Maintaining time of day synchronization
View Patent ↗A technique for maintaining time synchronization between network nodes involves broadcasting a timing signal from a source node to its neighbors. The timing signal is transmitted in a dedicated source time slot of a super epoch comprising a number of epochs each having a number of time slots. A reply timing signal is received at the source node from a neighboring node designated as the destination node, where the reply timing signal is received in a dedicated destination time slot of the super epoch. The clock drift and propagation delay between the source and destination nodes are computed at the source node. At the neighboring nodes other than the destination node, clock drift can be computed using previous estimates of propagation delay. RTS and CTS exchanges from PTP packets can also be used to estimate the propagation delay in the source node and the clock drift in the destination node.
1. A method for maintaining time synchronization between network nodes, the method comprising:
broadcasting a timing signal from a source node to neighboring nodes subsequent to an initial synchronization of the source node with respect to the neighboring nodes, the timing signal being broadcast in a dedicated source time slot of a super epoch comprising a plurality of epochs each having a plurality of fixed time slots, wherein the timing signal indicates a transmission time of the timing signal and a destination node designated for response;
receiving a reply timing signal at the source node from the neighboring node designated as the destination node, the reply timing signal being received in a dedicated destination time slot of the super epoch, wherein the reply timing signal indicates a transmission time of the reply timing signal;
computing at the source node a propagation delay between the source and destination nodes based on the transmission time of the timing signal and a reception time of the reply timing signal; and
computing at the source node a clock drift based on the transmission time of the timing signal, the transmission time of the reply timing signal, and the reception time of the reply timing signal.
2. The method of claim 1 , wherein the dedicated source time slot of the super epoch is adjacent to the dedicated destination time slot of the super epoch, and wherein the propagation delay and clock drift are computed further based on a time slot period.
3. The method of claim 1 , wherein broadcasting of the timing signal by the source node is scheduled according to a timer given by:
Timer= T a +random( T b *N nbr )
where T a is a minimum time separation for sending timing packets, N nbr is the number of neighboring nodes for the source node, and T b is a time parameter for controlling neighbor transmission.
4. The method of claim 1 , wherein the clock drift c is computed by:
c
=
2
T
3
-
T
4
-
T
1
-
S
2
where T 1 is the transmission time of the timing signal, T 3 is the transmission time of the reply timing signal, T 4 is the reception time of the reply timing signal, and S is a time slot period.
5. The method of claim 1 , wherein a running average of clock drift C k+1 is computed by:
C k+1 =αC k +(1−α) c
where α is a gain factor between 0 and 1, c is the clock drift and C k is a previously computed value of the running average of clock drift.
6. The method of claim 1 , wherein the propagation delay d t is computed by:
d
t
=
T
4
-
T
1
-
S
2
where T 1 is the transmission time of the timing signal, T 4 is the reception time of the reply timing signal, and S is a time slot period.
7. The method of claim 1 , wherein a running average of propagation delay D k+1 is computed by:
D k+1 =αD k +(1−α) d t
where α is a gain factor between 0 and 1, d t is the propagation delay, and D k is a previously computed value of the running average of propagation delay.
8. A communication device operable as a source node for maintaining time synchronization in a network, the communication device comprising:
a transmitter configured to broadcast a timing signal to neighboring nodes subsequent to an initial synchronization of the communication device with respect to the neighboring nodes, the timing signal being broadcast in a dedicated source time slot of a super epoch comprising a plurality of epochs each having a plurality of fixed time slots, wherein the timing signal indicates a transmission time of the timing signal and a destination node designated for response;
a receiver configured to receive a reply timing signal from the neighboring node designated as the destination node, the reply timing signal being received in a dedicated destination time slot of the super epoch, wherein the reply timing signal indicates a transmission time of the reply timing signal; and
a processor configured to compute: a propagation delay between the communication device and the destination node based on the transmission time of the timing signal and a reception time of the reply timing signal; and a clock drift based on the transmission time of the timing signal, the transmission time of the reply timing signal, and the reception time of the reply timing signal.
9. The communication device of claim 8 , wherein the dedicated source time slot of the super epoch is adjacent to the dedicated destination time slot of the super epoch, and wherein the propagation delay and clock drift are computed further based on a time slot period.
10. The communication device of claim 8 , wherein the communication device schedules broadcasting of the timing signal according to a timer given by:
Timer= T a +random( T b *N nbr )
where T a is a minimum time separation for sending timing packets, N nbr is the number of neighboring nodes for the communication device, and T b is a time parameter for controlling neighbor transmission.
11. The communication device of claim 8 , wherein the processor computes clock drift c by:
c
=
2
T
3
-
T
4
-
T
1
-
S
2
where T 1 is the transmission time of the timing signal, T 3 is the transmission time of the reply timing signal, T 4 is the reception time of the reply timing signal, and S is a time slot period.
12. The communication device of claim 8 , wherein the processor computes a running average of clock drift C k+1 by:
C k+1 =αC k +(1−α) c
where α is a gain factor between 0 and 1, c is the clock drift and C k is a previously computed value of the running average of clock drift.
13. The communication device of claim 8 , wherein the processor computes the propagation delay d t by:
d
t
=
T
4
-
T
1
-
S
2
where T 1 is the transmission time of the timing signal, T 4 is the reception time of the reply timing signal, and S is a time slot period.
14. The communication device of claim 8 , wherein the processor computes a running average of propagation delay D k+1 by:
D k+1 =αD k +(1−α) d t
where α is a gain factor between 0 and 1, d t is the propagation delay, and D k is a previously computed value of the running average of propagation delay.
15. A computer-readable memory storing instructions to maintain time synchronization between network nodes that, when executed by a processor, cause the processor to:
send instructions to broadcast a timing signal from a source node to neighboring nodes subsequent to an initial synchronization of the source node with respect to the neighboring nodes, the timing signal being broadcast in a dedicated source time slot of a super epoch comprising a plurality of epochs each having a plurality of fixed time slots, wherein the timing signal indicates a transmission time of the timing signal and a destination node designated for response;
process a reply timing signal received at the source node from the neighboring node designated as the destination node, the reply timing signal being received in a dedicated destination time slot of the super epoch, wherein the reply timing signal indicates a transmission time of the reply timing signal;
compute a propagation delay between the source and destination nodes based on the transmission time of the timing signal and a reception time of the reply timing signal; and
compute a clock drift based on the transmission time of the timing signal, the transmission time of the reply timing signal, and the reception time of the reply timing signal.