NSSF — Network Slice Selection Function — which slice, and therefore which AMF

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The core network

Where it sits

Sits atLevel 2 of the hierarchy · The core network · explained
Learn firstNetwork slice — 2 concepts in the full chain, see the paths
UnlocksNothing else depends on it directly.
Primary clauseTS 23.501 §6.2.14, §5.15.5
Used inref-core 6 · index 1 · ref-session 1 · ref-ue-state 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 Network Slice Selection Function — the function that turns which slice into which AMF. TS 23.501 §6.2.14, with the selection procedure in §5.15.5. It is consulted over N22, by an AMF, and it is not on the path of anything else.

The problem it solves. A device sends a Requested NSSAI — the set of S-NSSAIs it wants — in its Registration Request. But the AMF that received it was chosen by the RAN from partial information, and may not serve those slices. Something has to decide:

  1. Which of the requested S-NSSAIs this device is allowed in this serving network — producing the Allowed NSSAI, at most eight of them.
  2. Which AMF set can serve that combination — because an AMF must be common to every slice the device is registered in.
  3. Whether the initially-selected AMF must therefore be replaced, and the registration re-routed to one that qualifies.

The AMF-must-be-common constraint is what makes this a function rather than a lookup. A device may hold several slices at once, and it has exactly one AMF per access. So the AMF cannot be chosen per slice — it must be chosen for the intersection, and that is a constraint problem the RAN, which only saw the Requested NSSAI in an RRC message, is in no position to solve.

Note what the NSSF does not do: it does not find instances. Once the slice set is decided, which SMF or UPF instance serves it is answered by the NRF, because discovery already filters on S-NSSAI. The NSSF answers which slice; the NRF answers which instance. Keeping those apart is why neither is complicated.

Roaming. §5.15.5 provides for an NSSF in the visited network talking to one in the home network over N31, since the slice a subscriber is entitled to is a home-network fact and the slices that exist are a visited-network fact.

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 NSSF (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 slice
Immediately before NSSF: Network slice.
Keep going — where this sits on the route
◀Service-based architecturestep 17 · level 2
Step 18 of 118level 2 · The core network
PCFstep 19 · level 2▶
Nothing depends on this one directly
It is a leaf of the graph — follow the route rather than the branch.
↳ 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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