Moving from 4G to 5G: what transfers and what does not
If you know EPC well, a surprising amount of 5G is familiar. Here is the map from what you already know to what is genuinely new.
Engineers with solid EPC experience often approach 5G as if it were a fresh start. It is not. Much of it is the same problem solved with different plumbing, and knowing which parts are which saves months.
Maps almost directly
The MME splits into the AMF and part of the SMF, but the job of terminating NAS and managing mobility is recognisably the same. The SGW and PGW control planes become the SMF; their user planes become the UPF. The HSS becomes the UDM plus the UDR. The PCRF becomes the PCF.
If you can walk through an attach, you can walk through a registration. The steps line up more closely than the naming suggests.
Renamed, mostly the same
Bearers become QoS flows. It is a genuinely better model - flows are more granular and the mapping to radio bearers is explicit - but if you understand QCI and TFT, 5QI and QoS rules will take an afternoon.
APN becomes DNN. TAU becomes registration update. The concepts survive the rename.
Genuinely new
The service based interface. Diameter and GTP-C give way to HTTP/2 and JSON over a bus rather than point to point links. This is the biggest single shift, and it changes how you debug: you are reading REST calls, not decoding AVPs.
NF discovery. In EPC you configured peers. In 5G the NRF is a live registry, and instances come and go. The NF Set concept has no EPC equivalent at all.
Slicing as a first class concept. Not an afterthought bolted onto APNs but built into selection at every layer.
SUCI and identity concealment. The IMSI went over the air in the clear far too often in 4G. Concealment is new and it changes the authentication flow.
Where to spend your time
Skip the temptation to re-learn mobility from scratch. Spend the time on HTTP/2 and the SBI, on the NRF and discovery, and on PFCP - because CUPS was optional in 4G and is mandatory now. Those three are where EPC experience gives you the least head start, and they are exactly the areas interviews probe.