S-NSSAI — How a slice is named, and how many a device may hold at once
Sessions, flows and slices
Where it sits
What it is
What it is. Single Network Slice Selection Assistance Information — how a slice is named. TS 23.501 §5.15.2. It has two parts:
SST — Slice/Service Type, "which refers to the expected Network Slice behaviour in terms of features and services".
SD — Slice Differentiator, optional, "which complements the Slice/Service type(s) to differentiate amongst multiple Network Slices of the same Slice/Service type".
The standardised SST values, Table 5.15.2.2-1 — the only ones that mean the same thing in every network:
| SST | Slice/service type | For |
|---|---|---|
| 1 | eMBB | enhanced mobile broadband |
| 2 | URLLC | ultra-reliable low latency |
| 3 | MIoT | massive IoT |
| 4 | V2X | vehicular services |
| 5 | HMTC | high-performance machine-type communication |
| 6 | HDLLC | high data rate and low latency |
| 7 | GBRSS | guaranteed bit rate streaming |
SST 1, 2 and 3 are eMBB, URLLC and mMTC — the three service classes from TR 38.913, appearing here as slice types. The requirements study's categories became the core's vocabulary.
Two hard numbers, both checkable. §5.15.2.1: "There can be at most eight S-NSSAIs in Allowed NSSAI and Requested NSSAI sent in signalling messages between the UE and the Network." And §5.15.3: the UDM sends "at the most 16 Subscribed S-NSSAIs" to the AMF. So a subscription may entitle a device to sixteen slices, but it may only be actively registered in eight at once.
A non-standard S-NSSAI may not travel. §5.15.2.1: an S-NSSAI with a non-standard value "shall not be used by the UE in access stratum procedures in any PLMN other than the one to which the S-NSSAI is associated". An operator's private slice identifier is meaningless in someone else's network — which is why roaming needs an explicit mapping of serving-PLMN S-NSSAIs to home-PLMN ones, and why the standardised SST values exist at all.
The NSSAI family. An NSSAI is a collection of S-NSSAIs, and which collection matters: Configured (what the device has been given), Requested (what it asks for), Allowed (what the network granted), Rejected. The device asks; the NSSF decides; the answer is the Allowed NSSAI.
Read on
This concept is read as part of one argument in ref-session, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: