XBee‐PRO®900HP/XBee‐PRO®XSCRFModules
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Rejoining Nodes Which Have Lost Sync
Mesh networks get their robustness from taking advantage of routing redundancies which may be available in
a network. It is recommended to architect the network with redundant mesh nodes to increase robustness. If
a scenario exists such that the only route connecting a subnet to the rest of the network depends on a single
node, and that node fails -- or the wireless link fails due to changing environmental conditions (catastrophic
failure condition), then multiple subnets may arise while using the same wake and sleep intervals. When this
occurs the first task is to repair, replace, and strengthen the weak link with new and/or redundant modules to
fix the problem and prevent it from occurring in the future.
When the default DigiMesh sleep parameters are used, separated subnets will not drift out of phase with each
other. Subnets can drift out of phase with each other if the network is configured in one of the following ways:
• If multiple modules in the network have had the non-sleep coordinator sleep option bit dis-
abled and are thus eligible to be nominated as a sleep coordinator.
• If the modules in the network are not using the auto early wake-up sleep option.
If a network has multiple subnets that have drifted out of phase with each other, get the subnets back in
phase with the following steps:
1. Place a sleep support node in range of both subnets.
2. Select a node in the subnet that you want the other subnet to sync up with. Use this node to slightly change
the sleep cycle settings of the network (increment ST, for example).
3. Wait for the subnet’s next wake cycle. During this cycle, the node selected to change the sleep cycle
parameters will send the new settings to the entire subnet it is in range of, including the sleep support node
which is in range of the other subnet.
4. Wait for the out of sync subnet to wake up and send a sync. When the sleep support node receives this
sync, it will reject it and send a sync to the subnet with the new sleep settings.
5. The subnets will now be in sync. The sleep support node can be removed. If desired, the sleep cycle
settings can be changed back to what they were.
In the case that only a few nodes need to be replaced, this method can also be used:
1. Reset the out of sync node and set its sleep mode to cyclic sleep (SM = 8). Set it up to have a short sleep
cycle.
2. Place the node in range of a sleep support node or wake a sleeping node with the commissioning button.
3. The out of sync node will receive a sync from the node which is synchronized to the network and sync to the
network sleep settings.
DigiMesh Diagnostics
The following are useful in some applications when managing a sleeping router network:
Query current sleep cycle: the OS and OW commands can be used to query the current operational sleep
and wake times a module is currently using.
Sleep Status: the SS command can be used to query useful information regarding the sleep status of the
module. This command can be used to query if the node is currently acting as a network sleep coordinator, as
well as other useful diagnostics.
Missed Sync Messages Command: the MS command can be used to query the number of cycles that have
elapsed since the module last received a sync message.
Sleep Status API messages: when enabled with the SO command, a module configured in API mode will
output modem status frames immediately after a module wakes up and just prior to a module going to sleep.
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