> For the complete documentation index, see [llms.txt](https://gsm.learn-telecom.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gsm.learn-telecom.com/gsm-channels/intro.md).

# Intro

![](/files/3jBxYy26vlajav4M7pVO)

[TS GSM 05.02](https://www.etsi.org/deliver/etsi_gts/05/0502/05.00.00_60/gsmts_0502v050000p.pdf)

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