Method and system for time synchronization in communication networks

• US 7,813,382 B2
• Filed: 09/30/2005
• Issued: 10/12/2010
• Est. Priority Date: 09/30/2005
• Status: Active Grant

• Current Assignee: Robert Bosch GmbH
• Original Assignee: Robert Bosch GmbH
• Inventors: Srinivasan, Bhaskar, Venkatraman, Lakshmi, Manjeshwar, Arati
• Primary Examiner(s): Yao; Kwang B
• Assistant Examiner(s): DUDA, ADAM K
• Application Number:
  US11/241,298
• Publication Number:
  US 20070076762A1
• Time in Patent Office: 1,838 Days
• Field of Search: 370/503, 370/507, 370/509, 370/510, 370/512, 370/513, 370/514, 370/520, 370/350, 370/301, 370/326, 370/395.4, 370/508, 370/519
• US Class Current: 370/503
• CPC Class Codes: H04B 7/2696 Over the air autonomous synchronisation, e.g. by monitoring network activity H04B7/2693 takes precedence
• CPC Class Codes: H04J 3/0638 Clock or time synchronisation among nodes; Internode synchronisation synchronization for ring networks H04L12/422; data switching networks with synchronous transmission H04L12/43
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines

Abstract

A method of synchronizing a network includes transmitting a tone signal to convey time information and setting a local time according to the conveyed time information. The method may also include detecting an occurrence of a predefined aspect of the tone signal, and setting a local time based on the occurrence of the predefined aspect of the signal.

Claims

1. A method of synchronizing a receiver clock of a receiver to a transmitter clock of a transmitter within a network, comprising:

waking up the receiver at a wake-up time;

receiving by the receiver a tone signal transmitted by the transmitter, the tone signal beginning at a start time and ending at an end time that is different from the start time; and

at the end time, setting the receiver clock to a predefined time to synchronize the receiver clock to the transmitter clock;

wherein;

the wake-up time is a receiver clock time that is set such that a period extending from the wake-up time to the predefined time is at least as long as a maximum time skew of the receiver clock relative to the transmitter clock that occurs between two consecutive synchronizations; and

the predefined time is set at the receiver prior to the transmission of the tone signal.

2. The method of claim 1, wherein the signal includes a preamble followed by a start symbol.

3. The method of claim 1, wherein the tone signal does not include any data packets.

4. The method of claim 1, wherein the network includes wireless sensor nodes.

17. The method of claim 1, wherein the maximum time skew is due to clock crystal inaccuracies.

18. The system of claim 1, wherein the more often the receiver and transmitter clocks are synchronized, the shorter the period extending from the wake-up time to the predefined time.

5. A method for synchronizing a receiver clock of a receiver to a transmitter clock of a transmitter in a communication network, comprising:

scheduling the receiver to periodically wake up at wake-up times;

waking up the receiver at the wake-up times;

for each wake-up;

receiving by the receiver a tone signal from the transmitter, the tone signal beginning at a start time, and ending at an end time that is different from the start time;

detecting by the receiver an occurrence of a predefined aspect of the tone signal; and

setting the receiver clock to a predefined time at an instant when the receiver detects the occurrence of the predefined aspect of the signal to synchronize the receiver clock to the transmitter clock;

wherein;

the predefined aspect is the ending of the tone signal;

the wake-up time is a receiver clock time that is set such that a period extending from the wake-up time to the predefined time is at least as long as a maximum time skew between the receiver clock relative to the transmitter clock that occurs between two consecutive synchronizations.

6. The method of claim 5, wherein the tone signal is transmitted for at least a period selected based on a calculation whose output period length value is inversely proportionate to a frequency at which synchronizing is performed.

7. The method of claim 5, wherein the elements of the communication network form a chain topology and the method is repeated through the chain.

8. The method of claim 5, wherein elements of the communication network form a tree topology and the method is repeated through each branch of the tree.

19. The method of claim 5, wherein the maximum skew is due to clock crystal inaccuracies.

20. The method of claim 5, wherein the tone signal is transmitted for at least a period selected based on a calculation whose output period length value is dependent upon how often the receiver and transmitter clocks are synchronized.

21. The method of claim 20, wherein the more often the receiver and transmitter clocks are synchronized, the shorter the output period length.

9. A communication system, comprising:

a transmitter operating under a transmitter clock to transmit a signal having a start time and an end time that is different than the start time; and

a receiver operating under a receiver clock to;

wake up at a wake-up time;

receive the signal; and

at the end time of the signal, set the receiver clock to a predefined time to synchronize the receiver clock to the transmitter clock;

wherein;

the wake-up time is a receiver clock time that is set such that a period extending from the wake-up time to the predefined time is at least as long as a maximum time skew of the receiver clock relative to the transmitter clock that occurs between two consecutive synchronizations; and

the predefined time for synchronization is set at the receiver prior to the transmission of the signal.

10. The communication system of claim 9, wherein the signal is a tone.

11. The communication system of claim 10, wherein the tone includes a preamble followed by a start symbol.

12. The communication system of claim 9, wherein the receiver periodically wakes up from a sleep cycle to receive the signal, and the signal is transmitted for at least a period selected based on a calculation whose output period length value is inversely proportionate to a frequency at which the setting of the receiver clock to the predefined time is performed.

22. The system of claim 9, wherein the maximum time skew is due to clock crystal inaccuracies.

13. An apparatus operating under a local clock for synchronizing a communication network comprising:

a first arrangement to wake up at a wake-up time to receive a tone signal, the tone signal beginning at a start time, and ending at an end time that is different than the start time;

a second arrangement to detect an occurrence of a predefined aspect of the tone signal; and

a third arrangement to set the local clock to a predefined time at the occurrence of the predefined aspect of the tone signal;

wherein;

the wake-up time is a local clock time that is set such that a period extending from the wake-up time to the predefined time is at least as long as a maximum time skew of the local clock relative to a transmitter clock that occurs between two consecutive synchronizations;

the predefined aspect is an ending of the tone signal; and

the predefined time is set at the apparatus prior to the receipt of the tone signal.

14. A method of synchronizing a network, comprising:

predefining a first predefined time in a memory of a first receiver node;

waking up the first receiver node at a wake-up time;

receiving by the first receiver node a tone signal transmitted, subsequent to the predefining, by a transmitter node, the tone signal beginning at a start time, and ending at an end time that is different from the start time;

responsive to the received tone signal, setting a time of a local clock of the first receiver node to the stored first predefined time,

wherein the wake-up time is a local clock time that is set such that a period extending from the wake-up time to the predefined time is at least as long as a maximum time skew of the local clock relative to a transmitter clock that occurs between two consecutive synchronizations.

15. The method of claim 14, wherein the tone signal does not include data stating a time.

16. The method of claim 14, wherein:

the network includes a plurality of hierarchically arranged nodes so that the nodes communicate sequentially from a higher level to a lower level, the nodes including the transmitter and first receiver nodes; and

the method further comprises;

subsequent to the setting the time of the local clock, the first receiver node transmitting a tone signal to at least one second receiver node, the at least one second receiver node being of a hierarchically lower level than the first receiver node;

responsive to receipt of the tone signal transmitted by the first receiver node, setting, at each of the at least one second receiver node, a time of a respective clock of the respective second receiver node to a second predefined time later than the first predefined time.