Method, system and device for clock synchronization over time-varying and lossy networks
The present invention is directed towards a method, a system, and a device which allow an improved synchronization of a system-wide timing information over a network. Hence, slave clocks can be synchronized to a high quality clock, such as a master clock.
1. A method for synchronization of a client clock to a server clock over a network, the method comprising:
determining, within a provided time horizon, clock offsets by exchanging synchronization messages between the client and server clocks;
weighting the clock offsets by their measurement accuracy to obtain a weighted clock offset; and
adjusting the client clock as a function of the weighted clock offset such that the client clock is synchronized to the server clock,
wherein the weighted clock offset includes an estimated clock phase and clock frequency,
wherein the time horizon applies to one of a group of estimation techniques, the group comprising a Moving Horizon Estimation or line fitting over a sliding window technique,
wherein the client clock determines the clock offsets as a function of time stamps, each time stamp indicating a send or receive operation of the synchronization messages, and
wherein the time stamps are based on a local free-running oscillator.
2. The method according to claim 1 , wherein the clock offsets are determined as a function of a transmission time measured between sending and receiving the synchronization messages.
3. The method according to claim 1 , the weighting by the measurement accuracy is performed by selecting a measurement variance parameter as an inverse weighting parameter.
4. The method according to claim 3 , wherein in case all sending and receiving operations are performed properly a default value is provided for the variance parameter.
5. The method according to claim 3 , wherein in case the sending or the receiving operation is failed an infinite value is provided for the variance parameter.
6. The method according to claim 5 , wherein in case one determined clock offset differs from previously determined clock offsets to a specified extend an infinite value is provided for the variance parameter.
7. The method according to claim 6 , wherein the time horizon is determined by at least one of a group of techniques, the group comprising a number of previous message exchanges, preferably the last N messages, and a determined time span.
8. The method according to claim 7 , wherein determining the clock offsets considers asymmetric transmission times between the client and server clocks.
9. The method according to claim 1 , wherein in case the sending or the receiving operation is failed an infinite value is provided for a variance parameter.
10. The method according to claim 1 , wherein in case one determined clock offset differs from previously determined clock offsets to a specified extend an infinite value is provided for a variance parameter.
11. The method according to claim 1 , wherein the time horizon is determined by at least one of a group of techniques, the group comprising a number of previous message exchanges, preferably the last N messages, and a determined time span.
12. The method according to claim 1 , wherein determining the clock offsets considers asymmetric transmission times between the client and server clocks.
13. The method according to claim 1 , wherein the measurement accuracy is perceived as a function of at least one of a time stamping inaccuracy due to hardware and software delays in the clocks, a message loss in the network, and time-varying delays and delay asymmetries due to the network load and congestion.
14. A non-transitory computer-readable medium containing program instructions for causing a computer to perform the method of claim 1 .
15. A system for synchronization of a client clock to a server clock over a network, the system comprising:
a determining circuit being arranged to determine, within a provided time horizon, at least one clock offset by exchanging synchronization messages between the client and server clocks;
a weighting circuit being arranged to weight the determined at least one clock offset by their measurement accuracy, in order to obtain a weighted clock offset including an estimated clock phase and clock frequency; and
an adjustment circuit being arranged to adjust the client clock as a function of the weighted clock offset such that the client clock is synchronized to the server clock,
wherein the time horizon applies to one of a group of estimation techniques, the group comprising a Moving Horizon Estimation or line fitting over a sliding window technique,
wherein the client clock determines the at least one clock offset as a function of time stamps, each time stamp indicating a send or receive operation of the synchronization messages, and
wherein the time stamps are based on a local free-running oscillator.
16. A client being arranged to perform a synchronization of a client clock to a server clock over a network, the client comprising:
a determining circuit being arranged to determine, within a provided time horizon, at least one clock offset by exchanging synchronization messages between the client and server clocks;
a weighting circuit being arranged to weight the determined clock offsets by their measurement accuracy, to obtain a weighted clock offset including an estimated clock phase and clock frequency; and
an adjustment circuit being arranged to adjust the client clock as a function of the weighted clock offset such that the client clock is synchronized to the server clock, wherein the time horizon applies to one of a group of estimation techniques, the group comprising a Moving Horizon Estimation or line fitting over a sliding window technique,
wherein the client clock determines the at least one clock offset as a function of time stamps, each time stamp indicating a send or receive operation of the synchronization messages, and
wherein the time stamps are based on a local free-running oscillator.