> For the complete documentation index, see [llms.txt](https://help.boomerang3.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.boomerang3.com/s2-radio-system-manual/s2-system-description/s2-node/s2-communication-protocol/frame/header.md).

# Header

The first part of the frame is a header field which is always present. The header occupies fields 1 - 6 for UART and 1 - 7 for radio commands in the examples.

| 1      | 2       | 3       | 4   | 5  | 6     | 6(or 7) |
| ------ | ------- | ------- | --- | -- | ----- | ------- |
| Length | ID Info | Command | GRP | ID | To ID | CRC     |

 

<table><thead><tr><th width="132">Type</th><th>Description</th></tr></thead><tbody><tr><td>Length</td><td>The length of the message</td></tr><tr><td>ID Info</td><td>ID Info informs of the number of relays and a pointer of the active relay in radio communication. Set to 00 for UART command.<br>See more about this in section Relaying.</td></tr><tr><td>Command</td><td>Instruction on what to do.</td></tr><tr><td>GRP</td><td>Active group number.</td></tr><tr><td>ID</td><td>From ID (Or ‘To ID’ when the command is a UART command)</td></tr><tr><td>ID TO</td><td>To ID (for Radio commands.)</td></tr><tr><td>CRC</td><td>CRC-8 on header</td></tr></tbody></table>


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