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HCF medium access rules

HCF medium access rules. Wim Diepstraten Agere Systems WDiepstraten@Agere.com. Backgound. HCF can operate in the Contention Free Period (CFP) and the Contention Period (CP). All ESTA are pollable, but piggy-back Ack is not supported.

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HCF medium access rules

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  1. HCF medium access rules Wim Diepstraten Agere Systems WDiepstraten@Agere.com Wim Diepstraten, Agere Systems

  2. Backgound • HCF can operate in the Contention Free Period (CFP) and the Contention Period (CP). • All ESTA are pollable, but piggy-back Ack is not supported. • Within the Contention Free Bursts (CFB) contention free access rules are applied. • The HCF medium access rules should allow for EDCF contention during the CP. • However the current rules specify that the HC can access the medium after a PIFS. • A max duration of the CFB is defined, but with the PIFS rule the HC can immediately start a next CFB. • This allows for infinitely extended CFBs. • Clearly there is a conflict in these rules. Wim Diepstraten, Agere Systems

  3. HC access objectives • The HC shall allow for EDCF contention when operating in the CP. • The CFB duration should be limited to limit the access delay for EDCF access. • Given its role, and the traffic distribution the HC should be given extra access priority opportunities, to improve efficiency, and reduce contention overhead. Wim Diepstraten, Agere Systems

  4. Proposed CFB access rules • CFB access rules should allow EDCF contention opportunities when used during the CP. • CFB duration is limited to allow EDCF contention opportunities. • Between bursts the HC should use EDCF access procedures. • Thereby contending with ESTA using EDCF. • CFB can be allowed to piggyback its access on (E)DCF access from any station within the same BSS. • Only if that station belongs to the same BSS, to prevent overlap issues. • And the total duration of the burst is limited to the CFB max duration. • This gives the HC the opportunity to gain access without contention for every (E)DCF access of any station within that BSS. • Note that this also applies to legacy station. Wim Diepstraten, Agere Systems

  5. CFB start details in CP • Each CFB start shall allow EDCF contention. • The HC can start a CFB after any Data/Mgmt frame exchange from the same BSS with NF=0. • Note that this does in most cases traffic to the AP, which the AP needs to Ack. • The total burst duration should always be limited to CFB duration limit. Wim Diepstraten, Agere Systems

  6. HC access rules • At a start of a CFB the HC shall use the normal EDCF access rules. • Which allows immediate access when medium was free longer then the applicable AIFS. • Effectively the HC should enter a Postbackoff procedure at the end of a CFB. • If no traffic is Q’ed then a low priority AIFS and CWmin should be used. • If traffic is still Q’ed for transmission the appropriate AIFS and CWmin for that TC should be used for the postbackoff. • If a station wins the contention, then the HC can piggyback its access at the end of the frame exchange by the station. • So the HC will only experience a delay of one station access up to its TxOp limit. Wim Diepstraten, Agere Systems

  7. General issues: • What is the priority of a sequence-initiating Control frame? • E.g. RTS, CF-Poll, PS-Poll • RTS priority is the same as the data associated with it. • PS-Poll ?? • QoS no-data+CF-Poll ?? And what should the TCID field be? • should have the same priority then the data that it is associated with. • Note: CTS and Ack are response frames only. • What is the relation between: • TxOp limit. • Max CFB duration. • Probably want a separate limit for both. Wim Diepstraten, Agere Systems

  8. Normative text changes • Clause 9.10, • line 32: Delete “using higher medium access priority than ESTA’s”. • Add: The HCF shall perform the EDCF access procedure to start a CFB, or it can start a CFB at SIFS after a completed EDCF frame exchange (NF=0) of a (E)STA within the same BSS, provided that the total duration of the frame exchange + CFB is not longer then CFB limit. Wim Diepstraten, Agere Systems

  9. Simplified CFB limit rules • Independent TxOp and CFB limit. • HC does not need to dynamically decide what to put in its CFB, based on the previous (E)STA exchange. Wim Diepstraten, Agere Systems

  10. Motion to adopt following change: • Clause 9.10, • line 32: Delete “using higher medium access priority than ESTA’s”. • Add: The HCF shall perform the EDCF access procedure to start a CFB, or it can start a CFB at SIFS after a completed EDCF frame exchange (NF=0) of a (E)STA within the same BSS. • Note that the question what CWmin to use for HC EDCF access is a separate discussion. Wim Diepstraten, Agere Systems

  11. Effect of the change • Concerns have been raised that the HC should be given preferential access during the CP, to provide a low latency access capability. • It should be noted that the resulting characteristics of the proposed solution are: • The EDCF access rule means that an HC can effectively access the medium immediately after the medium has been free for AIFS. • If the medium is busy at that time, then the HC shall defer until the end of the frame, and then the piggyback access rule would apply, and allow the HC contention free access (when traffic is from own BSS). • Whether this is after SIFS or PIFS is not important as both will provide contention free access. • At the end of a CFB the HC should enter the Post-backoff procedure to allow EDCF access by contending stations, and provide contention resolution between overlapping BSS’s. • So only the post-backoff rule is new compared to the current draft. • While it needs clarification what post-backoff parameters should apply for the HC. • These parameters can be defined such that the HC can be given more preferential access during post-backoff. Wim Diepstraten, Agere Systems

  12. HC access rules • Existing rule: HC can access the medium after medium is free for a PIFS. • Existing rule: The CFB duration will be limited to CFB limit, which should be independent from TxOp limit. • New rule: After a CF-End and at the end of a CFB, or after defering for traffic from an other BSS, the HC shall enter the Post backoff procedure, with the following parameters: • IFS=PIFS • CWmin = dot11HcCWmin with a default of 15. • CWmax = dot11HcCWmax with a default of 31. Wim Diepstraten, Agere Systems

  13. Proposed normative text • Add: to appropriate clause in section 9. • After a CF-End frame or after the end of a CFB or after defering for traffic from an other BSS, the HCF shall perform the EDCF post backoff procedure with an IFS=PIFS, CWmin = dot11HcCWmin with a default of 15, and CWmax = dot11HcCWmax with a default of 31. Wim Diepstraten, Agere Systems

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