User's Manual

XBee®/XBee‐PRO®ZBRFModules
©2009DigiInternational,Inc. 75
Since the end device children are not guaranteed to be awake at a given time, the parent is
responsible for managing incoming data packets in behalf 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 will buffer the packet. The data packet will remain buffered until a
timeout expires, or until the end device sends a poll request to retrieve the data.
The parent can buffer one broadcast transmission for all of its end device children. When a
broadcast transmission is received and buffered, the parent sets a flag in its child table when each
child polls and retrieves the packet. Once all children have received the broadcast packet, the
buffered broadcast packet is discarded. If all children have not received a buffered broadcast
packet and a new broadcast is received, the old broadcast packet is discarded, the child table flags
are cleared, and the new broadcast packet is buffered for the end device children. This is
demonstrated in the figure below.
When an end device sends data to its parent that is destined for a remote device in the network,
the parent buffers the data packet until it can establish a route to the destination. The parent may
perform a route or 16-bit address discovery in behalf of its end device children. Once a route is
established, the parent sends the data transmission to the remote device.
End Device Poll Timeouts
To better support mobile end devices (end devices that can move around in a network), parent
router and coordinator devices have a poll timeout for each end device child. If an end device does
not send a poll request to its parent within the poll timeout, the parent will remove the end device
from its child table. This allows the child table on a router or coordinator to better accommodate
mobile end devices in the network.
Packet Buffer Usage
Packet buffer usage on a router or coordinator varies depending on the application. The following
activities can require use of packet buffers for up to several seconds:
• Route and address discoveries
• Application broadcast transmissions
• Stack broadcasts (e.g. ZDO "Device Announce" messages when devices join a network)
• Unicast transmissions (buffered until acknowledgment is received from destination or retries
exhausted)
• Unicast messages waiting for end device to wake.
Applications that use regular broadcasting or that require regular address or route discoveries will
use up a significant number of buffers, reducing the buffer availability for managing packets for
end device children. Applications should reduce the number of required application broadcasts,
and consider implementing an external address table or many-to-one and source routing if
necessary to improve routing efficiency.