This TS defines the Stage 2 service description for the Evolved Packet System (EPS).
The specification covers both roaming and non-roaming scenarios and covers all aspects, including mobility between E‑UTRAN and pre‑E‑UTRAN 3GPP radio access technologies, policy control and charging, and authentication.
to see a gradual view of Rel‑8 EPC architecture
Here are some links in support of this figure:
essential Table of Contents for TS 23.401 Word version: 16.6.0
each title links to the equivalent title in the full ToC