Dept. of Computer Science Embedded System Design for Network Time Synchronization So-Young Hwang, Dong-Hui Yu and Ki-Joune Li So-Young Hwang Dept. of Computer Science Pusan National Univ. 2004. 08. 25

Contents Introduction Embedded System for Network Time Synchronization General computer clock model Needs for time synchronization Approaches for time synchronization Embedded System for Network Time Synchronization Functional architecture A clock model Prototype implementation Experiments Plan and environment Experiment results Concluding Remarks 2004-08-25 EUC 2004

General Computer Clock Model Keep track of current time Various accounting purposes CPU utilization Disk I/O, and so on Consists of 3 component Quartz crystal Counter register Constant register Limits in accuracy and precision Inherent instability Environment elements Modifications of users, and errors of the system 2004-08-25 EUC 2004

Needs for Time Synchronization Critical piece of infrastructure for any distributed system Log file accuracy, auditing and monitoring Network fault diagnosis and recovery File time stamps Directory services Access security and authentication Distributed computing Scheduled operations Real-world time values / Interaction with users Purpose of synchronizing clocks Provide a global time base throughout a distributed system 2004-08-25 EUC 2004

Approaches for Time Synchronization Replaces a local clock with a higher cost, accurate and precise clock Found in mobile communication system PTS among the IS-95 CDMA base stations Request services from the external clocks that keep standard time Based on server-client model NTP on the Internet 2004-08-25 EUC 2004

Embedded System for Network Time Synchronization Design issues Acquire standard time Maintain accurate and precise local clock Distribute time information GPS Adapted as a reference clock source Time keeper Specified in clock model NTP Applied for time distribution Functional architecture 2004-08-25 EUC 2004

GPS Referenced Time Distribution Model Stratum 1 Server W/S NTP TIME SERVICE SNMP CONTROL GPS Satellites 2004-08-25 EUC 2004

Proposed Clock Model for Synchronization Time keeping mechanism System clock initialized by GPS Periodic signal of one second time interval is adapted as interrupt to synchronize the local clock 2004-08-25 EUC 2004

Prototype Implementation (1/2) Hardware 32bit RISC ARM7TDMI 100BaseT Ethernet GPS engine 10MHz VCTCXO Software OS: pSOSystem BSP TCP/IP protocol NTP, SNMP 2004-08-25 EUC 2004

Prototype Implementation (2/2) GPS engine Connected through RS-232C Sends position, status and date/time message every one second GPS message format Clock model implementation System clock and NTP clock are initialized synchronizing to the GPS reference clock 1PPS reference time interval Analyze error of local clock 2004-08-25 EUC 2004

Experiments Evaluation factor NTP is designed to produce 3 products Frequency of the local clock to evaluate accuracy and stability Use standard NTP query program to evaluate time distribution NTP is designed to produce 3 products Clock offset: amount to adjust the local clock Round-trip delay Dispersion: maximum error of the local clock  Timestamps exchange are used to determine 3 products client server ti-3 ti ti-2 ti-1 2004-08-25 EUC 2004

Evaluation Environment Time distribution Accuracy & stability HP53132A: Universal counter HP59551A: GPS synchronization module Linux W/S as NTP client Monitoring & data logging PC IEEE 488 GPIB, RS-232C interface 2004-08-25 EUC 2004

Evaluation (1/2) Frequency measurement result Traces the nominal frequency in about 0.5Hz error boundary Nominal frequency: ideal frequency Fig. Relationship between accuracy and stability 2004-08-25 EUC 2004

Evaluation (2/2) Variation of clock offset Variation of clock dispersion target system public server 1 public server 2 system peer 64% – 36% candidate 100% 2004-08-25 EUC 2004

Conclusion Computer clock has limits in accuracy and precision Inherent instability, environment elements, modifications of users, and errors of the system Needs to be synchronized with a standard clock Purpose of synchronizing clocks is to provide a global time base throughout a distributed system Design issues and implementation of an embedded system for network time synchronization Uses GPS as a standard reference time source Clock model to maintain accurate and precise time Offers UTC through the NTP 2004-08-25 EUC 2004

Future Work Clock model and clock discipline mechanism to maintain more accurate and stable time Wireless sensor networks Make extensive use of synchronized time Have unique requirements in the scope, lifetime, and precision of the synchronization achieved, as well as the time and energy required to achieve it Existing time synchronization methods need to be extended / modified to meet these new needs 2004-08-25 EUC 2004

How NTP works Multiple servers/peers provide redundancy and diversity NTP Messages Peer 1 Peer 2 Filter 1 Peer 3 Filter 2 Filter 3 Intersection and Clustering Algorithms Combining Algorithm Loop Filter LCO Timestamps P/F-Lock Loop Multiple servers/peers provide redundancy and diversity Clock filters select best from a window of eight clock offset samples Intersection and clustering algorithms pick best subset of peers and discard outliers Combining algorithm computes weighted average of offsets for best accuracy Loop filter and LCO implement hybrid phase/frequency-lock (P/F) feedback loop to minimize jitter and wander 2018/9/18

NTP Protocol Header and Timestamp Formats Strat Poll LI Mode VN NTP v3 and v4 Root Delay Root Dispersion Reference Identifier Reference Timestamp (64) Originate Timestamp (64) Receive Timestamp (64) Transmit Timestamp (64) Message Hash (64 or 128) NTP Protocol Header Format (32 bits) LI leap warning indicator VN version number (4) Strat stratum (0-15) Poll poll interval (log2) Prec precision (log2) Seconds (32) Fraction (32) NTP Timestamp Format (64 bits) Value is in seconds and fraction since 0h 1 January 1900 Key/Algorithm Identifier Cryptosum Authenticator (Optional) Extension Field 1 (optional) Extension Field 2… (optional) NTP v4 only Prec Extension Field (padded to 32-bit boundary) Field Length Field Type NTP v4 Extension Field Last field padded to 64-bit boundary authentication only 2004-08-25 EUC 2004