Aug 2004 doc IEEE 802 15 04 0446

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Project: IEEE P 802. 15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Explanation and Revision of Previous Time Synchronization Proposal to IEEE 802. 15. 4 b] Date Submitted: [August 23, 2004] Source: [Huai-Rong Shao, Hui Dai and Jinyun Zhang] Company [Mitsubishi Electric Research Labs] Address [8 th Floor, 201 Broadway, Cambridge, MA 02139 ] Voice: [617 -621 -7517], FAX: [617 -621 -7550], EMail: [shao@merl. com] Re: [Response to call for proposal of IEEE 802. 15. 4 b, Doc Number: 15 -04 -0239 -00 -004 b. ] Abstract: [Further explanation and revision of shared time-base distribution methods proposed in 15 -040313 -01 -004 b] Purpose: [Proposal to IEEE 802. 15. 4 b Task Group] Notice: This document has been prepared to assist the IEEE P 802. 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 P 802. 15. Submission 1 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Time Synchronization Proposal -Explanation and Revision for Shared Time-Base Distribution Methods Proposed in Document 15 -04 -0313 -01 Huai-Rong Shao, Hui Dai and Jinyun Zhang Mitsubishi Electric Research Laboratories Submission 2 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Outline • Motivation • Method – Basic Scheme – High Accuracy Options • Cases – Use beacon for synchronization – Use general packets for synchronization • Discussions • Message Sequence Charts Submission 3 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Motivation and Objectives • Time synchronization is important – – – Maintain superframe/slot synchronization among devices fine-tuned coordination of wake/sleep duty cycles to reduce power consumption Preserve the event orders Time synchronization is also important to security protocols since the clock reading is often used for encryption key generation Loop free routing (Robert Poor said) • Objective – A shared time-base distribution method with minimum modifications to 802. 15. 4 -2003 – Simple, Efficient and Flexible • Performance Consideration – Accuracy – Overhead Submission 4 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Method Summarization • Minimum changes to 802. 15. 4 -2003 • Shared time-base distribution method • Basic scheme for both beacon-enabled and non-beaconenabled network • Allow High Accuracy Options Submission 5 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Basic Scheme • Step 1 : At T 0 , sender obtains a timestamp • Step 2 : At T 1 , a synchronization packet containing timestamp is sent on wireless channel • Step 3 : At T 2 , receivers received the synchronization packet. They then set local clocks to the timestamp carried in synchronization packet Submission 6 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Basic Scheme (cont’d) Sender Receiver Step 1: Get timestamp at T 0 T 1 T 2 time Step 3: Receive packet at T 2. Receiver sets local clock to the timestamp carried in the packet Step 2: Send Synchronization frame at T 1 Submission 7 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b High Accuracy Option • Step 1 : Basic Scheme • Step 2 : Sender sends a second frame to receiver – The second frame contains the actual transmitting time of synchronization packet • Step 3 : Receiver adjusts the local clock again for higher accuracy Submission 8 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b High Accuracy Option (cont’d) Sender Receiver Step 1 a: Get timestamp at T 0 T 1 T 2 T 3 Step 1 b: Send Synchronization frame at T 1 Step 2: Send real Timestamp, i. e. T 1, at T 3 Submission Step 1 c: Receive Synchronization packet at T 2. Receiver sets local clock Step 3: Receiver adjusts local clock 9 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Implementation Choices Beacon Enabled Non-beacon enable Use Beacon • Beacons are transmitted regularly and periodically • Minimum modification Use General Packet Submission • Beacons are transmitted occasionally and indeterminately. • Require assistant mechanism to decide when to send synchronize packet • Need to add Primitives and attributes • Synchronization can be initiated by higher level 10 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Case I : Use Beacon As Synchronization Packet • Beacon provides natural synchronization embodiment in beacon-enabled network. • Use of beacon can minimize modification to 802. 15. 42003 • Beacon-enabled WPAN – Beacons are transmitted periodically • Non-beacon WPAN – Beacons are transmitted occasionally and indefinitely – Require assistance to schedule beacon transmission for synchronization Submission 11 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Case I : Interfaces and Parameters • Add symbol offset captured position attribute to MIB because this could be vendor specific – mac. Sync. Symbol. Offset • Receiver reports time to next higher layer – Timestamp can be carried in the beacon payload. – mac. Beacon. Tx. Time in page 136, Table 71 stores the actual beacon transmitted time – MLME-BEACON-NOTIFY. indication is already specify in 802. 15. 4 -2003, Page 75 § 7. 1. 5. 1 Submission 12 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Case II : Use General Packet As Synchronization Packet • Beacon-enabled and Non-beacon-enabled WPAN – Require higher layer to schedule synchronization packet transmission – More primitives are needed • Up to higher level to select configurations – Synchronization Frequency – Accuracy Submission 13 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Case II : Interface and Parameters • Add symbol offset captured position attribute to MIB because this could be vendor specific – mac. Sync. Symbol. Offset • Receiver reports timestamp to next higher layer – Add timestamp argument to MCPS-DATA. indication, page 60 § 7. 1. 1. 3 • Sender reports the actual Tx time to next high layer – Add timestamp argument to MCPS-DATA. confirm, page 59 § 7. 1. 1. 2 – OR Add mac. Sync. Tx. Time to MIB to record the synchronization packet transmission time Submission 14 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Discussion: Timestamp in Basic Scheme • Timestamp carried in synchronization packet – A synchronization packet’s actual transmitting time can’t be carried in itself. Because actual transmitting time is captured during packet transmission according to 802. 15. 4 -2003. – A timestamp different from actual transmitting time is put into synchronization packet at MAC layer or above. – The error can be possibly reduced by estimation. • Detailed error analysis and estimation are presented at the previous proposal – 15 -04 -0313 -01 -004 b-enhancements-to-ieee-802 -15 -4 Submission 15 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Discussion: Other High Accuracy Options • SNTP-like Method – Step 1 : Basic Scheme – Step 2 : Receivers send response frame to the sender • Response frame contains received timestamp and the previous sync frame sequence number – Step 3 : Sender sends back calculation to reduce the propagation error • Problem – Too much overhead for WPAN – It mainly focus on reducing propagation error, which is NOT the major error source in 802. 15. 4 Submission 16 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Discussion: SNTP-Like High Accuracy Option Sender Receiver Step 1 a: Get timestamp at T 0 T 1 T 2 T 3 T 4 Step 1 c: Receive Synchronization packet at T 2. Step 1 b: Send Synchronization frame at T 1 Step 3: Sender sends feedback to reduce the propagation error at T 4 Submission Step 2: Receiver send response to sender at T 3 17 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b Conclusion • • Works with both absolute clock and relative clock Totally compatible with existing mechanisms Minimum changes to 802. 15. 4 -2003 Can be easily extended to multi-hops cases Submission 18 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b MSC : Basic Scheme (Use General Packet) SENDER Next High Layer RECEIVER MAC Layer Next High Layer MCPS-DATA. request Synchronization Packet MCPS-DATA. indication With timestamp Submission 19 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b MSC : High Accuracy Option (Use General Packet) SENDER Next Higher Layer RECEIVER MLME SENDER MLME Next Higher Layer MCPS-DATA. request Synchronization Packet MCPS-DATA. confirm (with time stamp) MCPS-DATA. request MCPS-DATA. indication (with time stamp) Synchronization Follow Up MCPS-DATA. indication (with time stamp) Submission 20 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b MSC : Basic Scheme (Use beacon) SENDER Next Higher Layer RECEIVER MLME SENDER MLME Next Higher Layer Beacon ( with Synchronize timestamp) MLME-BEACON-NOTIFY. indication (with time stamp) Submission 21 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

Aug. 2004 doc. : IEEE 802. 15 -04 -0446 -01 -004 b MSC : High Accuracy Option (Use beacon) SENDER Next Higher Layer RECEIVER MLME SENDER MLME Beacon ( with Synchronize timestamp) Synchronization Follow Up (with mac. Sync. Tx. Time) Submission 22 Next Higher Layer MLME-BEACON-NOTIFY. indication (with time stamp) MCPS-DATA. indication H. Shao, H. Dai, J. Zhang, Mitsubishi Electric