EPC

The EPC is the 4G core — the all-IP brain behind the radio. Its defining design choice was the clean split between control plane and user plane. Signalling (who you are, where you are, what you're allowed to do) flows through the MME and HSS; your actual packets flow through the Serving Gateway and PDN Gateway. Keeping those apart is what made the architecture scale and, later, what made it natural to virtualise.

The pieces you'll meet on the job:

• MME — control-plane anchor: authentication, mobility, paging, bearer setup.
• S-GW — user-plane anchor that holds your session steady across eNB handovers.
• P-GW — the door to the outside world; allocates your IP and enforces policy.
• HSS — the subscriber database with credentials and service profiles.
• PCRF — decides QoS and charging rules per service flow.

When 5G arrived, the EPC didn't vanish. In non-standalone deployments the 5G radio still anchors on the EPC, and only standalone 5G moves to the service-based 5G Core (5GC). Plenty of live networks still run an EPC underneath their 5G layer.