Transmission, addressing, and routing Data transmission
XBee/XBee-PRO ZigBee RF Modules User Guide 57
Fragmentation
Each unicast transmission may support up to 84 bytes of RF payload. Enabling security or using source routing
can reduce this number. See the NP command for details. However, the XBee ZB firmware supports a ZigBee
feature called fragmentation that allows a single large data packet to be broken up into multiple RF
transmissions and reassembled by the receiver before sending data out its UART. This following image show this.
The API transmit frame can include up to 255 bytes of data, which will be broken up into multiple transmissions
and reassembled on the receiving side. If one or more of the fragmented messages are not received by the
receiving device, the receiver will drop the entire message, and the sender will indicate a transmission failure in
the Tx Status API frame. ZB firmware can only support one fragmented packet at a time at the receive node, due
to memory constraints.
Applications that do not wish to use fragmentation should avoid sending more than the maximum number of
bytes in a single RF transmission. See Maximum RF payload size on page 67 for details.
Devices will not receive or reassemble fragmented RF packets if RTS flow control is enabled (D6 command).
Note Broadcast transmissions do not support fragmentation. Maximum payload size = up to 84 bytes.
Data transmission examples
AT firmware
To send a data packet in AT firmware, the DH and DL commands must be set to match the 64-bit address of the
destination device. DH must match the upper 4-bytes, and DL must match the lower 4 bytes. Since the
coordinator always receives a 16-bit address of 0x0000, a 64-bit address of 0x0000000000000000 is defined as the
coordinator's address (in ZB firmware). The default values of DH and DL are 0x00, which sends data to the
coordinator.
Example 1: send a transmission to the coordinator.
Note In this example, a \r refers to a carriage return character.
A router or end device can send data in two ways. First, set the destination address (DH and DL commands) to
0x00.
1 Enter command mode (+++).
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