When the radio is being used in a transparent (AT) mode, packet size is irrelevant because the data will be fragmented as needed. However, when using API frames, it is important not to exceed the allowable payload size.
Maximum payload size varies depending on what parameters are set on the radio. In order to determine what the payload size is, you can query the NP parameter. NP will indicate the maximum payload size (as a hexadecimal value,) based on the parameters that are currently set on the radio.
If you are using a ZigBee XBee, NP will always return 0xFF. This is because the ZigBee protocol supports fragmentation: regardless of what settings are being used, you can put a 255 byte payload into the transmission frame and it will fragment the data as needed. If you want to avoid fragmentation, the payload size without encryption will be 84 bytes, 66 bytes if encryption is enabled (EE parameter), and 92 bytes if the transmission is a broadcast. Broadcast transmissions do not use encryption.
The NP parameter is not available for the following radios, payload sizes are included as a reference:
Note: This only applies to the point-to-multipoint (non-DigiMesh) firmware.
|802.15.4 (Series 1) XBee