Feb. 2002 doc.: IEEE 802.15-02/076r0 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: Modified CTRB Parameters Date Submitted:
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Feb. 2002 doc.: IEEE 802.15-02/076r0 Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: Modified CTRB Parameters Date Submitted: 6 February, 2002 Source: Dr. William Shvodian Company: XtremeSpectrum, Inc. Address: 8133 Leesburg Pike, Suite 700, Vienna, VA 22182 Voice: 703-269-3047 , FAX: 703-269-3092 , E-Mail: [email protected] Re: [ ] Abstract: This points out problems with the current CTRB parameters and proposes a solution that will help to simplify the 802.15.3 MAC. Purpose: To provide information for comment resolution of LB12, Comment Number 1333, 1491 and 792 Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15 Submission Slide 1 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Overview • Problems with Current CTRB parameters • Proposed Modified CTRB • Proposed 1 s resolution for CTR, CTA and piconet synchronization timing Submission Slide 2 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Problems with Current CTRB Parameters • The Allocation Period currently does not have an integer relationship to the Superframe Duration. The Allocation Period can be any allowed time value So: • Either the PNC has to adjust the superframe duration based on the application needs – which will create interoperability problems • Or, the GTS position changes on a superframe by superframe basis. Consequently, pseudostatic GTS slots are not possible in this scenario. It was shown in submission 01/436 that pseudo static GTS slots are needed for isochronous streams to avoid transmission stoppage due to corrupted Beacons. Submission Slide 3 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Problems with Current CTRB Parameters (continued) • Maximum Allocation Delay attempts to put a tight bound on delay variation, but it can only control the delay variation of the GTS slot, not the actual transmitted frames (details to follow) • Allocation for GTS slots with an allocation period greater than a superframe is not possible given the CTRB in Raju’s proposed text change for EPS. Submission Slide 4 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Adjusting the Superframe Duration to fit the most QoS demanding application • This is asking for interoperability problems. – What if two simultaneous applications are running that require different superframe durations? (One wants a slot every 33 ms +/- 1 us, the other wants one every 32 ms +/- 1 us.) These applications can not be both satisfied simultaneously. “Hey, my camcorder works great except when my kids are watching a DVD in the next room.” • Recommendation: Don’t adjust the SF duration to match application requirements Submission Slide 5 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Sources of Delay Variation - Clock Error • The application clock and the PNC clock are close, but not perfectly synchronized. • The clocks will drift relative to each other, causing delay variation at the receiver. Submission Slide 6 G T S Beacon G T S Beacon G T S Frame arrives too late and has to wait an entire superframe Beacon Beacon Frame arrives just in time for GTS slot G T S Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Sources of Delay Variation – Channel throughput must be greater than application throughput • Because of clock uncertainty, the allocated slot time must occur slightly more often than source arrivals in order to ensure the queue doesn’t grow to infinity • Also, if ACKs are used, extra throughput is required for potential retransmissions. • Hence, the queue will periodically be empty during an assigned slot and the slot will go unused resulting in delay variation Submission G T S Slide 7 G T S Beacon Frame arrives too late and has to wait an entire superframe Beacon G T S Beacon Beacon Frame arrives just in time for GTS slot G T S Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Frame delay variation versus GTS slot Maximum Allocation Delay • Summary of delay variation sources: – Any retransmissions at all will cause the frame delay variation to exceed the Maximum Allocation Delay for the GTS slot assignments. – Retransmissions require that the GTS throughput is actually higher than the application throughput to handle retransmissions. – The applications will not be synchronized with the PNC clock – and they should not be. This will cause slippage between the packet arrivals and the slots. • Recommendation: Eliminate Maximum Allocation Delay from the CTRB Submission Slide 8 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Bounding the superframe duration bounds the delay variation • Limiting the superframe duration limits the delay variation to: – 1 X Superframe duration for static GTS, – 2 x Superframe duration for dynamic GTS Submission Slide 9 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Delay Variation Smoothing • In order to compensate for MAC delay variation, the receiver will use a smoothing buffer at the convergence layer so that packets are passed smoothly to the application. Submission Slide 10 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Change to GTS Minimum and Desired GTS Time • Minimum and Desired GTS Times don’t tell PNC the units of time to be allocated. If the PNC allocates more than the minimum but less than the desired, it needs to know what increment of time to allocate • Replace Minimum and Desired GTS time with GTS time Unit, Minimum GTS time units and Desired GTS time units Submission Slide 11 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Sub rate slot allocations • The GTS slot per superframe field in the CTRB allows for sub rate allocations • The value of the “GTS Interval” field determines how often the GTS occurs. – 0 indicates “give me as much channel time as is available.” – 1 indicates a GTS every superframe, – 2 indicates a GTS every other superframe, – 3 or more indicates a GTS every 3rd, 4th, … superframe Submission Slide 12 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 CTRB Comparison Baseline CTRB (from Raju’s doc) octets: 1 1 2 2 2 2 Target DEVID Stream Index Allocation Period Minimum GTS Time Desired GTS Time Maximum Allocation Delay Proposed CTRB octets: 1 1 1 2 1 1 Target DEVID Stream Index GTS Interval GTS Time Unit Minimum GTS Time Units Desired GTS Time Units Submission Slide 13 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Limiting SF Duration (Comments 792, 1491) • Baseline slot resolution is 8 s. This is 800 bits at 100 Mbps • Currently superframe duration is limited to 100 ms. • Changing the resolution to 1 s will limits the superframe duration to 65.535 ms which will reduce the potential delay variation, allow more efficient slot assignments and more efficiently use the 16 bits of resolution Submission Slide 14 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Recommendation Summary • PNC chooses SF duration • Adopt new CTRB parameters – DEV chooses GTS Time unit – DEV requests Minimum and Desired number of time units – DEV requests GTS Interval (allows for sub-superframe rate allocations) instead of Allocation Period – Maximum Allocation delay deleted • DEV requests enough time for retransmissions • Jitter Buffer used to smooth out the delay variation • Change CTR and CTA, and piconet synchronization time parameters to 1 s resolution instead of 8 s Submission Slide 15 Dr. William Shvodian Feb. 2002 doc.: IEEE 802.15-02/076r0 Conclusions • Delay variation is limited by superframe size and is minimized by a smoothing buffer at the receiver • New CTRB parameters greatly simplify the PNC’s slot request/allocation processing Submission Slide 16 Dr. William Shvodian