doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 1 A Hybrid Coordination Function for QoS Michael Fischer Intersil Corporation Medical Drive San Antonio, TX voice: fax:

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 2 Overview A Hybrid Coordination Function (HCF) combines the enhanced DCF with selected PCF features to simplify the QoS conformance model: –Levels 1 & 2 merge into a single level for prioritized QoS. –The mandatory ESTA functionality is identical for ALL levels. This applies to receptions from the WM and service interface functions. ESTA implementations may choose not to initiate particular frames or frame exchange sequences that are allowed under the HCF. –The HCF is more robust than the EPCF in the presence of interference from overlapping BSSes, while also being friendler to legacy STAs. –The HCF makes more efficient use of the WM than the EPCF when handling periodic traffic at service rates faster than the CFP repetition rate. It appears that the HCF could provide all of the centralized functions needed for QoS control, replacing the current EPCF. –However, there is still a "Level 3" to distinguish between EAPs which support parameterized QoS and EAPs which only support prioritized QoS.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 3 Origins Many of concepts incorporated in the HCF originated in one of the following submissions: –Suggested PCF Enhancements and Contention Free Bursts (document 00/113), by Maarten Hoeben and Menzo Wentink. –p-DCF for Prioritised MAC Service (document 00/367r1), by Jin-Meng Ho, Sid Schrum, and Khaled Turki. –An Integrated QoS Model with Point-controlled Contention Arbitration (document 00/448), by Robert Meier.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 4 Terminology HCF: Hybrid coordination function –A coordination function which combines a prioritized DCF and centrally controlled channel access by a QoC coordinator to provide most of the benefits of an EPCF. QC: QoS Coordinator –The entity controlling QBSS operation under the HCF. The QC is a type of point coordinator, but uses different rules than both the PC or EPC. TxOp –A bounded-duration time interval during which an ESTA may transmit on the WM. The ESTA obtains each TxOp either by winning an instance of EDCF contention or by receiving a (+)CF-Poll from the QC {or EPC}. CFB: Contention Free Burst –A sequence of PIFS-separated frame exchanges each of which is either from the QC or in direct response to a (+)CF-Poll from the QC. Autonomous Burst –A sequence of SIFS-separated frame exchanges from a plurality of ESTAs, where the first ESTA receives a (+)CF-Poll from the QC, and the right to transmit is conveyed directly between successive ESTAs.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 5 General Properties of the HCF The HCF can be used in conjunction with any of the current EDCF proposals (vDCF, pDCF, or TCMA). –The basic requirement is for the channel access mechanism to provide a means by which the QC can obtain priority access to the WM with low latency and a high probability of success. The priority access mechanism which is compatible with the legacy DCF and the EDCF proposals is for the QC to commence transmission after detecting the WM to be free (both CCA and NAV) for a PIFS interval. –A QC could coexist with an EPC if there are reasons to use HCF and EPCF simultaneously (which is doubtful). The HCF can benefit from BSS overlap mitigation, especially for the purpose of reducing beacon collisions. However, in typical, low- or medium-density environments, the EDCF should be able to handle QBSS overlap interference without additional mechanisms.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 6 Summary of Changes to QoS Baseline Changes to frame formats and contents of certain fields: –Additional QoS information in Beacon and Probe Response frames –Transfer of information from Duration/ID field to TCID field –Modified formats for CC and RR frames Changes to QoS frame exchange sequences and timing: –Rules for using PIFS and SIFS during the CP –Rules for NAV coverage (duration values) during the CP –Rules for using QoS Data frames during the CP –Rules for using CC and RR frames during the CP and extra uses for RR

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 7 QoS Information in Beacon TxOp limit –Maximum time of medium occupancy by an ESTA per non-polled TxOp This limit bounds the temporal extent of EDCF TxOps, and does not override the DCF rule of sending one MPDU or MMPDU per successful contention. –If =0, TxOp limit is 1 maximum size MSDU at data rate Autonomous Burst limit –Maximum time that an autonomous burst may continue, following issuance by the QC of a QoS-(+)CF-Poll –If =0, autonomous burst limit equals TxOp limit (e.g. no bursts allowed) If the EDCF mechanism uses a feedback function, the monitoring entity is collocated with the QC and provides a feedback information element.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 8 Contents of Duration/ID and TCID Fields All QoS-specific frame subtypes (QoS Data, CC, RR, Container) are permitted during the CP, where a duration value is required, so the Duration/ID field reverts to its usage as specified in The QoS information moves to the low-order 12 bits of the TCID field: –The encoding and units have been simplified from those in the QoS baseline.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 9 Modified CC and RR Frame Formats CC Frames: –A TCID field (with the format shown above for CC frames) is added immediately after the BSSID field to convey the priority limit, CCI length, and CCOP length. –This change also applies for CC+Cf-Ack frames, although it is unclear whether an HC will ever send such frames. RR Frames: –The TCA field is replaced with a TCID field (of the format shown above for RR frames) to convey queue state in a uniform manner. A provision is added to identify whether the reported length is the total amount of queued traffic or the size of the frame at the head of the queue. –A separate, 2-octet field containing the AID of the requesting ESTA is added immediately after the TCID field to identify the request source.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 10 IFS Usage for HCF QoS Transfers SIFS –Restricted to use within frame exchange sequences and between frame exchange sequences during individual TxOps and/or autonomous bursts. Autonomous bursts can be viewed as very long frame exchange sequences, because the recipient of a +CF-Poll has to respond within SIFS. PIFS –Used for priority channel access by the QC –Used between successive frame exchanges during a CFB to permit detection of inter-BSS CFB collisions The CFB ends immediately if CCA(busy) is detected between frame exchanges. DIFS/EIFS –The minimum used for all other inter-frame spaces during the CP.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 11 NAV Usage for HCF QoS Transfers Under HCF the NAV is used to protect TxOps, not the entire CFB. –The continuation of a CFB is based on the use of PIFS by the QC. –This eliminates the need for a frame to signal the end a CFB, allowing each burst to end after any frame exchange if interference is detected. The duration value is set as follows: –The duration value in frames which include (+)CF-Poll covers the full duration of the TxOp granted by that CF-Poll, plus one PIFS time. –ESTAs reduce the duration value used in the subsequent frame by the duration of the current frame and its IFS. This is identical to the duration handling required in for CTS frames and for ACK frames within fragment bursts. –Duration in frames of non-polled TxOps is set as specified in CF-Poll NAV from CF-Poll SIFSTxOp from CF-PollPIFS

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 12 HCF Frame Exchange Rules at ESTA A TxOp obtained by winning EDCF contention can be used to send a single MPDU or MMPDU with total medium occupancy time not exceeding the TxOp limit from the QoS information in the beacon. A TxOp pursuant to a +CF-Poll includes a specified duration limit. ESTAs use QoS data format for all exchanges to/from HC/EAP, and may use QoS data format for ESTA-to-ESTA transfers. –Traffic monitor and HC assume all non-QoS frames are best effort traffic ESTAs may use (+)CF-Ack to QC/EAP –Can only use +CF-Ack for ESTA-to-ESTA if the ability to detect +CF-Ack is known to exist at acknowledgement target. This requires further discussion. ESTAs must respond to a (+)CF-Poll –A response of QoS-Null(+CF-Ack) is used when there is nothing to send or when the frame to send will not fit in the TxOp. The HC can distinguish these cases by the queue state in the TCID field.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 13 CC/RR Usage Changes The QC can send a CC at any time (e.g. during the CP) –The duration of the CC covers the CCI period plus one SIFS plus CF-Poll time. ESTAs send RR frames in response to CCs as currently specified. ESTAs may also send RRs during any TxOp: –A TxOp longer than the limit in the beacon can be requested by sending an RR after winning EDCF contention. The QC must respond to this RR with a CF-Poll frame, but is not required to grant a longer TxOp. –ESTAs which support parameterized QoS may an RR frame (to a QC which supports parameterized QoS) during any TxOp to request allocation of a periodic service TxOp.

doc.: IEEE /452 Submission December, 2000 Michael Fischer, Intersil Slide 14 Autonomous Bursts There are a number of subtle problems associated with the autonomous burst concept. It is appropriate to consider whether the complexity of the autonomous burst facility is justified. Only QoS data frames and container frames may be sent during autonomous bursts –This is the simplest rule to overcome the problem that legacy stations cannot participate properly in a burst, both because of duration field update and because of inability to respond to RTS during a burst (because their NAV will be set). The total duration of the burst must be bounded or the ability for the QC to provide periodic TxOps is destroyed. –A total burst duration limit in the beacon is dependent on the subsequent ESTAs having received the original (+)CF-Poll from the QC. An ESTA sending a (+)CF-Poll within a burst is responsible for informing the QC of non-response (presumably using a QoS-Null).