Specifications
XBee®/XBee‐PRO®SERFModules
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RF Packet Buffering Timeout
When a router or coordinator receives an RF data packet intended for one of its end device
children, it buffers the packet until the end device wakes and polls for the data, or until a packet
buffering timeout occurs. This timeout is settable using the SP command. The actual timeout is
(1.2 * SP), with a minimum timeout of 1.2 seconds and a maximum of 30 seconds. Since the
packet buffering timeout is set slightly larger than the SP setting, SP should be set the same on
routers and coordinators as it is on cyclic sleep end devices. For pin sleep devices, SP should be
set as long as the pin sleep device can sleep, up to 30 seconds.
Note: In pin sleep and extended cyclic sleep, end devices can sleep longer than 30 seconds. If end
devices sleep longer than 30 seconds, parent and non-parent devices must know when the end
device is awake in order to reliably send data. For applications that require sleeping longer than 30
seconds, end devices should transmit data when they wake to alert other devices that they can
send data to the end device.
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 assume the end
device has moved out of range and will remove the end device from its child table. This allows
routers and coordinators to be responsive to changing network conditions. The NC command can
be issued at any time to read the number of remaining (unused) child table entries on a router or
coordinator.
The child poll timeout is settable 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
actual timeout is calculated as (3 * SP * SN), with a minimum of 5 seconds. For networks
consisting of pin sleep end devices, the SP and SN values on the coordinator and routers should be
set such that SP * SN matches the longest expected sleep period of any pin sleep device. The 3
multiplier ensures the end device will not be removed unless 3 sleep cycles pass without receiving
a poll request. The poll timeout is settable up to a couple of months.
Transmission Timeout
As mentioned in chapter 4, when sending RF data to a remote router, since routers are always on,
the timeout is based on the number of hops the transmission may traverse. This timeout it
settable using the NH command. (See chapter 4 for details.)
Since end devices may sleep for lengthy periods of time, the transmission timeout to end devices
also includes some allowance 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 and an acknowledgment has not been received, the source device will resend the
transmission until an acknowledgment is received, up to two more times.
The transmission timeout per attempt is:
3 * ((unicast router timeout) + (end device sleep time)), or
3 * ((50 * NH) + (1.2 * SP)), where SP is measured in 10ms units.
For best results, SP should be set on routers and coordinator devices to match the SP setting on
the end devices.
Note: The NH command is used to determine the timeout when transmitting to routers.
Putting it all Together
Short Sleep Periods
Pin and cyclic sleep devices that sleep less than 30 seconds can receive data transmissions at any
time since their parent device(s) will be able to buffer data long enough for the end devices to
wake and poll to receive the data. SP should be set the same on all devices in the network. If end