“Statelessness” of the 5G Network Functions is often mentioned as one of the concepts for better network optimization and improving its reliability and resilience, compared to the “earlier Gs”. A stateful network element needs to store certain user/connection/association data (a “context”), and in a case of its failure the whole maintained connection obviously collapses. In contrast, connectionless elements – not storing any related data locally – are therefore not a point of failure: a different network element with relevant capabilities can be used instead, thus guaranteeing continuity of the association/connection in question.

The 5G UDSF

The obvious benefits of de-coupling compute from storage have resulted in a new database functionality defined in the 5G standards, to act as a storage of dynamic connection-related contexts for various NFs handling the actual connections: “The Unified Data Management (UDM): the 5GC supports Data Storage architecture for Compute and Storage separation. The Unified Data Repository (UDR) is the master database. The Unstructured Data Storage Function (UDSF) is introduced to store dynamic state data.”  [3GPP TS 21.915].

At the moment (R15) the AMF seems to be the main beneficiary of the UDSF’s introduction. With a remote storage to place UE-related data different AMF Instances can be used at different moments in time to serve a particular UE, creating a more flexible and a better-balanced network environment.

Implementation choices for the 5G NFs

There are four possible levels of statelessness of NFs than an operator can implement:

  • No state: no UE state information stored in the NF (e.g. the UDM Front End);
  • Stateless: the relevant UE state information is pulled by the NF from the UDSF for the duration of the transaction only (e.g. PCF);
  • State efficient: the relevant UE state information is pulled by the NF from the UDSF and kept there during periods of high UE activity, cached for a few seconds or minutes, with the stable UE state information stored by the NF in the UDSF at the end of a certain procedure (e.g. AMF or SMF);
  • Stateful: the UE state information is stored permanently in the NF (e.g. UDR).

R15 3GPP guidelines for Compute-Storage separation

For operators interested in deploying a UDSF functionality in their network, the 3GPP suggests conforming with the following guidelines [TS 23.501, Annex C]

  • The UDSF can be used as either Primary or Secondary Storage. In the first case the NF does not need to be stateful, in the second case the UDSF acts as a backup storage area;
  • 3GPP does not standardise triggers for storing unstructured data, but recommends for the transfer to be performed once the state information is stable;
  • Implementation-dependant measures should be taken to handle race conditions (multiple NFs attempting access at the same time);
  • AMF Instances that can be used interchangeably (i.e. AMF Instances belonging to the same AMF Set) must have access to the same storage entity;
  • UDSF usage is a pre-requisite for AMF planned removal and AMF auto-recovery network features.

Implementing the UDSF

Deployment details, and placement of the UDSF will be operator-specific, under the constraint that the NF using the UDSF and the UDSF itself must be in the same MNO’s (Mobile Network Operator) PLMN. Operators might decide to co-locate the UDSF with the UDR, or to place it close to a NF (or group of NFs) that it serves.

If you are interested in the various 5G Core Network functions, have a look at our 5G Core Network Architecture course.

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