1. 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.

2. 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
EUC 2004

3. 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
EUC 2004

4. 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
EUC 2004

5. 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
EUC 2004

6. 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
EUC 2004

7. GPS Referenced Time Distribution Model
Stratum 1 Server
W/S
NTP TIME SERVICE
SNMP CONTROL
GPS Satellites
EUC 2004

8. 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
EUC 2004

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

10. 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
EUC 2004

11. 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
EUC 2004

12. 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
EUC 2004

13. 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
EUC 2004

14. 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%
EUC 2004

15. 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
EUC 2004

16. 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
EUC 2004

17. 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

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
EUC 2004