Packet Structure
All messages sent over the air take the form of a packet. Packets encapsulate the actual user data payload with information required for the network to operate. They are similar to their 802.15.4 counterparts, but extended to allow larger payloads.
Headers
These go at the start of the packet, before the user payload. Going from outermost to innermost header:
Synchronization
While this is not technically part of the data, it's used by the underlying radio subsystem to recognize the frame. The sync header is the very first part of the packet to be transmitted, and is always the same. It's first a 32-bit preamble sequence (generated and specified by the radio PHY) followed by a fixed synchronization word – this is specified by the network, with a default value of 0xF6.
PHY
It is 2 bytes, and is used by the PHY layer to know how large of a packet it should receive. The length counter is a 10 bit quantity, meaning packets up to 1024 bytes (0 byte packets are not permitted) are allowed.
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Reserved (set to 0) | Length Counter | ||||||||||||||
MAC
Next, a 6 byte header is used to indicate the source and destination address of the packet, as well as some additional flags.
Footers
Additional information needed to validate packets goes after the payload data.
Checksum
Contains a 16-bit checksum over all headers (starting with the PHY header) and payload data.
This checksum is to be a 16-bit CRC, using the CCITT polynomial with a seed of 0xFFFF. CRCs are calculated LSB first, and output most significant byte (and bit in the byte) first.
The radio layer shall discard all received packets where the checksum footer's value does not match the computed packet checksum. Packet headers shall not be interpreted until the checksum has been validated, as they may have been corrupted otherwise.