Manage End Devices Short sleep periods
Digi XBee® 3 Zigbee® RF Module
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Child poll timeout
Router and coordinator devices maintain a timestamp for each end device child indicating when the
end device sent its last poll request to check for buffered data packets. If an end device does not send
a poll request to its parent for a certain period of time, the parent will discard the packet.
Set the child poll timeout with the SP and SN commands. SP and SN should be set such that SP * SN
matches the longest expected sleep time of any end devices in the network. The device calculates the
actual timeout as (3* SP * SN), with a minimum of five seconds. For networks consisting of pin sleep
end devices, set the SP and SN values on the coordinator and routers so the SP * SN matches the
longest expected sleep period of any pin sleep device.
Adaptive polling
PO (Polling Rate) determines the regular polling rate. However, if RF data has been recently received
by an end device, it is likely that more RF data could be on the way. Therefore, the end device polls at
a faster rate, gradually decreasing its adaptive poll rate until polling resumes at the regular rate as
defined by the PO command.
Transmission timeout
When you are sending RF data to a remote router, because routers are always on, the timeout is
based on the number of hops the transmission may traverse. Set the timeout using NH (Maximum
Unicast Hops). For more information, see Transmission, addressing, and routing.
Since end devices may sleep for lengthy periods of time, the transmission timeout to end devices also
allows for the sleep period of the end device. When sending data to a remote end device, the
transmission timeout is calculated using the SP and NH commands. If the timeout occurs with no
acknowledgment received, the source device re-sends the transmission until it receives an
acknowledgment, up to two more times.
The transmission timeout per attempt is:
3 * ((unicast router timeout) + (end device sleep time))
3 * ((50 * NH) + (1.2 * SP)), where SP is measured in 10 ms units.
Short sleep periods
Pin and cyclic sleep devices that sleep less than 30 seconds can receive data transmissions at any
time since their parent devices are able to buffer data long enough for the end devices to wake and
poll to receive the data. Set SP the same on all devices in the network. If end devices in a network
have more than one SP setting, set SP on the routers and coordinators to match the largest SP
setting of any end device. This ensure the RF packet buffering, poll timeout, and transmission
timeouts are set correctly.
Extended sleep periods
Pin and cyclic sleep devices that might sleep longer than 30 seconds cannot receive data
transmissions reliably unless you take certain design approaches. Specifically, the end devices should
use I/O sampling or another mechanism to transmit data when they wake to inform the network they
can receive data. SP and SN should be set on routers and coordinators such that (SP * SN) matches
the longest expected sleep time.
As a general rule, SP and SN should be set the same on all devices in almost all cases.
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