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: [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:[+82-42-860-1668], FAX: [+82-42-869-1712], E-Mail:[shincs@etri.re.kr] Re: [IEEE P802.15.4e Call For proposal] Abstract: [This document proposes time synchronization solution for IEEE802.15.4e MAC] Purpose: [This document is a response to call for proposals] 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. ChangSub Shin et al

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

3. Motivation • TG4e MAC requirements for industrial applications • Low latency • Robustness • Low energy consumption • IEEE802.15.4-2006 MAC cannot support TG4e requirements • TDMA based frequency hopping MAC • => need time synchronization mechanism ChangSub Shin et al

4. Time synchronization requirements for TDMA • 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 ChangSub Shin et al

5. WSN Time Synchronization Problems • 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 ChangSub Shin et al

6. Directional Time Diffused Synchronization (DTDS) (1/10) • 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. Source: The Flooding Time Synchronization Protocol [Miklos Maroti et al / SenSys’04] ChangSub Shin et al

7. Directional Time Diffused Synchronization (DTDS) (2/10) • 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 Insert Timestamp Backoff CCA Transmitting Sender MAC PHY Get Timestamp Receiving Receiver PHY ChangSub Shin et al

8. Directional Time Diffused Synchronization (DTDS) (3/10) • 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 Slide 8 ChangSub Shin et al

9. Directional Time Diffused Synchronization (DTDS) (4/10) • Network-Wide Time Synchronization (1) Sync-source A Depth-1 (3) C (2) B Depth-2 F D E (5) (6) Depth 3 (4) G J H I Slide 9 ChangSub Shin et al

10. Directional Time Diffused Synchronization (DTDS) (5/10) • 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 65 65 70 75 70 75 70 65 Method 2 Method 1 Slide 10 ChangSub Shin et al

11. Directional Time Diffused Synchronization (DTDS) (6/10) • 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 n : the number of sampled value Tk : current time Te : estimated time Te = Re-sync interval Re-sync interval Compensate time value without re-sync frame Get a time value at n times and average ChangSub Shin et al

12. Directional Time Diffused Synchronization (DTDS) (7/10) • 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 ChangSub Shin et al

13. Directional Time Diffused Synchronization (DTDS) (8/10) • Design issues • Clock accuracy • Time duration of basic time slot • Error boundary • Guard time of basic time slot • Re-synchronization interval ChangSub Shin et al

14. Directional Time Diffused Synchronization (DTDS) (9/10) • Design time slot unit • 2400–2483.5 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 => 4.256 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) ChangSub Shin et al

15. Directional Time Diffused Synchronization (DTDS) (10/10) • 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 - 0.05ms) / (20ppm) = 48s • 5 hop Re-sync interval < ( 1ms - 0.05ms – 0.004 ms) / (20ppm) = 47.3s ChangSub Shin et al

16. Procedure of Synchronization A B C D E ChangSub Shin et al

17. Define PIB, Primitives • PIB • Primitives ChangSub Shin et al

18. Define Frame Format (1/2) • 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 ChangSub Shin et al

19. Define Frame Format (2/2) • Sync request frame : request synchronization to neighbor nodes in case of loss synchronization • Diffusion depth : sync requesting node’s hop count from PAN Coordinator ChangSub Shin et al

20. Thank you very much for your attention Any Questions? ChangSub Shin et al