Where to gain knowledge on the E1 digital stream: theory, practice?

Question

Electronics

Interested in how it works, application, as well as everything related to equipment, internal arrangement, manuals.
End goal: write a driver for the E1 IC transceiver with knowledge of the topic.

Answer 1

Obviously, in Google, starting with the search for simple things and moving on to complex ones.
Start with Wikipedia:
https://ru.wikipedia.org/wiki/%D0%A6%D0%B8%D1%84%D...
https://en.wikipedia.org/wiki/E-carrier