5.19.5.2 AMF Overload Control

Under unusual circumstances, if AMF has reached overload situation or there is high probability that signalling storm will happen based on received analytics of signalling storm caused by NF(s) abnormal signalling or set of UEs in specific group, slice or priority as described in clause 6.22 in TS 23.288 [86], the AMF activates NAS level congestion control as specified in Clause 5.19.7 and AMF restricts the load that the 5G-AN node(s) are generating, if the 5G-AN is configured to support overload control. N2 overload control can be achieved by the AMF invoking the N2 overload procedure (see TS 38.300 [27] and TS 38.413 [34]) to all or to a proportion of the 5G-AN nodes with which the AMF has N2 connections. The AMF may include the S-NSSAI(s) in NGAP OVERLOAD START message sent to 5G-AN node(s) to indicate the Network Slice(s) with which NAS signalling is to be restricted. To reflect the amount of load that the AMF wishes to reduce, the AMF can adjust the proportion of 5G-AN nodes which are sent NGAP OVERLOAD START message and the content of the overload start procedure.

When NGAP OVERLOAD START is sent by multiple AMFs or from the same AMF set in the same PLMN towards the 5G-AN, it should be ensured that the signalling load is evenly distributed within the PLMN and within each AMF set.

A 5G-AN node supports restricting of 5G-AN signalling connection when a signalling connection establishment are attempted by certain UEs (which are registered or attempting to register with the 5GC), as specified in TS 38.331 [28] and TS 36.331 [51]. Additionally, a 5G-AN node provides support for the barring of UEs as described in TS 22.261 [2]. These mechanisms are further specified in TS 38.331 [28] and TS 36.331 [51]. For 3GPP Access Type, the signalling connection establishment attempt includes a RRC Connection Resume procedure from RRC_INACTIVE.

By sending the NGAP OVERLOAD START message, the AMF can request the 5G-AN node to apply the following behaviour for UEs that the AMF is serving:

• Restrict 5G-AN signalling connection requests that are not for emergency, not for exception reporting and not for high priority mobile originated services; or
• Restrict 5G-AN signalling connection requests for uplink NAS signalling transmission to that AMF;
• Restrict 5G-AN signalling connection requests where the Requested NSSAI at AS layer only includes the indicated S-NSSAI(s) in the NGAP OVERLOAD START message. This applies also to RRC_INACTIVE Connection Resume procedure where the Allowed NSSAI in the stored UE context in the RAN only includes S-NSSAIs included in the NGAP OVERLOAD START.
• only permit 5G-AN signalling connection requests for emergency sessions and mobile terminated services for that AMF; or
• only permit 5G-AN signalling connection requests for high priority sessions, exception reporting and mobile terminated services for that AMF;

The above applies for RRC Connection Establishment procedure and RRC Connection Resume procedures over 3GPP access, as well as for the UE-N3IWF connection establishment over untrusted Non-3GPP access and for the UE-TNGF connection establishment over trusted Non-3GPP access.

The AMF can provide a value that indicates the percentage of connection requests to be restricted in the NGAP OVERLOAD START and the 5G-AN node may consider this value for congestion control.

When restricting a 5G-AN signalling connection, the 5G-AN indicates to the UE an appropriate wait timer that limits further 5G-AN signalling connection requests until the wait timer expires.

During an overload situation, the AMF should attempt to maintain support for emergency services and for MPS.

When the AMF is recovering, the AMF can either:

• send a NGAP OVERLOAD START message with a new percentage value that permits more connection requests to be successful, or
• send a NGAP OVERLOAD STOP message.

to the same 5G-AN node(s) the NGAP OVERLOAD START was previously sent.