Transmission, addressing, and routing Encrypted transmissions
Digi XBee® 3 Zigbee® RF Module
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Disable route records
If an aggregator collects route records from the nodes of the network and no longer needs route
records sent (which consume network throughput) :
1. Set Bit 6 of DO to Enable High RAM Concentrator mode. High RAM mode means the aggregator
has sufficient memory to hold route records for its potential destinations.
2. Set AR to 0x00 for a one-time broadcast (which some nodes might miss), or a value in the
range of 0x01 to 0xFE (in units of 10 seconds) to periodically send a broadcast to inform the
network that the aggregator is operating in High RAM Concentrator mode and no longer needs
to receive route records.
3. Use Create Source Route - 0x21 to load the route record for a destination into the local
device's source route table.
4. Send a unicast to the destination. The route record embeds in the payload and determines the
sequence of routers to use in transmitting the unicast to the destination. After receiving the
unicast, the destination no longer sends route records to the aggregator, now that it has
confirmed the High RAM Concentrator aggregator 'knows' its route record.
Clear the source route table
To clear the source route table, change the AR setting from a non-0xFF setting to 0xFF and complete
an AC command. To re-establish periodic aggregator broadcasts, change the AR setting to a non-0xFF
setting and complete an AC command.
Encrypted transmissions
Encrypted transmissions are routed similar to non-encrypted transmissions with one exception. As an
encrypted packet propagates from one device to another, each device decrypts the packet using the
network key and authenticates the packet by verifying packet integrity. It then re-encrypts the
packet with its own source address and frame counter values and sends the message to the next hop.
This process adds some overhead latency to unicast transmissions, but it helps prevent replay
attacks. For more information see Zigbee security.
Maximum RF payload size
The NP command returns the maximum payload size in bytes. The actual maximum payload is a
function of the following:
n message type (broadcast or unicast)
n AP setting
n APS encryption option
n Source-routing
Broadcasts, which cannot be encrypted with APS or fragmented have a maximum payload of 0x54
bytes (84 bytes).
For broadcast messages and unicast messages when AP==0, the maximum payload is 0x54 bytes.
For unicast messages, the maximum payload is 0xFF (255 bytes) bytes. If the combination of payload
and optional APS encryption overhead (EE1, TxOption 0x20) is too high, the message fragments into a
maximum of five fragments. The firmware encrypts and transmits each fragment separately. The
destination radio reassembles the fragments into a full message.
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