Data transmission

You can send Zigbee data packets as either unicast or broadcast transmissions. Unicast transmissions route data from one source device to one destination device, whereas broadcast transmissions are sent to many or all devices in the network.

Broadcast transmissions

Broadcast transmissions within the Zigbee protocol are intended to be propagated throughout the entire network such that all nodes receive the transmission. To accomplish this, the coordinator and all routers that receive a broadcast transmission retransmits the packet three times.

Note When a router or coordinator delivers a broadcast transmission to an end device child, the transmission is only sent once (immediately after the end device wakes and polls the parent for any new data). For more information, see Parent operation.

Each node that transmits the broadcast also creates an entry in a local broadcast transmission table. This entry to keeps track of each received broadcast packet to ensure the packets are not transmitted endlessly. Each entry persists for 8 seconds, and the broadcast transmission table holds 8 entries, effectively limiting network broadcast transmissions to once per second.

For each broadcast transmission, the Zigbee stack reserves buffer space for a copy of the data packet that retransmits the packet as needed. Large broadcast packets require more buffer space. Users cannot change any buffer spacing; information on buffer space is for general knowledge only. The XBee/XBee-PRO Zigbee RF Module handles buffer spacing automatically.

Since each device in the network retransmits broadcast transmissions, use broadcast messages sparingly to avoid network congestion.

Unicast transmissions

Unicast transmissions are sent from one source device to another destination device. The destination device could be an immediate neighbor of the source, or it could be several hops away. Unicast transmissions sent along a multiple hop path require some means of establishing a route to the destination device. For more information, see RF packet routing.