Before we talk about 5G Core, it is worth understanding what 4G Core was built to do — and why it worked so well.
4G Core, also known as EPC, was designed to handle mobile data in a more efficient and packet-focused way than earlier generations.
Its job was to manage mobility, session control, and user data flow while keeping the network stable and scalable.
At a high level, the major functions were handled by nodes such as:
MME for mobility and signaling control.
SGW for user plane anchoring and routing.
PGW for internet breakout and external connectivity.
HSS for subscriber data and authentication.
PCRF for policy and charging decisions.
From an engineering perspective, the 4G Core introduced a clean separation between control and user plane functions, which was a major step forward.
It supported LTE growth very effectively and became the backbone of mobile broadband for years.
But even with that success, the architecture was still built for a different era.
In the next part, I’ll break down where the limitations started to appear — and why 5G Core needed a new design approach.
This is a very good explanation of EPC and have a very good understanding of the network but I will do small adjustments to make it perfect.The SGW—→PGW interface is not S6a it is S5/S8 interface if both nodes are in Same PLMN then it is S5 and different PLMN(VoLTE Roaming) it is S8 and thats why Volte Roaming is called S8 Home Routing(S8HR).And that interface itself is the one deviate from Control and User plane sepearation in EPC and Both Control and User plane flows via Same path.Usually All sessions are Anchored in PGW not SGW and thats why all the Policy related triggers Gx and Charging related triggers are originating from PGW.And Convergence of Control plane and user plane in PGW can be understand easily if you see the real network digram where most of the time SGW/PGW reside in the same box.This is the issue which is corrected in 5G SA to complete the CUPS(Control & USER Plane Seperation) in Packet Core. AMF/MME, SGW-C/PGW-C/SMF, and PGW-U/UPF are devided so control and User plane follow completely different path from start to End.
