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

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [ Explanation and Revision of Previous Time Synchronization Proposal to IEEE 802.15.4b ] Date Submitted: [ August 23, 2004 ]

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

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Explanation and Revision of Previous Time Synchronization Proposal to IEEE 802.15.4b] Date Submitted: [August 23, 2004] Source: [Huai-Rong Shao, Hui Dai and Jinyun Zhang] Company [Mitsubishi Electric Research Labs] Address [8th 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 802.15.4b, Doc Number: 15-04-0239-00-004b.] Abstract: [Further explanation and revision of shared time-base distribution methods proposed in 15-04-0313-01-004b] Purpose: [Proposal to IEEE 802.15.4b Task Group] 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. H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  2. 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 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  3. Outline • Proposal Summary • Motivation and Objectives • Method • Basic Scheme • High Accuracy Options • Cases • Use beacon for synchronization • Use general packets for synchronization • Discussions • Message Sequence Charts H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  4. Proposal Summary • Minimum changes to 802.15.4-2003 • Add macSyncSymbolOffset attribute to MIB • Shared time-base distribution method • Basic scheme for both beacon-enabled and non-beacon network • Allow High Accuracy Options H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  5. 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 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  6. Basic Scheme • Step 1 : At T0 , sender obtains a timestamp • Step 2 : At T1 , a synchronization packet containing timestamp is sent on wireless channel • Step 3 : At T2 , receivers receive the synchronization packet. They then adjust local clocks according to the timestamp carried in synchronization packet H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  7. Basic Scheme (cont’d) Sender Receiver Step1: Get timestamp at T0 T0 T1 T2 time Step3: Receive packet at T2. Receiver adjusts local clock according to the timestamp carried in the packet Step2: Send Synchronization frame at T1 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  8. 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 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  9. High Accuracy Option (cont’d) Sender Receiver Step1 a: Get timestamp at T0 T0 T1 T2 T3 Step1 b: Send Synchronization frame at T1 Step 1 c: Receive Synchronization packet at T2 .Receiver adjusts local clock Step 2: Send real Timestamp, i.e. T1, at T3 Step 3: Receiver adjusts local clock H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  10. Implementation Choices H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  11. Case I : Use Beacon As Synchronization Packet • Beacon provides natural synchronization embodiment in beacon-enabled network. • Use of beacon can minimize modification to 802.15.4-2003 • Beacon-enabled WPAN • Beacons are transmitted periodically • Nonbeacon-enabled WPAN • Ad hoc Beacons can be used • Association requires Beacon anyway • Synchronization frequency is implementation specific H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  12. Case I : Interfaces and Parameters • Add symbol offset captured position attribute to MIB because this could be vendor specific • macSyncSymbolOffset • No other modification • Timestamp can be carried in the beacon payload. • macBeaconTxTime (page 136, Table 71) stores the actual beacon transmitted time • MLME-BEACON-NOTIFY.indicationis already specified in 802.15.4-2003, Page 75§7.1.5.1 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  13. Case II : Use General Packet As Synchronization Packet • Beacon-enabled and Nonbeacon-enabled WPAN • Require higher layer to schedule synchronization packet transmission • More primitives are needed • Up to higher level to select configurations • Synchronization Frequency • Accuracy H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  14. Case II : Interface and Parameters • Add symbol offset captured position attribute to MIB because this could be vendor specific • macSyncSymbolOffset • 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 macSyncTxTime to MIB to record the synchronization packet transmission time H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  15. 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 • 15-04-0313-01-004b-enhancements-to-ieee-802-15-4 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  16. 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 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  17. Discussion: SNTP-Like High Accuracy Option Sender Receiver Step1 a: Get timestamp at T0 T0 T1 T2 T3 T4 Step 1 c: Receive Synchronization packet at T2 . Step1 b: Send Synchronization frame at T1 Step 3: Sender sends feedback to reduce the propagation error at T4 Step 2: Receiver send response to sender at T3 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  18. 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 H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  19. MSC : Basic Scheme (Use beacon) SENDER RECEIVER RECEIVER MLME Next Higher Layer SENDER MLME Next Higher Layer Beacon ( with Synchronize timestamp) MLME-BEACON-NOTIFY.indication (with time stamp) H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  20. MSC : High Accuracy Option (Use beacon) SENDER RECEIVER RECEIVER MLME Next Higher Layer SENDER MLME Next Higher Layer Beacon ( with Synchronize timestamp) MLME-BEACON-NOTIFY.indication (with time stamp) Synchronization Follow Up (with macSyncTxTime) MCPS-DATA.indication H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  21. MSC : Basic Scheme (Use General Packet) SENDER RECEIVER MAC Layer Next High Layer MAC Layer Next High Layer MCPS-DATA.request Synchronization Packet MCPS-DATA.indication With timestamp H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

  22. MSC : High Accuracy Option (Use General Packet) SENDER RECEIVER RECEIVER MLME Next Higher Layer SENDER MLME Next Higher Layer MCPS-DATA.request Synchronization Packet MCPS-DATA.confirm (with time stamp) MCPS-DATA.indication (with time stamp) MCPS-DATA.request Synchronization Follow Up MCPS-DATA.indication (with time stamp) H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

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