Why Most Telecom Engineers Can't Explain a 5G SA Registration — And How to Fix That
Here's something I've noticed after years in telecom: most engineers can tell you the difference between NSA and SA at a high level, but ask them to walk through a 5G SA Initial Registration call flow message by message — AMF selection, SUCI decryption via AUSF/UDM, the NAS Security Mode procedure, the N2 Initial Context Setup — and they freeze.
It's not because they're bad engineers. It's because the 3GPP specs are designed as reference documents, not learning material. TS 23.502 assumes you already understand the context. TS 24.501 throws NAS message structures at you without explaining why they exist.
The gap isn't knowledge — it's structure.
Here's what actually works for learning call flows:
1. Start with the network functions, not the messages. Before you trace a single arrow on an MSC, understand what the AMF, SMF, and UPF actually do. The messages make sense once you know each node's responsibility.
2. Compare 4G and 5G side by side. If you already understand LTE Attach, 5G SA Registration is 80% familiar. The MME split into AMF + SMF, the S1AP → NGAP transition, the EPS bearer → PDU Session mapping — it all clicks when you see the evolution.
3. Draw the call flow yourself. Reading an MSC is passive. Drawing one from memory forces you to understand the sequence and the reason behind each step.
I built CallFlow Academy specifically for junior and mid-level engineers who want to go from "I sort of know the architecture" to "I can trace any call flow end to end." It's structured, sequential, and covers 4G LTE, 5G NSA, and 5G SA from the ground up.