doc.: IEEE e SubmissionSlide 1 Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Network-wide Time Synchronization for TDMA MAC] Date Submitted: [September, 2008] Source: [ChangSub Shin, Wun-Cheol Jeong, Soyoung Hwang, Anseok Lee, Seong-Soon Joo] Company [ETRI] Address [161 Gajeong-dong Yuseong-gu, Daejeon, Korea] Voice:[ ], FAX: [ ], Re: [IEEE P e Call For proposal] Abstract:[This document proposes time synchronization solution for IEEE e MAC] Purpose:[This document is a response to call for proposals] 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 ChangSub Shin et al September 2008

doc.: IEEE e SubmissionChangSub Shin et alSlide 2 Network-wide Time Synchronization for TDMA MAC ChangSub Shin, Wun-Cheol Jeong, Soyoung Hwang, Anseok Lee, Seong-Soon Joo ETRI September 2008 This work has been supported by the Ministry of Knowledge Economy (MKE) of the Republic of Korea under Grants 2008-F-052.

doc.: IEEE e Submission TG4e MAC requirements for industrial applications Low latency Robustness Low energy consumption IEEE MAC cannot support TG4e requirements TDMA based frequency hopping MAC => need time synchronization mechanism Motivation ChangSub Shin et alSlide 3 September 2008

doc.: IEEE e Submission Network-wide multi-hop time synchronization Support mesh topology Simple and reliable Guarantee time accuracy within error boundary Robust in change of topology Error recovery from a fail node Low control traffic for synchronization Time synchronization requirements for TDMA ChangSub Shin et alSlide 4 September 2008

doc.: IEEE e Submission Clock Drift Problem Frequency change with time Message-Delivery Uncertainty Problem send, access, transmission, propagation, reception and receive time Multi-hop Time-Synchronization Problem Error Increases with hop distance Dynamic topology change Overhead of control packet for sync Unreliable & unstable WSN Time Synchronization Problems ChangSub Shin et alSlide 5 September 2008

doc.: IEEE e Submission Decomposition of the message delivery uncertainties (1) Send Time—time used to assemble the message and issue the send request to the MAC layer on the transmitter side. Nondeterministic, hundreds of milliseconds error. (2) Access Time—waiting time to access channel for transmitting frame. Nondeterministic, varying from milliseconds up to seconds. (3) Transmission Time—the time it takes for the sender to transmit the message. Deterministic, tens of milliseconds. (4) Propagation Time—the time it takes for the message to transmit from sender to receiver once it has left the sender. highly deterministic, less than one microsecond (for ranges under 300 meters). (5) Reception Time—the time it takes for the receiver to receive the message. It is the same as the transmission time. The transmission and reception times overlapping. (6) Receive Time—time to process the incoming message and to notify the receiver application. Its characteristics are similar to that of send time. Directional Time Diffused Synchronization (DTDS) (1/10) ChangSub Shin et al September 2008 Slide 6 Source: The Flooding Time Synchronization Protocol [Miklos Maroti et al / SenSys’04]

doc.: IEEE e Submission Pair-wise synchronization Sender Insert timestamp value after backoff+CCA Send timestamp value in the sync frame Receiver Get timestamp value(defined 15.4 MAC) at receiving sync frame Synchronize with sender’s time Directional Time Diffused Synchronization (DTDS) (2/10) Backoff MAC Insert Timestamp CCATransmitting PHY Receiving PHY Sender Receiver Get Timestamp ChangSub Shin et al Slide 7 September 2008

doc.: IEEE e Submission Network-Wide Time Synchronization PAN Coordinator (sync source) Start synchronization by broadcasting sync frame Diffusion count value of Synchronization frame should be increased by one for each hop May offer additional information such as superframe structure Coordinator and end device Synchronization frame including the lowest diffusion depth or the latest timestamp value is accepted for time synchronization Keep its own diffusion depth counter for sync frame Reliable broadcasting sync frame using broadcast-ack address Refrain backward flooding with the diffusion count value Directional Time Diffused Synchronization (DTDS) (3/10) ChangSub Shin et alSlide 8 September 2008

doc.: IEEE e Submission Network-Wide Time Synchronization Directional Time Diffused Synchronization (DTDS) (4/10) ChangSub Shin et alSlide 9 A C B D F G IH J E Depth-1 Depth-2 Depth 3 Sync-source September 2008 (1) (2) (3) (4) (5) (6)

doc.: IEEE e Submission Select sync reference node from neighbor nodes Wait [X]time from first received sync packet Select time (2) Method 1 : Select the only the earliest time Method 2 : Average time value of all received sync time Directional Time Diffused Synchronization (DTDS) (5/10) ChangSub Shin et alSlide Method 1 Method 2 September 2008

doc.: IEEE e Submission Compensate clock method Get a time value from neighbor nodes at n times Average the consecutive time values Compensate own time value without re-sync frame Directional Time Diffused Synchronization (DTDS) (6/10) ChangSub Shin et al Re-sync interval Get a time value at n times and average September 2008 Slide 11 Re-sync interval Compensate time value without re-sync frame T e = n : the number of sampled value T k : current time T e : estimated time

doc.: IEEE e Submission advantage Simple Robust in failure of device Independent on network topology Flexible and adaptable Low traffic overhead Low synchronization error using MAC-layer timestamp Can support of global time information Directional Time Diffused Synchronization (DTDS) (7/10) ChangSub Shin et alSlide 12 September 2008

doc.: IEEE e Submission Design issues Clock accuracy Time duration of basic time slot Error boundary Guard time of basic time slot Re-synchronization interval Directional Time Diffused Synchronization (DTDS) (8/10) ChangSub Shin et al September 2008 Slide 13

doc.: IEEE e Submission Design time slot unit 2400– MHz O-QPSK PHY : max 133 bytes Preamble : 4 bytes SFD : 1 bytes PHR : 1 bytes Max PSDU size : 127 bytes Pure max Tx time => ms (133 bytes) We have to consider proper value in implementation => 1 tx + turn around time + 1 ack + 2 guard time Example value of slot unit => 625 symbols (10ms in 2.4) Directional Time Diffused Synchronization (DTDS) (9/10) ChangSub Shin et al September 2008 Slide 14

doc.: IEEE e Submission Error boundary and re-sync interval For example, error boundary : +/-1ms synchronization error : 50us Clock accuracy : +/-10ppm Per hop accumulated error : 0.8 us (average backoff delay(40ms)) 1 hop Re-sync interval < ( 1ms ms) / (20ppm) = 48s 5 hop Re-sync interval < ( 1ms ms – ms) / (20ppm) = 47.3s Directional Time Diffused Synchronization (DTDS) (10/10) ChangSub Shin et al September 2008 Slide 15

doc.: IEEE e Submission Procedure of Synchronization ChangSub Shin et al September 2008 Slide 16 A BC DE

doc.: IEEE e Submission PIB Define PIB, Primitives ChangSub Shin et al September 2008 Slide 17 attributeTypedescription macSyncInterval3 bytesThe number of interval times for re- synchronization. macReceivedTimestamp3 bytesTime of receiving the first bit of the SFD field. A symbol unit macReceivedTimestampListListA consecutive timestamp value of receiving sync frame. For S/W time conpensation Primitives NameRequestIndicationResponseConfirmdescription MLME-START-SYNCOOStart synchronization by PAN Coordinator MLME-SYNC-FAILONotify sync fail to the next higher layer

doc.: IEEE e Submission Sync frame : propagate timestamp for synchronization Timestamp : time value for synchronization Sync interval : the number of interval times for re- synchronization Diffusion depth : hop count from PAN Coordinator for preventing backward time update Define Frame Format (1/2) ChangSub Shin et al September 2008 Slide 18 4 bytes3 bytes 1 bytes2 bytes Timestamp_se cond Timestamp_usSync intervalDiffusion depthAck address

doc.: IEEE e Submission Sync request frame : request synchronization to neighbor nodes in case of loss synchronization Diffusion depth : sync requesting node’s hop count from PAN Coordinator Define Frame Format (2/2) ChangSub Shin et al September 2008 Slide 19 1 bytes Diffusion depth

doc.: IEEE e Submission Thank you very much for your attention Any Questions? ChangSub Shin et alSlide 20 September 2008