GTP-C and GTP-U Finally Separated Control from Subscriber Traffic
GTP made more sense once I stopped treating it as one protocol and separated tunnel-control signalling from the packets that actually carry user traffic.
EPC
Open5GS Made the EPC Feel Like a Set of Services Instead of One Big Core
Breaking the EPC into MME, HSS, SGW, PGW and PCRF roles made LTE core traffic much easier to follow in captures and logs.
Walking a VoLTE Call End to End Connected the EPC, IMS and RTP Pieces
Following one VoLTE call from LTE attachment through IMS registration, SIP session setup, bearer creation and RTP finally connected the year's separate labs.
APNs, PGW Selection and the Path to the Data Network
An APN is more than a label on the handset; it influences how a subscriber session reaches a packet gateway and an external data network.