Manage End Devices; End Device Operation; Parent Operation - Digi XBee S2C Zigbee User Manual

Manage End Devices

End device operation

When an end device joins a Zigbee network, it must find a router or coordinator device that is
allowing end devices to join. Once the end device joins a network, it forms a parent-child relationship
with the end device and the router or coordinator that allowed it to join. For more information, see
Zigbee networks.
When the end device is awake, it sends poll request messages to its parent. When the parent receives
a poll request, it checks a packet queue to see if it has any buffered messages for the end device. It
then sends a MAC layer acknowledgment back to the end device that indicates if it has data to send to
the end device or not.
If the end device receives the acknowledgment and finds that the parent has no data for it, the end
device can return to idle mode or sleep. Otherwise, it remains awake to receive the data. This polling
mechanism allows the end device to enter idle mode and turn its receiver off when RF data is not
expected in order to reduce current consumption and conserve battery life.
The end device can only send data directly to its parent. If an end device must send a broadcast or a
unicast transmission to other devices in the network, it sends the message directly to its parent and
the parent performs any necessary route or address discoveries to route the packet to the final
destination.
The parent of the receiving device does not send the network ACK back to the originator until the
sleeping end device wakes and polls the data or until the timeout occurs.

Parent operation

Each router or coordinator maintains a child table that contains the addresses of its end device
children. A router or coordinator that has unused entries in its child table has end device capacity, or
the ability to allow new end devices to join. If the child table is completely filled (such that the
number of its end device children matches the number of child table entries), the device cannot allow
any more end devices to join.
Since the end device children are not guaranteed to be awake at a given time, the parent is
responsible for managing incoming data packets of its end device children. If a parent receives an RF
data transmission destined for one of its end device children, and if the parent has enough unused
buffer space, it buffers the packet. The data packet remains buffered until a timeout expires, or until
the end device sends a poll request to retrieve the data.
XBee/XBee-PRO® S2C Zigbee® RF Module User Guide
End device operation
146