Time distribution scheme for wireless mesh networks
A wireless mesh network includes a mesh of continuously-powered devices (CPDs) and a mesh of battery-powered devices (BPDs). The BPDs are organized into hop layers based on hopping distance to the mesh of CPDs. The CPDs transmit time beacons to BPDs in a first hop layer during a first receive window associated with the first hop layer. The BPDs in the first hop layer then transmit time beacons to BPDs in a second hop layer during a second receive window. In this manner, the wireless mesh network propagates time values throughout the BPD mesh. Based on these time values, the BPDs power on during short time intervals to exchange data with neighboring BPDs, and then power off for longer time intervals, thereby conserving battery power. The techniques described herein for conserving battery power for BPDs may also be applied to conserve power consumption of CPDs.
1. A computer-implemented method for coordinating time across a mesh network, the method comprising:
determining a first receive window associated with each node of a plurality of nodes included in a first hop layer of the wireless mesh network, wherein a first node resides within the first hop layer;
determining a second receive window associated with each node of a plurality of nodes included in a second hop layer of the wireless mesh network, wherein a second node resides within the second hop layer, and the first receive window is different than the second receive window;
receiving a first set of time beacons during the first receive window;
generating a first time beacon based on the first set of time beacons; and
transmitting the first time beacon from the first node to the second node during the second receive window.
2. The computer-implemented of claim 1 , wherein receiving the first set of time beacons during the first receive window comprises:
causing the first node to enter into a sniff mode during a first subinterval of time within the first receive window;
detecting energy associated with a first time beacon included in the first set of time beacons;
in response to detecting the energy, causing the first node to enter into a receive mode during the first subinterval of time, wherein operating in the receive mode consumes more power than operating in the sniff mode; and
receiving the first time beacon during the first receive window.
3. The computer-implemented method of claim 2 , wherein receiving the first set of time beacons during the first receive window further comprises:
causing the first node to enter into a sleep mode during a second subinterval of time included in the first receive window, wherein operating in the sleep mode consumes less power than operating in the sniff mode;
causing the first node to enter into the sniff mode during a third subinterval of time included in the first receive window;
detecting energy associated with another time beacon included in the first set of time beacons;
in response to detecting the energy, causing the first node to enter into the receive mode during the third subinterval of time; and
receiving the another time beacon during the first receive window.
4. The computer-implemented method of claim 1 , further comprising:
randomly selecting a first transmit time within the second receive window; and
transmitting the first time beacon to the second node at the first transmit time.
5. The computer-implemented method of claim 1 , wherein generating the first time beacon based on the first set of time beacons further comprises:
parsing each time beacon included in the first set of time beacons to extract a time estimate and an uncertainty estimate corresponding to the time estimate;
weighting each time estimate based on a corresponding uncertainty estimate to generate a plurality of weighted time estimates; and
combining the weighted time estimates included in the plurality of weighted time estimates to generate a first time estimate that is included in the first time beacon.
6. The computer-implemented method of claim 5 , further comprising: combining each uncertainty estimate to generate a first uncertainty estimate associated with the first time estimate that also is included in the first time beacon.
7. The computer-implemented method of claim 1 , wherein the first node performs the steps of determining, receiving, generating, and transmitting.
8. The computer-implemented method of claim 1 , wherein the first receive window comprises a first interval of time, the second receive window comprises a second interval of time, and the first interval of time occurs before the second interval of time.
9. The computer-implemented method of claim 1 , wherein each of the first node and the second node comprises a battery-powered node that resides within a battery-powered portion of the mesh network.
10. The computer-implemented method of claim 1 , wherein the first node receives the first set of time beacons from one or more continuously-powered devices that reside within a continuously-powered portion of the mesh network.
11. A first node, comprising:
a memory storing program instructions; and
a processor coupled to the memory and configured to:
determine a first receive window associated with each node of a plurality of nodes included in a first hop layer of the wireless mesh network, wherein the first node resides within the first hop layer,
determine a second receive window associated with each node of a plurality of nodes included in a second hop layer of the wireless mesh network, wherein a second node resides within the second hop layer, and the first receive window is different than the second receive window,
receive a first set of time beacons during the first receive window,
generate a first time beacon based on the first set of time beacons, and
transmit the first time beacon from the first node to the second node during the second receive window.
12. The first node of claim 11 , wherein the processor, upon executing the program instructions, is configured to:
determine the first receive window;
determine the second receive window;
receive the first set of time beacons;
generate the first time beacon based on the first set of time beacons; and
transmit the first time beacon to the second node.
13. The first node of claim 11 , wherein the processor is configured to receive the first set of time beacons during the first receive window by:
causing the first node to enter into a sniff mode during a first subinterval of time within the first receive window;
detecting energy associated with a first time beacon included in the first set of time beacons;
in response to detecting the energy, causing the first node to enter into a receive mode during the first subinterval of time, wherein operating in the receive mode consumes more power than operating in the sniff mode; and
receiving the first time beacon during the first receive window.
14. The first node of claim 13 , wherein the processor is further configured to receive the first set of time beacons during the first receive window by:
causing the first node to enter into a sleep mode during a second subinterval of time included in the first receive window, wherein operating in the sleep mode consumes less power than operating in the sniff mode;
causing the first node to enter into the sniff mode during a third subinterval of time included in the first receive window;
detecting energy associated with another time beacon included in the first set of time beacons;
in response to detecting the energy, causing the first node to enter into the receive mode during the third subinterval of time; and
receiving the another time beacon during the first receive window.
15. The first node of claim 11 , wherein the processor is further configured to:
randomly select a first transmit time within the second receive window; and
transmit the first time beacon to the second node at the first transmit time.
16. The first node of claim 11 , wherein the processor is configured to generate the first time beacon based on the first set of time beacons by:
parsing each time beacon included in the first set of time beacons to extract a time estimate and an uncertainty estimate corresponding to the time estimate;
weighting each time estimate based on a corresponding uncertainty estimate to generate a plurality of weighted time estimates; and
combining the weighted time estimates included in the plurality of weighted time estimates to generate a first time estimate that is included in the first time beacon.
17. The first node of claim 16 , wherein the processor is further configured to combine each uncertainty estimate to generate a first uncertainty estimate associated with the first time estimate that also is included in the first time beacon.
18. The first node of claim 11 , wherein the first receive window comprises a first interval of time, the second receive window comprises a second interval of time, and the first interval of time occurs before the second interval of time.
19. The first node of claim 11 , wherein each node in the first hop layer resides one hop away from an access point, and each node in the second hop layer resides two hops away from the access point, and wherein each node included in the first hop layer and the second hop layer comprises a battery-powered node.
20. A non-transitory computer-readable medium storing program instructions that, when executed by a processor, cause the processor to coordinate time across a mesh network, by performing the steps of:
determining a first receive window associated with each node of a plurality of nodes included in a first hop layer of the wireless mesh network, wherein a first node resides within the first hop layer;
determining a second receive window associated with each node of a plurality of nodes included in a second hop layer of the wireless mesh network, wherein a second node resides within the second hop layer, and the first receive window is different than the second receive window;
receiving a first set of time beacons during the first receive window;
generating a first time beacon based on the first set of time beacons; and
transmitting the first time beacon from the first node to the second node during the second receive window.