Digi XBee-PRO 900 Instruction Manual page 19

XBee‐PRO® 900 OEM RF Modules
Changing Sleep Parameters
Use the following steps to change sleep parameters:
1. Enter the SP command with the sleep time interval represented in units of 10 msec.
2. Enter the ST command with the wake time interval represented in units of 1 msec.
3. Verify the values of the SP and ST commands.
Note: the value of the ST configuration parameter will be automatically adjusted to a minimum
effective value if the value of SP is increased enough. However, ST will not be automatically
adjusted downwards should the value of SP be decreased. Enter the apply changes command AC.
At the start of the next cycle, a special overriding synch message will be broadcast across the
network. It will override any synch message coming from any sleep coordinator. Then, a cycle
later, the network will wake and operate according to the new ST and SP values.
Rejoining Subnets
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, but out of phase with each other due to clock
drift. The first task is to repair, replace, and strengthen the weak link with new and/or redundant
nodes so this problem doesn't arise again. The second task is to get the subnets back in phase
(rejoin subnets).
To get the subnets back in phase, follow these steps:
1. Power up a node in normal mode, and position it next to a node in the subnet you want to use
as a reference.
2. Wait a cycle (plus a bit more for safety margin) for it to synch with the network.
3. Enable its preferred sleep coordinator option. This node will now become the sleep coordinator
for the network. A synch from a normal node overrides synch messages from any sleeping node
and will cause any sleep coordinator node to disable its own synch message.
To place each out-of-phase sleeping node, follow these steps:
1. Place the sleep coordinator node in range of the sleeping node and toggle power for the
sleeping node off and back on.
2. When the sleeping coordinator node powers up, it sees that it is out of synch, and requests a
synch message. The sleep coordinator node will receive the request and transmit a special unicast
synch response. The sleeping node will return to sleep and wake again in synch with the rest of
the network.
3. Power down and store away the normal node for safe keeping.
© 2008 Digi International, Inc. 19
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