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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 A. Dasylva, Z. Yao, D.Y. Montuno, W. Chen, M. Ouellette, J. Aweya, and K. Felske Nortel Networks Abel Dasylva, Nortel Networks
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
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
Motivation for CCP • The need for a simple and effective QoS provisioning mechanism • EDCA does not provide good isolation of real time traffic from UDP best effort traffic • 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
ECP Scheduling (Informative) Abel Dasylva, Nortel Networks
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
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
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
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 • 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
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
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
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
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
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
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
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
MAC sublayer functional description 4/5 ECP allocation examples Abel Dasylva, Nortel Networks
MAC sublayer functional description 5/5 ECP allocation examples Abel Dasylva, Nortel Networks
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
MAC sublayer Mgmt/ IBSS operation Currently not supported with CCP Abel Dasylva, Nortel Networks
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
Conclusion • A simple framework for effective QoS provisioning • A wide variety of bandwidth allocation and QoS policies supported • Full back 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