Network slice — One physical network, several logical ones
Sessions, flows and slices
Where it sits
What it is
What it is. One physical network presented as several logical ones, each with its own functions, its own policy and its own promises. TS 23.501 §5.15. A slice is identified by an S-NSSAI; an instance of one is a Network Slice Instance.
What is sliced and what is not — the part that is usually got wrong:
| Per slice? | |
|---|---|
| SMF, UPF, PCF | Yes — different instances per slice |
| AMF | No — one AMF serves all of a device's slices |
| UDM, AUSF | No — the subscription is not per slice |
| The radio | Shared — one air interface, differentiated by QoS and scheduling |
The AMF being common is the constraint everything else bends around. A device has one NAS connection and therefore one AMF, but may be registered in several slices at once. So the AMF must be able to serve the intersection of the device's slices — which is exactly the problem the NSSF exists to solve, and the reason slice selection is a function rather than a lookup.
The radio is not sliced, and this is worth being honest about. There is one carrier, one resource grid, one scheduler. Slice differentiation at the radio is achieved by the QoS profile and by the operator's scheduling policy — not by partitioning spectrum. A slice is a core network construct with radio consequences, and marketing material that shows the air interface cut into coloured slabs is describing something the specification does not do.
Why it only became feasible in 5G. Standing up another logical core means standing up more instances of control functions. Under LTE's node model, each new instance needed its peers' addresses configured — cost growing with slices × interfaces. Under the service-based architecture the instances register themselves with the NRF and are found by discovery, filtered on S-NSSAI. Slicing is not a feature bolted onto the SBA; it is the SBA's most direct consequence.
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: