5G SA EPC LTE Mobile Networks

CUPS – Control and User Plane Separation in LTE & NR with PFCP (Sx & N4)

A look at PFCP for the Sx and N4 Interfaces

3GPP release 14 introduced the concept of CUPS – Control & User Plane Separation, and the Sx interface, this allows the control plane (GTP-C) functionality and the user plane (GTP-U) functionality to be separated, and run in a distributed fashion, allowing the node a user’s GTP-U traffic flows through to be in a different location to where the Control / Signalling traffic (GTP-C) flows.

In practice that means for an LTE EPC this means we split our P-GW and S-GW into a minimum of two network elements each.

A P-GW is split and becomes a P-GW-C that handles the P-GW Control Plane traffic (GTPv2-C) and a P-GW-U speaking GTP-U for our User Plane traffic. But the split doesn’t need to stop there, one P-GW-C could control multiple P-GW-Us, routing the user plane traffic. Sames goes for S-GW being split into S-GW-C and S-GW-U,

This would mean we could have a P-GW-U located closer to a eNB / User to reduce latency, by allowing GTP-U traffic to break out on a node closer to the user.

It also means we can scale better too, if we need to handle more data traffic, but not necessarily more control plane traffic, we can just add more P-GW-U nodes to handle this.

To manage this a new protocol was defined – PFCP – the Packet Forwarding Control Protocol. For LTE this is refereed to as the Sx reference point, it’ also reused in 5G-SA as the N4 reference point.

When a GTP-C “Create Session Request” comes into a P-GW for example from an S-GW, a PFCP “Session Establishment Request” is sent by the P-GW-C to the P-GW-U with much the same information that was in the GTP-C request, to setup the session.

So why split the Control and User Plane traffic if you’re going to just relay the GTP-C traffic in a different format?

That was my first question – the answer is that keeping the GTP-C interface ensures backward compatibility with older MMEs, PCRFs, charging systems, and allows a staged roll out and bolting on extra capacity.

GTP-C disappears entirely in 5G Standalone architecture and is replaced with the N4 interface, which uses PFCP for the Control Plan and GTP-U again.

Here’s a capture from Open5Gs core showing GTPv2C and PFCP in play.

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