meta)

  4. May 2023 Next: Extending R-TWT for Multi-AP deployments in UHR doc.: IEEE 802.11-23/0291r0 Several contributions in UHR have discussed R-TWT enhancements with multi- AP coordination 11-23/0226r0: Coordination of R-TWT for Multi-AP Deployment, Abdel K. Ajami (Qualcomm) et.al. 11-23/0250r0: AP coordination with R-TWT, Liwen Chu (NXP) et.al. 11-23/0970r0: rTWT for Multi-AP, Laurent Cariou (Intel) et.al. This contribution shares our view and introduces further discussion on this topic Submission Slide 4 M. Kumail Haider (Meta)

  5. May 2023 Extending R-TWT with Multi-AP Coordination Solution doc.: IEEE 802.11-23/0291r0 R-TWT in 11be focuses on building the basic signaling and solution for single BSS case. It s a first step. To scale up its effectiveness in wider deployments, we should consider tackling the problem further in multiple BSS environments R-TWT schedules outside of multiple or co-hosted BSSID sets are currently not advertised inside an AP s BSS Interference from OBSS could result in delay in medium acquisition by member R- TWT STAs from SP start time, if OBSS traffic continues into the SP, delaying delivery of latency sensitive traffic The peer-to-peer(s) link, esp. when none of the STAs are associated with the infra- AP, also falls into this multi-BSS/OBSS scenario, and would benefit from a solution in this direction R-TWT SPs in neighboring BSSs could overlap in time, resulting in reduced medium access efficiency. Coordination in usage of SPs will help mitigate contention and improve predictable delivery of low latency traffic Submission Slide 5 M. Kumail Haider (Meta)

  6. May 2023 doc.: IEEE 802.11-23/0291r0 R-TWT Schedule Coordination Neighboring APs may exchange R-TWT schedule information and advertise OBSS schedules in their respective BSS R-TWT supporting STAs in neighboring BSSs follow the R-TWT SP start time protection rule, suppressing OBSS interference An AP may consider OBSS schedules when devising schedules in its own BSS (implementation dependent or further rules TBD) AP coordination for allocation of resources within SPs Submission Slide 6 M. Kumail Haider (Meta)

  7. May 2023 doc.: IEEE 802.11-23/0291r0 Modes of Schedule Coordination Two types of schedule coordination: 1. OBSS schedules: Announcing schedules from OBSS for SP start time protection R-TWT supporting STAs end any TXOP at SP start Non-interfering transmissions may still happen within the BSS (spatial reuse) AP1 announces AP2 s R-TWT schedules in its BSS in beacons *AP1 and AP2 may exchange schedule info on the same or different channel, or on a backhaul link R-TWT SP start AP1 may respond to STA11 if its inter-BSS NAV is not set beacon AP1 BA BA AP1 AP2 STA11 initiates TXOP as it is not suppressed by AP2 s TX DATA STA11 STA12 ensures its TXOP ends at R-TWT SP start boundary STA12 doesn t initiate further TXOP as it defers to transmissions in BSS2 DATA STA12 R-TWT SP STA12 AP2 BA+DATA Trigger STA2 STA11 BA DATA STA2 *Assuming all APs and STAs support R-TWT Submission Slide 7 M. Kumail Haider (Meta)

  8. May 2023 doc.: IEEE 802.11-23/0291r0 Modes of Schedule Coordination Two types of schedule coordination: 1. OBSS schedules: Announcing schedules from OBSS for SP start time protection R-TWT supporting STAs end any TXOP at SP start Non-interfering transmissions may still happen within the BSS (spatial reuse) 2. Multi BSS schedules: APs may devise and announce coordinated schedules Reduces number of schedules being advertised/devised Schedule has the same TWT parameters (e.g., start time, duration, period) and advertised in respective BSSs STAs from multiple BSS may be members of this schedule (negotiated with respective APs) TXOP-level coordination for efficient medium access Submission Slide 8 M. Kumail Haider (Meta)

  9. May 2023 doc.: IEEE 802.11-23/0291r0 Modes of Schedule Coordination Further thoughts on Multi-BSS schedules Having a joint schedule, which serves STAs in multiple BSS, may improve medium efficiency compared to advertising separate OBSS schedules where R-TWT supporting STAs have to end TXOP at start boundary of each schedule APs may coordinate using other features (e.g., enhanced spatial reuse) to allow multiple R- TWT scheduled STAs to be served concurrently in their respective BSSs to enhance efficiency (beyond the scope of R-TWT operation) AP coordination and triggered access will be key to prioritizing latency sensitive traffic delivery and reducing contention It may also be helpful with mobility/roaming. E.g., STA switching between APs in an ESS may remain member of the same schedule while handed over between APs. Any associated QoS Char IE etc. may also be retained (further signaling needed between APs) TXOP-level allocation within SPs can further improve STA s power saving and devise resource allocations that closely fulfil STA s requirements for latency sensitive traffic delivery Submission Slide 9 M. Kumail Haider (Meta)

  10. May 2023 doc.: IEEE 802.11-23/0291r0 Signaling for Schedule Information APs may acquire schedule information in other BSSs: 1. Passively: By listening to beacons from neighboring BSSs 2. Actively: By sharing schedule information via a new individually addressed frame Frame could carry additional information about schedules helping with coordination Schedules may be shared and coordinated on before announcing in respective BSSs Could also be shared via a central management entity in ESSs TWT element already provides basis for schedule announcement o o o Restricted TWT Schedule Info field provides further info about advertised schedules value description Value 3 could be expanded to include OBSS/Multi- o BSS schedules o For such schedules, TWT element may carry an identifier for the originating BSS, or to identify between OBSS and Multi-BSS schedules 0 Idle 1 Active 2 Active, Full 3 Active, belongs to co-hosted/multiple BSSID set Submission Slide 10 M. Kumail Haider (Meta)

  11. May 2023 doc.: IEEE 802.11-23/0291r0 Further thoughts on gaps in signaling Several signaling gaps were discussed in 802.11be and deferred to next round TID indication in Trigger frames and BSR Control field Basic Trigger frame can only indicate a Preferred AC, and not directly indicate a TID for soliciting UL For enhanced SP coordination within BSS and OBSS to solicit UL traffic, TID indication in Trigger frames will be useful Similar issue with AC level buffer reporting in BSR Control Explicit signaling for SP management becomes necessary R-TWT schedules may need to be extended in case R-TWT TID traffic is not delivered within the SP When coordinating and managing schedules in multiple BSS, this problem becomes even more relevant and critical Explicit signaling helps with better schedule management (SP termination and extension) Enhanced support for p2p traffic Member STA s p2p traffic could be provisioned via AP e.g., via enhanced TXOP sharing for more efficient medium utilization and coordination between infra and p2p link Signaling enhancements are needed to indicate requirements for p2p traffic and its delivery Submission Slide 11 M. Kumail Haider (Meta)

  12. May 2023 doc.: IEEE 802.11-23/0291r0 Conclusion We introduced concepts and solutions for R-TWT schedule coordination between multiple APs in neighboring BSS to enhance R- TWT operation in UHR Two modes/levels of schedule coordination are identified We discuss existing and new signaling for advertisement of such schedules between APs and between AP and their associated STAs Extending R-TWT operation for multi-AP coordination will facilitate in mitigating OBSS interference during SPs and providing predictable latency for low latency traffic Submission M. Kumail Haider (Meta) Slide 12

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