XBee®/XBee‐PRO® ZB SMT RF Modules
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.
Non-Parent Device Operation
Devices in the ZigBee network treat data transmissions to end devices differently than transmissions to other
routers and coordinators. Recall that when a unicast transmission is sent, if a network acknowledgment is not
received within a timeout, the device resends the transmission. When transmitting data to remote coordinator or
router devices, the transmission timeout is relatively short since these devices are powered and responsive.
However, since end devices may sleep for some time, unicast transmissions to end devices use an extended timeout
mechanism in order to allow enough time for the end device to wake and receive the data transmission from its
parent.
© 2010 Digi International, Inc.
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DCD [ DIGITAL CORE DESIGN ]
