Maximum RF payload size

The maximum payload size on the XBee 3 Zigbee RF Module is a function of the following:

• Message type: broadcast or unicast
• AES encryption (EE command)
• APS security (TO bit 4)
• Secure Session (TO bit 5)
• Source Routing

The maximum payload size of a single packet is:

Message type	Unicast	Broadcast
Unencrypted (EE = 0)	84 bytes	92 bytes
Encrypted (EE = 1)	66 bytes	74 bytes
APS Security (EE = 1, TO bit 5)	57 bytes	N/A

When operating in Transparent mode (AP = 0), all outgoing transmissions are sent as non-fragmented messages.

When sending a unicast transmission in API mode or through MicroPython, the maximum payload is 255 bytes. If the combination of payload and optional APS security overhead is too high, the message fragments into a maximum of five fragments. The firmware encrypts and transmits each fragment separately. The destination device reassembles the fragments into a full message.

Broadcast transmissions are sent as non-fragmented messages and cannot use APS security.

• Enabling encryption (EE = 1) reduces maximum payload size by 18 bytes.
• Enabling APS security (TO bit 5) reduces maximum payload size by 9 bytes.
• Enabling Secure Session (TO bit 4) reduces maximum payload size by 5 bytes.

Using source routing will further reduce payload size depending on how many hops are being traversed. When an aggregator (AR < 0xFF) sends a source-routed message, it embeds the route into the message as overhead, or into each fragment of the message if fragmentation is necessary. If you use APS security (EE 1, Tx Option 0x20), it reduces the number further.

The route overhead is two bytes plus two bytes per hop. The bytes are:

• One byte for the number of hops.
• One byte is an index into the route list that increments in value at each hop.
• For each hop, two bytes are used for the 16-bit network address of each routing device.

Aggregator source-routed payload maximums do not apply to messages that are sourced by non-aggregator nodes (AR = 0xFF).

The following table shows the aggregator source-routed payload maximums (in bytes) as a function of hops and APS security:

Hops	Maximum payload	Maximum APS-encrypted payload
1	255	255
2	255	255
3	255	255
4	255	255
5	255	255
6	255	215
7	250	205
8	240	195
9	230	185
10	220	175
11	210	165
12	200	155
13	190	145
14	180	135
15	170	125
16	160	115
17	150	105
18	140	95
19	130	85
20	120	75
21	110	65
22	100	55
23	90	45
24	80	35
25	70	25