CUPS Architecture
Side-by-side comparison of Control / User Plane Separation in the evolved EPC (3GPP TS 23.214) and the fully service-based 5G Core (TS 23.501). Dashed lines are control-plane signalling, solid lines are user-plane traffic.
EPC with CUPS
5G Core (SBA)
PFCP — Packet Forwarding Control Protocol
PFCP (3GPP TS 29.244) is the single protocol that lets a control-plane node program a user-plane node. In EPC with CUPS it runs on Sxa (SGW-C↔SGW-U), Sxb (PGW-C↔PGW-U) and Sxc (TDF-C↔TDF-U). In 5GC it runs on N4 (SMF↔UPF). Messages are UDP on port 8805 and carry PDRs (Packet Detection Rules), FARs (Forwarding Action Rules), QERs (QoS Enforcement Rules) and URRs (Usage Reporting Rules). The same machinery powers both architectures — which is why multi-vendor UPFs can be re-used across 4G and 5G deployments.
Comparison
| Feature | Pre-CUPS EPC | EPC with CUPS | 5GC |
|---|---|---|---|
| Control / User split | Combined in SGW & PGW | Separated (Sx) | Separated (N4) |
| Protocol | GTP-U + Diameter / GTP-C | PFCP (Sxa / Sxb / Sxc) | PFCP over N4 |
| User-plane NF | SGW / PGW | SGW-U / PGW-U | UPF |
| Service framework | Reference points | Reference points | Service-Based (SBI, HTTP/2) |
| Scalability | Monolithic | CP/UP scale independently | CP fully stateless micro-services |
| Typical deployment | 3G/4G legacy | 4G + fixed-mobile conv. | 5G SA, MEC, slicing |
About CUPS
Before Release 14 the SGW and PGW in LTE bundled control-plane signalling with user-plane packet forwarding on the same box. Operators that wanted to scale throughput (say, for a sudden video streaming spike) had to scale the control plane too, and had to place the heavy data-plane hardware exactly where the signalling needed to sit. CUPS — introduced in TS 23.214 — broke that coupling. The control plane (SGW-C, PGW-C, TDF-C) stays centralised while the user plane (SGW-U, PGW-U, TDF-U) can be pushed to the network edge, closer to RAN aggregation sites and MEC platforms. The glue between them is PFCP.
5G Core takes the same idea and bakes it in from day one. There is no such thing as a combined SMF/UPF — the SMF is always control plane, the UPF always user plane, and the N4 interface between them is just PFCP again. The payoff is low-latency URLLC and edge-local breakout: the same SMF in a central data centre can steer a UE onto a UPF running inside a factory, stadium or base of a cell tower.
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