NSSAI
Network Slice Selection Assistance Information: a collection of S-NSSAI values that identifies the network slices a UE is authorized to use or is requesting to use.
NSSAI is just the container for a UE's slice identities — a set of S-NSSAIs rather than a single one, because a device can be on several slices at the same time. What makes it confusing the first time is that the same word shows up in several flavours during signalling, and each means something specific:
- Requested NSSAI — what the UE asks for at registration.
- Allowed NSSAI — what the network actually grants it in the current area, after the AMF (sometimes with the NSSF) decides.
- Configured NSSAI — what's provisioned on the UE for a PLMN.
- Subscribed S-NSSAIs — what the subscription permits.
They often don't match: a UE can request more than it's allowed, and the allowed set can change as it moves between areas where different slices are available. When you debug slicing, half the work is figuring out which of these lists you're looking at and why the granted set differs from the requested one.
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