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.

The parent can buffer one broadcast transmission for all of its end device children. When the parent receives and buffers a broadcast transmission, it sets a flag in its child table when each child polls and retrieves the packet. Once all children have received the broadcast packet, the parent discards the buffered broadcast packet. If all children have not received a buffered broadcast packet and the parent receives a new broadcast, it discards the old broadcast packet, clears the child table flags, and buffers the new broadcast packet for the end device children as shown in the following figure.

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 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 within 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 removes 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 (for example 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 use up a significant number of buffers, reducing the buffer availability for managing packets for end device children. Applications can 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.