1. Aug. 2004
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Explanation and Revision of Previous Time Synchronization Proposal to IEEE b]
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 ]
Voice:[ ], FAX: [ ],
Re: [Response to call for proposal of IEEE b, Doc Number: b.]
Abstract: [Further explanation and revision of shared time-base distribution methods proposed in b]
Purpose: [Proposal to IEEE b Task Group]
Notice: This document has been prepared to assist the IEEE P 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
H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

2. Aug. 2004
Time Synchronization Proposal -Explanation and Revision for Shared Time-Base Distribution Methods Proposed in Document
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 Cases
Aug. 2004
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
Aug. 2004
Proposal Summary
Minimum changes to
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
Aug. 2004
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
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
Aug. 2004
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
Aug. 2004
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
Aug. 2004
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)
Aug. 2004
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
Aug. 2004
Implementation Choices
Beacon Enabled
Nonbeacon-enabled
Use Beacon
Beacons are transmitted regularly and periodically
Minimum modification
Using Ad Hoc Beacon
Synchronization frequency is implementation specific
Use General
Packet
Need to add more primitives and attributes
Synchronization can be initiated by higher level, assistant mechanism is needed
H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

11. Case I : Use Beacon As Synchronization Packet
Aug. 2004
Case I : Use Beacon As Synchronization Packet
Beacon provides natural synchronization embodiment in beacon-enabled network.
Use of beacon can minimize modification to
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
Aug. 2004
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.indication is already specified in , Page 75 §
H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

13. Case II : Use General Packet As Synchronization Packet
Aug. 2004
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
Aug. 2004
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 §
Sender reports the actual Tx time to next high layer
Add timestamp argument to MCPS-DATA.confirm, page 59 §
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
Aug. 2004
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
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
b-enhancements-to-ieee
H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

16. Discussion: Other High Accuracy Options
Aug. 2004
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
H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

17. Discussion: SNTP-Like High Accuracy Option
Aug. 2004
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
Aug. 2004
Conclusion
Works with both absolute clock and relative clock
Totally compatible with existing mechanisms
Minimum changes to
Can be easily extended to multi-hops cases
H. Shao, H. Dai, J. Zhang, Mitsubishi Electric

19. MSC : Basic Scheme (Use beacon)
Aug. 2004
MSC : Basic Scheme (Use beacon)
SENDER
RECEIVER
Next Higher
Layer
SENDER
MLME
RECEIVER
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)
Aug. 2004
MSC : High Accuracy Option (Use beacon)
SENDER
RECEIVER
Next Higher
Layer
SENDER
MLME
RECEIVER
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)
Aug. 2004
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)
Aug. 2004
MSC : High Accuracy Option (Use General Packet)
SENDER
RECEIVER
Next Higher
Layer
SENDER
MLME
RECEIVER
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