Method for synchronization in networks
The invention relates to a method for synchronization in networks, whereby the local time (t loc ) which is valid at the particular node, is updated at different nodes. For that purpose, timing messages are regularly transmitted by a freely selectable superior node (N 1 ; N 3 ; N 6 ) and only by a superior node to an inferior node (N 2 , N 3 ; N 4 -N 6 ; N 7 ), which receives the timing messages (M 1 -M 8 ) and analyzes said messages for updating the local time (t loc ) thereof. A minimum propagation time (d min ) is determined for a timing message (M 1 -M 8 ) between an inferior node (N 1 ; N 3 ; N 6 ) and a superior node (N 2 , N 3 ; N 4 -N 6 ; N 7 ). When the inferior node (N 2 , N 3 ; N 4 -N 6 ; N 7 ) receives a timing message (M 1 -M 8 ), said inferior node extracts the local time of the superior node (N 1 ; N 3 ), which is contained in said timing message (M 1 -M 8 ) and adds the minimum propagation time (d min ) thereto, in order to generate a reference time (t comp ,1 -t comp ,8 ). Said reference time (t comp ,1 -t comp ,8 ) is then compared with the proper local time (t loc ). If the reference time is retarded in relation to the proper local time (t loc ), said proper local time (t loc ) is not updated. If said reference time is advanced in relation to the proper local time (t loc ).
1. A method for synchronization in a network having a plurality of nodes, the plurality of nodes comprising higher-level nodes and lower-level nodes and the plurality of nodes including a first lower-level node, the method comprising:
the first node regularly receiving time messages transmitted from one or more higher-level nodes of the plurality of nodes, including receiving a time message from a higher-level node, wherein the time message comprises a local time of the higher-level node, and wherein a minimum delay time is defined for time messages between higher-level nodes and lower-level nodes;
the first lower-level node evaluating the time message received from the higher-level node, wherein said evaluating comprises:
on reception of the time message, reading the local time of the higher-level node, and adding the defined minimum delay time to the local time of the higher-level node, thereby generating a comparison time;
comparing the comparison time with the first lower-level node's local time;
if the comparison time is newer than the first lower-level node's local time, updating the first lower-level node's local time in accordance with the comparison time and
if the comparison time is older than the first lower-level node's local time, the first lower-level node's local time is not updated;
monitoring the time since the last update of the local time of the first lower-level node;
comparing the time since the last update of the local time of the first lower-level node with a time interval, which can be predetermined;
if the time since the last update of the local time of the first lower-level node exceeds the time interval, producing a virtual local time at the first lower-level node wherein the virtual local time lags behind the actual local time at the first lower-level node;
if the comparison time is newer than the virtual local time which has been produced at the first lower-level node; updating the first lower-level node's local time in accordance with the comparison time.
2. The method of claim 1 , wherein the time messages received by the first lower-level node are first time messages, the method further comprising:
the first lower-level node regularly sending second time messages to one or more lower-level nodes of the plurality of nodes, including sending a third time message to a lower-level node, wherein the third time message comprises a local time of the first lower-level node;
wherein the third time message is useable by the lower-level nodes to update the lower-level node's local time by:
the lower-level node evaluating the time message received from the first lower-level node, comprising:
on reception of the third time message, reading the local time of the first lower-level node, and adding the defined minimum delay time to the local time of the first lower-level node, thereby generating another comparison time;
comparing the other comparison time with the lower-level node's local time;
if the other comparison time is newer than the lower-level node's local time, updating the lower-level node's local time in accordance with the other comparison time;
wherein, if the other comparison time is older than the lower-level node's local time, the lower-level node's local time is not updated.
3. The method of claim 2 , further comprising:
determining which nodes of the plurality of nodes act as upper-level nodes, and which nodes act as lower-level nodes, wherein only nodes acting as higher-level nodes send time messages.
4. The method of claim 1 , wherein said updating the lower-level node's local time in accordance with the other comparison time comprises setting the lower-level node's local time to the comparison time.
5. The method of claim 1 , wherein the minimum delay time is:
predetermined;
determined by a delay time measurement performed at the start of the method; or
determined by a delay time measurement performed during the method.
6. The method of claim 1 , wherein each node of the plurality of nodes comprises a respective local clock, wherein the speed of the respective local clock at lower-level nodes is slower than the speed of the respective local clock at higher-level nodes.
7. The method of claim 6 , wherein the speed of the respective local clocks at the lower-level nodes can be adjusted, and in which, after each update of the local time at a respective lower-level node, the speed of the local clock at the respective lower-level node is adjusted to be precise as a function of the speed of the local clock before the time of the update of the local time, and as a function of the time interval of the update to the local time.
8. The method of claim 7 , wherein the speed of the local clock at the respective lower-level node is adjusted to be precise further as a function of a time interval which has passed between two updates of the local time for the respective lower-level node.
9. The method of claim 1 , wherein the network has a number of stages, comprising two or more sub-networks via which individual nodes are connected to one another, wherein each node in a first subset of the plurality of nodes can act only as higher-level nodes, wherein a second subset of the plurality of nodes can act only as lower-level nodes, and wherein a third subset of the plurality of nodes can act both as higher-level nodes and as lower-level nodes; and
wherein synchronization of a node acting as a lower-level node is carried out with respect to its higher-level node via at least one of the two or more sub-networks, but is decoupled from any synchronization of the higher-level node with respect to the higher-level node's higher-level node.
10. The method according to claim 9 , wherein an independent local clock is provided at a second node which acts both as a higher-level node and as a lower-level node, wherein when a time message is sent from the second node to a lower-level node, both an instantaneous value of the independent local clock and a difference between the instantaneous value of the independent local clock and the local time derived from its higher-level node are sent, and wherein on reception of such a time message, the comparison time is generated from the instantaneous value of the independent local clock contained in the time message and wherein a map of the reference time of the higher-level node is derived from the difference between the instantaneous value of the independent local clock and the local time derived from its higher-level node at the lower-level node.
11. The method according to claim 9 , wherein, when a time message is sent from a higher-level node to a lower-level node, not only is the sum of all the extents of updates performed since the last time message sent, but also the sum of all the corrections made since the last time message sent to the speed at which the time passes, and wherein, on reception of a time message from a higher-level node at the lower-level node, the local time of the lower-level node is first changed by the sum of all the extents of updates contained in the time message, and furthermore, the speed at which time passes at the lower-level node is changed by the sum of all the speed corrections contained in the time message prior to the comparison time being generated at the lower-level node.