PCF — Policy Control Function — the rules a session must obey

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The core network

Where it sits

Sits atLevel 2 of the hierarchy · The core network · explained
Learn firstNetwork function — 2 concepts in the full chain, see the paths
UnlocksPolicy control
Primary clauseTS 23.501 §6.2.4, TS 23.503 §6.1
Used inref-core 5 · ref-session 3 · index 1
Scanned from the notes at page load and joined with terms.json; nothing on this card is typed by hand.

What it is

What it is. The Policy Control Function — where the operator's intent enters the system. TS 23.501 §6.2.4, with the detail in TS 23.503 §6.2.1. Its list in the architecture specification is four lines:

Supports a unified policy framework to govern network behaviour · provides policy rules to control plane functions to enforce them · accesses subscription information relevant for policy decisions in a UDR · supports PDU Set based handling.

NOTE: "The PCF accesses the UDR located in the same PLMN as the PCF."

"Provides policy rules to control plane function(s) to enforce them" is the shape of the whole thing. The PCF decides nothing at packet level and enforces nothing. It writes rules, and other functions carry them out:

Rule Goes to Which then
PCC rules the SMF programs the UPF over N4 to detect and treat traffic
URSP rules the device, over NAS decides which app's traffic goes on which PDU session and slice
AM policy the AMF applies service-area and RFSP restrictions

URSP is the interesting direction, because policy reaches into the device. Policy is usually imagined as something the network does to traffic. URSP is the operator telling the device how to sort its own traffic before any of it is sent — which application matches which route, and therefore which slice. Without it, slicing would need the network to classify traffic it may not be able to see, and the device would have no way to know which of its sessions to use.

It is a policy decision point. The enforcement points are the UPF (for packets) and the device (for routing). Keeping the decision in one function is what lets an operator change behaviour without touching either — the same separation the CP/UP split makes for forwarding.

Read on

This concept is read as part of one argument in ref-core, alongside the rest of its group.

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Before this concept, the hierarchy says to learn the following — the full chain, in order:

To understand PCF (level 2) you first need 2 other concepts. Read them in this order — everything on one line can be read in any order, but no line before the one above it:
Level Learn these
0 5G Core
1 Network function
Immediately before PCF: Network function.
Keep going — where this sits on the route
◀NSSFstep 18 · level 2
Step 19 of 118level 2 · The core network
S-NSSAIstep 20 · level 2▶
Level 2 → 3 · this unlocks
Policy control level 12
↳ all 9 concepts at level 3 · level 2 in full · the whole route
The route is every concept in the folder ordered by level, so nothing here needs anything after it. Computed at page load from terms.json; the same numbering as the route page.
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