5.31.14.3 Small Data Rate Control

The (H-)SMF may consider, e.g. based on operator policy, subscription, DNN, S-NSSAI, RAT type etc. to determine whether to apply Small Data Rate Control or not. The (H-)SMF can send a Small Data Uplink Rate Control command to the UE using the PCO information element. The (H-)SMF informs the UPF or NEF of any Small Data Rate Control that shall be enforced.

The Small Data Rate Control applies to data PDUs sent on that PDU Session by either Data Radio Bearers or Signalling Radio Bearers (NAS Data PDUs).

The rate control information is separate for uplink and downlink and in the form of:

• an integer 'number of packets per time unit' and
• an integer 'number of additional allowed exception report packets per time unit' once the rate control limit has been reached.

The UE shall comply with this uplink rate control instruction. If the UE exceeds the uplink 'number of packets per time unit', the UE may still send uplink exception reports if allowed and the 'number of additional allowed exception reports per time unit' has not been exceeded. The UE shall consider this rate control instruction as valid until it receives a new one from (H-)SMF.

When a PDU Session is first established, the (H-)SMF may provide the configured Small Data Rate Control parameters to the UE and UPF or NEF.

When the PDU Session is released, the Small Data Rate Control Status (including the number of packets still allowed in the given time unit, the number of additional exception reports still allowed in the given time unit and the termination time of the current Small Data Rate Control validity period) may be stored in the AMF so that it can be retrieved for a subsequent re-establishment of a new PDU Session.

At subsequent establishment of a new PDU Session, the (H-)SMF may receive the previously stored Small Data Rate Control Status and if the validity period has not expired, it provides the parameters to the UE in the PCO and to the UPF/NEF as the initially applied parameters, in addition to the configured Small Data Rate Control parameters. If the initially applied parameters are provided, the UE and UPF or NEF shall apply them and shall use the SMF provided configured Small Data Rate Control parameters once the initially applied Small Data Rate Control validity period expires.

NOTE 1: Storage of Small Data Rate Control Status information for very long time intervals can be implementation specific.

For the UPF and NEF, Small Data Rate Control is based on a 'maximum allowed rate' per direction. If (H-)SMF provided the 'number of additional allowed exception report packets per time unit', then the 'maximum allowed rate' is equal to the 'number of packets per time unit' plus the 'number of additional allowed exception report packets per time unit', otherwise the 'maximum allowed rate' is equal to the 'number of packets per time unit'.

The UPF or NEF may enforce the uplink rate by discarding or delaying packets that exceed the 'maximum allowed rate'. The UPF or NEF shall enforce the downlink rate by discarding or delaying packets that exceed the downlink part of the 'maximum allowed rate'.

NOTE 2: It is assumed that the Serving PLMN Rate is sufficiently high to not interfere with the Small Data Rate Control as the Small Data Rate Control, if used, is assumed to allow fewer messages. NAS PDUs related to exception reports are not subject to the Serving PLMN Rate Control.

For NB-IoT the AMF maintains an "MO Exception Data Counter" which is incremented when the RRC establishment cause "MO exception data" is received from NG-RAN. The AMF reports whether the UE accessed using "MO exception data" RRC establishment cause, to all (H-)SMFs which have PDU Sessions that are subject to Small Data Rate Control and if the UE is accessing using "MO exception data" then the "MO Exception Data Counter" is also provided by the AMF. The SMF indicates each use of the RRC establishment cause "MO Exception Data" by including the related counter on the charging information.

NOTE 3: Since Exception Data PDUs and normal priority PDUs cannot be distinguished within an RRC connection, the AMF is only counting the number of RRC Connection establishments with "MO Exception data" priority.

If the UE moves to EPC then the UE and the PGW-U+UPF store the current Small Data Rate Control Status for all PDU Sessions that are not released. If the UE moves back to 5GC the stored Small Data Rate Control Status is restored and continues to apply to PDU Session(s) that are moved from EPC to 5GC, taking into account remaining validity period of the stored Small Data Rate Control Status. When the UE moves to EPC the Small Data Rate Control Status for all PDU Session(s) may also be stored in the AMF if the PDU Session is released while the UE is connected to EPC and re-established when the UE moves to 5GC. The time to store the Small Data Rate Control Status information is implementation specific.