Operation
Operation
XBee 868LP RF Modules User Guide
68
Operation
The XBee 868LP RF Module uses a multi-layered firmware base to order the flow of data, dependent
on the hardware and software configuration you choose. The following configuration block diagram
shows the host serial interface as the physical starting point and the antenna as the physical endpoint
for the transferred data. A block must be able to touch another block above or below it for the two
interfaces to interact. For example, if the device uses SPI mode, Transparent mode is not available as
shown in the following image:
The command handler code processes commands from AT Command Mode or API Mode; see
. The command handler also processes commands from remote devices; see
Listen Before Talk and Automatic Frequency Agility
This device implements Listen Before Talk (LBT) and Automatic Frequency Agility (AFA). The advantage
of LBT with AFA is that the device bypasses the Duty Cycle requirement imposed by European
standards. LBT+AFA requires that you use at least two frequencies for transmission.
This feature provides a level of fairness to the devices in a given area. Before this device transmits, it
senses a channel to determine if there is activity by taking an RSSI measurement for 5 ms. If the
measurement is below the threshold, the device transmits on that channel. If there is activity, that
channel is not used, and the device listens for at least 5 ms to allow transmissions to be received.
After the device transmits on a channel, it will not transmit on that channel again until the minimum
TX off time has been met, which is greater than 100 ms. It is useful to have many channels in your
channel mask, so transmissions are less likely to be delayed.
European requirements also state that only 100 seconds of transmission may occur over the period of
an hour on 200 kHz of spectrum. This method simplifies and optimizes the calculations of spectrum
use over the period of one hour. The standard states that the more channels you have, the more
transmission time you have in a one hour period. Calculate the effective duty cycle based on the
number of available channels enabled as follows:
