S-NSSAI and 5G Slice Usage

As illustrated by 3GPP standard TR 28.801 Study on management and orchestration of network slicing for next-generation network (Release 15), one Network Slice Instance can be consumed by many Communication Service Instances. And one Communication Service Instance can consume more than on NSI. This is as per the logical model shown in Figure 4.9.3.1 of 3GPP TR 28.801.

This relationship is shown in the Information model of Figure 4.2.2.1 of 3GPP TR28.801

A specific example of this is illustrated in my contributions to TMForum IG1224

The S-NSSAI is created during the 5G Network Slice Creation Process. The specific requirements for the slice (based on the service profile) are provided by the OSS/BSS system when ordering a slice from the Network Slice Management function (NSMF). This includes the S-NSSAI used to identify the appropriate Network Slice Instance based on Service Profile.

How the S-NSSAI is used depends on how the 5G slicing network is configured and available network functions like

  • NSSF
  • URSP
  • pre-configured S-NSSAIs based on specific requirements of network slice
  • default S-NSSAIs as defined by 3GPP for specific types of services and applications

The NSSF is not a mandatory 5G Core Network function and plays a role in 5G slicing either

a) when AMF cannot provide the requested NSSAI (made up of S-NSSAIs)

b) when there is more than one slice with the same NSSAI

Without an NSSF function in the network, the S-NSSAI would be pre-configured in the network. This would be part of the slice creation workflow. In this case, the S-NSSAI can be selected based on the preconfigured policies without needing an NSSF.

In cases where there is no NSSF function in the network and pre-configured policies have not been defined, the default N-SSAI is used.

Multiple consuming services on a slice

For the scenario in which more than one Communication Service Instance consumes one Networks Slice Instance, each Communication Service Instance would have its own Resource Facing Service Instance related to the NSI. Because the provisioning of the RFS uses parameters specific to the RFS and specified in the RFSSpec.

As shown below multiple RFSs relate to one NSI.

Consuming service requires multiple slices

For the scenario where one Communication Service needs more than one NSI, then the CFS would decompose to multiple RFSs, with each RFS instance related to its respective NSI.

RFS instance tracks service usage and characteristics of service usage per slice consumer.