Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: IEEE802.15.3: SSID and PNID proposal Date Submitted: 07 October, 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: bshvodian@REMOVETHISxtremespectrum.com Re: [LB 19 comments 56, 349, 350, 351, 352, 353, 354, 355, 387, 407, 513 ] Abstract: MTS and Slotted Aloha comment resolution Purpose: To provide information for LB 19 comment resolution for MTS comments 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 Dr. William Shvodian

2. Overview • MTS History • Directed MTSs • CSMA/CA issues in 802.15.3 D11 • Slotted Aloha vs. CSMA/CA in a finite slot Dr. William Shvodian

3. MTS history • MTSs have been in the draft for over a year and Raju Gubbi was the only voter with outstanding comments on them from the previous two drafts. The other comments should be rejected since they are on things that were in the draft since LB 12. • Never the less… Dr. William Shvodian

4. Directed MTSs • Directed MTSs provide a mechanism for assigning CFP time for PNC to DEV or DEV to PNC contention free transmission. • Contention free transmission of command frames is highly desirable. Dr. William Shvodian

5. Comments on CSMA/CA as specified in 802.15.3 • Is RIFS required at the start of a CAP? It shouldn’t be. Text doesn’t specifically address the start of a CAP. • Currently RIFS is required even after an immediate ACK. It shouldn’t be. Dr. William Shvodian

6. Slotted Aloha vs. slotted CSMA/CA • The performance of Slotted Aloha is well known, well studied and well understood. • Slotted Aloha combined with TDMA has been in the literature for decades. CSMA combined with TDMA has not. • I couldn’t find a single paper analyzing the performance of CSMA/CA in a restricted size TDMA slot. Dr. William Shvodian

7. Slotted Aloha efficiency • The minimum size for a slotted aloha Association MTS is the size of the association command plus ACK, two SIFSs and guard time. • CSMA/CA requires a CAP size larger than the above minimum in order to be effective. Dr. William Shvodian

8. Slotted CSMA/CA • In 802.11, CSMA takes up most of, or at least a large chunk of the superframe • In 802.15.3 it is desirable to be able to use as much of the superframe as possible for isochronous streams. • If the CAP is set to the minimum size ( the size of one association, plus ACK plus two SIFS) all association frames transmitted in the CAP must begin precisely at the start of the CAP in order to fit. • The carrier sense mechanism serves no purpose if the CAP is set to this size. • The performance of CSMA/CA becomes like a poorly designed slotted aloha. Dr. William Shvodian

9. Association Time • What is the time for an association frame plus ACK? • 2*preamble+2*header+association payload+FCS+2*SIFS=2*10*16*90.9 ns + 2*14*8*90.9/2 ns + 17*8*90.9/2 ns +2* 8*90.9/2 ns + 2*10us = 29088 + 10180.8 + 6181.2 + 727.2 + 20000 ns = 66.18 s • What is the backoff time? • 0-7,0-15,0-31, 0-63 => 5.8*7  s = 0-40.6 s, 0-87 s, 0-179.8 s, 0-365.4 s • First backoff is always less than 1 minimum association time Dr. William Shvodian

10. CSMA/CA in TG3 • Can a CAP be sized for the minimum association time? • “The DEV first waits a RIFS duration, 8.4.1, from when the medium is determined to be idle before beginning the backoff algorithm.” • This man not apply to the start of CAP. The text is unclear. • Impact: Even if the CAP is idle, the DEV must perform a back-off and cannot transmit in the first CAP if the CAP is sized for one association frame and an ACK. • If RIFS (or BIFS?) is not required at the start of CAP, 1/8 probability that backoff = 0 and the DEV will transmit in CAP2. 7/8 of the time the backoff time will be >0 and the transmission will happen in CAP3. • Backoff will complete during the CAP2, but transmission must wait until the next CAP • Collisions will ALWAYS occur if two DEVs generate a frame during the same CFP or busy CAP, and the next CAP is idle First Transmission always starts here if previous CAP idle Frame arrives at Tx DEV during CFP or busy CAP Backoff CFP & Beacon CFP & Beacon CFP & Beacon CAP2 CAP3 CAP1 Dr. William Shvodian

11. CSMA/CA in TG3 • CSMA/CA with a CAP sized for the minimum time required for an association will perform worse than Slotted Aloha because the first backoff interval is less than a slot and the maximum backoff is only 6 slots. Dr. William Shvodian

12. Summary • Directed MTS allow for un-contended access for command frames. There is no other documented description of the PNC assigning GTSs to DEVs for commands. • Slotted Aloha for association outperforms an association sized CSMA/CA CAP. • Some SG3a proposers do not believe UWB can perform CCA (see 02/379). Depending on the UWB proposal chosen by SG3a, slotted aloha may be required for association. • Association, open and directed MTSs have been in the draft for over a year now and should not be removed. Dr. William Shvodian