# HDLC

AI-generated

High-Level Data Link Control,
the framing [AX.25](../ax25/index.html.md) borrows:
a frame starts and ends with the `01111110` flag,
a 0 is stuffed after five 1s in a row so data never looks like a flag,
and a 16-bit FCS (a CRC) at the end catches corruption.
On air the bits are NRZI-coded,
a 0 as a change of tone or level and a 1 as no change.

## References

* [High-Level Data Link Control on Wikipedia](https://en.wikipedia.org/wiki/High-Level_Data_Link_Control)
* [AX.25 Link Access Protocol for Amateur Packet Radio, v2.2](https://www.tapr.org/pdf/AX25.2.2.pdf) (TAPR), section 3