P-GW
PDN Gateway: the EPC node providing connectivity to external packet data networks, performing IP address allocation, policy enforcement, and charging functions.
The PDN Gateway is the edge of the mobile network — the point where your traffic crosses from the operator's world into the actual data network you asked for, whether that's the public internet or a corporate APN. It hands you your IP address, and it's the natural place to enforce everything that needs to look at your traffic as a whole: policy from the PCRF, per-flow QoS, packet inspection, lawful intercept, and charging.
A useful mental model: the S-GW is the mobility anchor (it moves with you across eNBs), while the P-GW is the session anchor (it stays fixed for the life of your PDN connection, which is why your IP doesn't change as you roam). The Gi or SGi interface on its far side is the boundary to external networks. As with the S-GW, the 5G core absorbs the P-GW's forwarding role into the UPF and pulls its decision-making out into the SMF and PCF — but the architectural job is unchanged.
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