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Class-based Contention Periods (CCP) for the 802.11n MAC

Class-based Contention Periods (CCP) for the 802.11n MAC. A. Dasylva, Z. Yao, D.Y. Montuno, W. Chen, M. Ouellette, J. Aweya, and K. Felske Nortel Networks. Acronyms. HC: Hybrid Coordinator CP: Contention Period CFP: Contention Free Period CCP: Class-based Contention Periods

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Class-based Contention Periods (CCP) for the 802.11n MAC

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  1. Class-based Contention Periods (CCP) for the 802.11n MAC A. Dasylva, Z. Yao, D.Y. Montuno, W. Chen, M. Ouellette, J. Aweya, and K. Felske Nortel Networks Abel Dasylva, Nortel Networks

  2. Acronyms • HC: Hybrid Coordinator • CP: Contention Period • CFP: Contention Free Period • CCP: Class-based Contention Periods • ECP: Explicit Contention Period. A type of CP explicitly allocated by the HC and where a subset of the ACs may contend. It starts and ends with specific control frames • LCP: Legacy Contention Period. A traditional CP. • STA: Station. Unless mentioned other wise an STA indicates a legacy STA (with or without QoS support) • HT STA: High Throughput STA, an STA supporting the new features described in this document and other 802.11n features. • HT AP: High Throughput AP, an AP supporting the features described in this document and other 802.11n features. Abel Dasylva, Nortel Networks

  3. General description of CCP • Two types of contention periods • Explicit CPs (ECPs) allocated by the HT AP • Legacy CPs (LCPs) • In each ECP a subset of ACs contend according to EDCA rules • ECPs are delimited by • ECP-Start • ECP-End or • ECP-Start+ECP-end frames • Two access modes for ECPs • Default mode: a channel access function can access the channel within an ECP if its AC is allowed in the ECP • QoS negotiation mode: the HT AP grants access to the channel access function after a QoS negotiation phase Abel Dasylva, Nortel Networks

  4. Motivation for CCP • The need for a simple and effective QoS provisioning mechanism • Improve the throughput efficiency of EDCA by • Allowing non-QoS (TCP) traffic to contend more aggressively for the available bandwidth • Maintain and improve the performance of QoS traffic (with EDCA) by better isolation from non-QoS flows • The complexity and polling overhead (especially the associated preamble+PLCP overhead) of HCCA • The difficulty of accurate QoS provisioning with EDCA • Solution CCP: blend features of HCCA and EDCA • Centralized allocation and scheduling of ECPs • Distributed channel access within ECPs • QoS provided by the proper selection of ECP lengths and scheduling Abel Dasylva, Nortel Networks

  5. ECP Scheduling (Informative) Abel Dasylva, Nortel Networks

  6. Control frames/ ECP-Start frame • A frame sent by the HT AP to initiate a new ECP • Fields: • RA: set to the broadcast group address • ECP type: 1 byte field giving the ECP type. Each ECP type maps to a subset of ACs. • Duration: this field is set to the length of the ECP Abel Dasylva, Nortel Networks

  7. Control frames/ ECP-End frame • A frame sent by the HT AP to end an ECP • Fields: • RA: set to the broadcast group address • Duration: frame duration Abel Dasylva, Nortel Networks

  8. Control frames/ ECP-End+ECP-Start frame • A frame sent by the HT AP to end the current ECP and start the next ECP • Fields: • RA: set to the broadcast group address • Duration: duration of the ECP • ECP type: type of the next ECP Abel Dasylva, Nortel Networks

  9. Control frames/ ECP-Access Req. frame • A frame sent by a HT STA to the AP to request access to ECPs of one or more types • The ECP access request is on a per-flow basis. It is not doe for each data frame • Fields: • Duration: frame duration • TA: address of the requesting HT STA • ECP type n: n-th ECP type for which access is requested • TSPEC n: TSPEC of the traffic to be transmitted in ECPs of type n Abel Dasylva, Nortel Networks

  10. Control frames/ ECP-Access Req. ACK. frame • A frame sent by an HT AP to an HT STA to acknowledge the receipt of an ECP-access request frame • Fields: • Duration: frame duration • RA: address of the requesting HT STA • Request number: request number assigned by the HT AP Abel Dasylva, Nortel Networks

  11. Control frames/ ECP-Access Resp. frame • A frame sent by the HT AP to a requesting HT STA in response to an ECP access request access • Fields: • Duration: frame duration • RA: address of the requesting HT STA • ECP type n: n-th ECP type for which access is requested • Resp n: admission decision for ECP n Abel Dasylva, Nortel Networks

  12. Mgmt frames/ ECP capability element • Information element advertising ECP capability by the HT AP or HT STAs • Fields: • ECP capability: bit indicating whether the HT AP is able to allocate ECPs, or HT STAs are able to interpret ECP frames • ECP length: maximum ECP length • Num. ECP types: the number of ECP types that are supported by the HT AP Abel Dasylva, Nortel Networks

  13. Mgmt frames/ ECP parameter element • An information element giving the parameters of an ECP type • Fields: • ECP type: type of the ECP between 0 and 255 • Mode: access mode for the ECP type, i.e. default (0) or through QoS negotiation (1) • AC mask: ACs that are allowed to contend for channel access Abel Dasylva, Nortel Networks

  14. MAC sublayer functional description 1/5 • ECP allocation and scheduling • The HT AP allocates ECPs by sending ECP-Start or ECP-Start+ECP-End frames • The duration of a new ECP is set in the duration field of the corresponding ECP-Start, or ECP-Start + ECP-End frame • The length of an ECP cannot exceed the value of MAX_ECP_LENGTH (larger than the CFP length) set in the ECP length field of the ECP capability element • An ECP-Start or ECP-End+ECP-Start frame may be allocated • A PIFS interval after the completion of a CFP or ECP • A PIFS interval after the transmission of a frame in LCPs • Channel access during ECPs • Essentially EDCA rules with minor modifications: • A frame exchange sequence initiated within an ECP must complete within that ECP • A TXOP obtained within an ECP must complete within the ECP • The HCCA function cannot obtained polled TXOPs within an ECP Abel Dasylva, Nortel Networks

  15. MAC sublayer functional description 2/5 • Channel access during LCPs • All HT STAs may contend according to EDCA rules • The HCCA function may allocate polled TXOPs • Interaction with the power save feature • Consider a HT STA emerging from power-save mode • This HT STA may not have knowledge of the current CFP/ECP/LCP • The HT STA resets an ECP-length timer with the value MAX_ECP_LENGTH and waits for of the following events to occur: • The timer expires: then the HT STA concludes that it is within an LCP, and the states of the channel access functions are set accordingly • A CFP-End, ECP-Start, ECP-End or ECP-Start+ECP-End frame is received and the states of the channel access functions may be properly set Abel Dasylva, Nortel Networks

  16. MAC sublayer functional description 3/5 • Support of frame aggregation (class-based frame aggregation): it is possible to transmit aggregate frames within ECPs subject to the following limitations: • For an ECP with no required QoS negotiation: only frames of ACs allowed within the ECP may form an aggregate • For an ECP with required QoS negotiation: only frames from channel access functions of the allowed ACs that have been granted access to the ECP type by the HC Abel Dasylva, Nortel Networks

  17. MAC sublayer functional description 4/5 ECP allocation examples Abel Dasylva, Nortel Networks

  18. MAC sublayer functional description 5/5 ECP allocation examples Abel Dasylva, Nortel Networks

  19. MAC sublayer Mgmt/ ECP capability • ECP capability information: included by the HT AP in association or re-association messages with the following info • Whether ECPs are supported • The maximum ECP length (MAX_ECP_LENGTH) • The number of supported ECP types • ECP parameters: included by the HT AP in association or re-association messages with the following info for each ECP type • The mode: default or through QoS negotiation • AC mask: allowed ACs Abel Dasylva, Nortel Networks

  20. MAC sublayer Mgmt/ IBSS operation Currently not supported with CCP Abel Dasylva, Nortel Networks

  21. MAC sublayer mgmt/ Coexistence with legacy STAs • Legacy STAs: STAs not able to interpret ECP control frames • Channel access within ECPs: the setting of the duration field in ECP-Start, and Ecp-Start+ECP-End frames ensure that legacy STAs do not interfere with ECP traffic • Channel access within LCPs: all STAs including legacy ones may contend according to EDCA • No conflict with HCCA Abel Dasylva, Nortel Networks

  22. Simulations 1/10 Settings Abel Dasylva, Nortel Networks

  23. Simulations 2/10 Settings Abel Dasylva, Nortel Networks

  24. Simulations 3/10 AC mapping Abel Dasylva, Nortel Networks

  25. Simulations 4/10 ECP scheduling • Two ECP types are defined • An ECP type for AC_VO+AC_VI • An ECP type for AC_BK+AC_BE • Within an ECP type • EDCA rules • Strict priority among the ACs allowed (differet AIFS, and internal contention resolution) • A round-robin schedule • The ECPs alternate and are delimited by ECP-Start+ECP-End control frames Abel Dasylva, Nortel Networks

  26. Simulations 5/10 Other • The PHY rate is small at 54Mbps • CCP does not support ad-hoc traffic • The traffic demand of the SSs have been slightly modified as follows • To avoid excessive overload by QoS traffic: some QoS sources have been turned off • To allow a fair comparison with EDCA: ad-hoc traffic has been removed • The same traffic demand is used for EDCA and EDCA+CCP Abel Dasylva, Nortel Networks

  27. Simulations 6/10 Comparison criteria • CC3: the overall system throughput (QoS+non-QoS) is increased by CCP. The increase is substantial in SS4 • CC18: in all SSs the non-QoS throughput is significantly increased by CCP • CC19: the QoS throughput is about the same with EDCA and with CCP. The best possible case is the EDCA throughput where QoS traffic has strict priority over non-QoS traffic Abel Dasylva, Nortel Networks

  28. Simulations 7/10 Comparison criteria • CC20: CCP does not affect the performance of QoS traffic. However, in SS1 and SS6 there is an overload of RT traffic (at a PHY rate of 54Mbps) and some large HDTV and SDTV flows do not meet their QoS requirements. • CC24: the throughput efficiency is significantly increased with CCP Abel Dasylva, Nortel Networks

  29. Simulations 8/10 SS1 • CCP increases the non-QoS throughput and the overall throughput • The effect is minimal on the QoS of real-time flows Abel Dasylva, Nortel Networks

  30. Simulations 9/10 SS4 • CCP significantly increases the non-QoS throughput and the overall throughput • The effect is minimal on the QoS of real-time flows Abel Dasylva, Nortel Networks

  31. Simulations 10/10 SS6 • CCP significantly increases the non-QoS throughput and the overall throughput • The effect is minimal on the QoS of real-time flows Abel Dasylva, Nortel Networks

  32. Conclusion • A simple framework for effective QoS provisioning • A wide variety of bandwidth allocation and QoS policies supported • Full backward compatibility with 802.11/802.11e • The requirement for admission control to ensure QoS within real time ECPs (beyond the scope of this work) Abel Dasylva, Nortel Networks

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