G)Fsk Packet Format; Figure 8-4: Fixed-Length Packet; Figure 8-5: Variable-Length Packet - Semtech LR1121 User Manual

8.4.3 (G)FSK Packet Format

The (G)FSK packet format provides a conventional packet format for application in proprietary NRZ coded, low energy
communication links. The packet format has built in facilities for CRC checking of the payload, dynamic payload size and
packet acknowledgement. Whitening based upon pseudo random number generation can be enabled. Two principle
packet formats are available in the (G)FSK protocol: fixed-length and variable-length.
8.4.3.1 Fixed-Length Packet
If the packet length is fixed and known on both sides of the link then knowledge of the packet length does not need to be
transmitted over the air. Instead the packet length can be written to the parameter PacketLength which determines the
packet length in bytes:
•
0 to 255 if address filtering is not activated
•
0 to 254 if address filtering is activated
It is usually recommended to use a minimum of 16 bits for the preamble to guarantee a valid reception of the packet on the
receiver side.
The CRC operation, packet length and preamble length are defined in command SetPacketParams(...).
8.4.3.2 Variable-Length Packet
This field encodes the payload length in bytes.
If the packet is of uncertain or variable size, information about the payload length must be transmitted within the packet.
The format of the variable-length packet is shown in the figure below:
• the packet length is 0 to 254 bytes if address filtering is activated
• the Length field is 1 byte long, except for SX128x compatibility, when it is 9 bits (see
LR1121
User Manual Rev 1.1
UM.LR1121.W.APP Mar 2023

Figure 8-4: Fixed-Length Packet

Figure 8-5: Variable-Length Packet

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