5G Network Slicing Concepts

5G Network Slicing Concepts

prasanna sahu
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In 5G network communication infrastructure is not just confined to mobile voice/text communication, it is now segregated and very diversified to different services like Industrial IoT, Smart home domestic IoT, Low latency Medical communication, high bandwidth mobile broadband etc. And each of these services require different data behavior and QoS from network infrastructure.

In 5G each network node is equipped with special features to serve the purpose of one or multiple services and the kind of service supported by a particular node is defined in NSSF(Network Slice Selection Function). For any particular service request from UE, is served by a set of network entities associated with that Service and called a slice.

NSSAI(Network Slice Selection Assistance Information) Structure and Fundamentals

  • Network Slice configuration Information can have multiple NSSAI
  • Each PLMN can have at most one configured NSSAI
  • Each NSSAI has multiple S-NSSAI slices.
  • Each S-NSSAI slice has multiple DNNs configured.
  • A configured NSSAI can be configured by a serving PLMN or default NSSAI configured by HPLMN.
  • If Serving PLMN doesn’t have specific configured PLMN then it uses default configured NSSAI from HPLMN.
  • UE is pre-configured/provisioned by signalling message with default configured NSSAI by HPLMN.
  • UE is only configured with a set of subscribed S-NSSAIs out of the default configured NSSAI, which is a subset of the S-NSSAIs configured inside default configured NSSAI in HPLMN.
  • Allowed S-NSSAIs provided to the UE can have values, which are not served by Serving PLMN, in that case Serving PLMN updates the allowed S-NSSAI list with mapping to corresponding S-NSSAI of the HPLMN.

S-NSSAI and it’s Structure

Each Slice is identified by S-NSSAI (single network slice selection identifier)

  • SST is required value where was SD is optional
  • SST refer to expected behaviour of the slice.
  • SD is optional and differentiates among multiple slices with same SST.

  • UE during Registration and PDU session Establishment sends S-NSSAI value and optionally HPLMN NSSAI value, if in visiting area.
  • The requested NSSAI signalled by UE to network allows the network to select appropriate serving AMF, Network slice and network slice instance.
  • Based on the subscription data, one UE can have subscription to multiple S-NSSAIs and one of them can be marked as default S-NSSAI.
  • Subscription information for each S-NSSAI may have multiple DNN and one of them is default DNN.

Services provided by NSSF

Nnssf_NSSelection_Get service operation

  • May be invoked during Registration, for serving AMF selection and re-allocation.
  • PDU session establishment procedure, for SMF selection.
  • UE configuration update procedure, to update allowed S-NNAIs to UEs in current serving PLMN.


  • Nnssf_NSSAIAvailability_Update : In this process, AMF updates NSSF with S-NSSAIs supported by AMF per TA and   gets back availability of S-NSSAIs for each TA.
  • Nnssf_NSSAIAvailability_Notify  : AMF notify NSSF with restricted S-NSSAIs per TA using this procedure.

AMF Re-allocation Procedure

During UE registration procedure, if AMF doesn’t support one or more requested S-NSSAIs which is allowed by SPLMN/HPLMN then it request NSSF to provide the appropriate AMF to redirect the registration request from UE.

prasanna sahu

Telecom Professional


Barış Posted on8:01 am - May 26, 2019

Hi Prasanna, I kindly need your assistance for some slicing-specific issues and would be very grateful if you could help.

I am adding my questions below.

In 3GPP 28.541 spec – Management and orchestration; 5G Network Resource Model (NRM)), we found some ambigious points that need to be clarified.

1-) Does slice related info comes from orchestration? If so from which element? OSS or EMS?

2-)How will NSSF get this info? (Does it make request or it is periodically/automatically updated by OSS/EMS)

3-)After the slice/network config is modified and updated ( eg. by operator) how will NSSF be informed? What will be the method for the transfer of updated info to NSSF side?
4-)We also need some info for the transfer of Yang files, what is the transfer method? (Which interface and protocol etc.etc.) ( Between OSS and EMS? or other parts?)
​ 5-)Also we would like to know the full exact content of slice specific data that is transferred between OSS/EMS and NSSF. (What is included in data model? NSI,NSSAI, PLMN ıd etc.)

Many thanks for your generous contribution.
Best regards.

Huseyin Y. Posted on9:27 am - May 27, 2019

Is there a governing rule about the number of nssf instances per plmn in 5g core?
can we say that there is at most one NSSF instance per PLMN?
thanks in advance

    Venkatesh (Venky) Aravamudhan Posted on8:42 am - August 12, 2019

    This is a good question, NSSF is a PLMN level entity in most cases ( TS 23.003 specification allows for NSSF FQDN which includes PLMN and other details). For roaming, H-NSSF may be determined using PLMN.

    Can NSSF be part of a slice, quiet possible, as no one seems to restrict that option too with initial registration process performed by PLMN level NSSF and slice related queries handled by slice level NSSF (all this enabled through NRF)>

Huseyin Y. Posted on9:47 am - May 27, 2019

can you please elaborate which network function hosts SNSSAI – nsi mapping?
for example when AMF asks for a slice instance with a snssai to the NSSF , how does the nssf retrieve the relevant slice related attributes such as nsiId of nssai information?(or from where does the nssf retrieve that info ?) . Thanks in advance.

    Venkatesh (Venky) Aravamudhan Posted on8:39 am - August 12, 2019

    3GPP specification indicates that NSI is a deployment option. Mapping of S-NSSAI with NSI may need to be performed when slice orchestration comes in picture, i.e., when a NSI is created, it would indiciate the S-NSSAIs that it would support. Please refer to TS 29.532 for information. Have to say, specifications is still under work.

    In the current information, S-NSSAI to NSI would be at the most, operator configured.

swensens Posted on4:41 pm - February 28, 2020

why don’t we use NRF instead ? because NRF also know the NSSAI for each NFs?

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