> 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

![](https://1516040574-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2Fg081KOVSE5kjkOj8LvCP%2Fuploads%2Fb1xtoUyiHFHHYzNfNiyV%2F2.JPG?alt=media\&token=b0eb8343-327b-4bc5-9f5e-6ea78128ec87)

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

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