5G System Learning - System Overload (Initiation)

1. System Overload: In 5G networks, "overload" refers to excessive traffic or too many devices attempting to connect simultaneously, overwhelming network resources and leading to congestion, slow speeds, or connection failures. Strategies for addressing this overload include releasing more licensed spectrum, allocating resources through network slicing and core network functions, and implementing mechanisms such as throttling, exit timers, and overload messages to effectively control and manage traffic.

2. The overload initiation process notifies the NG-RAN node to reduce the signaling load directed to the associated AMF. This initiation process uses non-UE-associated signaling. As shown in Figure 8.7.7.2-1 below, the initiation process includes:

• An NG-RAN node receiving an overload initiation message should assume that the receiving AMF is in an overloaded state.
• If the Overload Start message contains the Overload Action IE and the AMF Overload Response IE, the NG-RAN node shall use it to identify the relevant signaling traffic. This information is used when the Overload Action IE is set to:
• “Reject RRC connection establishment for non-emergency mobile originated data transfers” (i.e., reject traffic corresponding to the RRC causes “mo-data”, “mo-SMS”, “mo-VideoCall”, and “mo-VoiceCall” in TS 38.331 or “mo-data” and “mo-VoiceCall” in TS 36.331), or
• “Reject RRC connection establishment for signaling” (i.e., reject traffic corresponding to the RRC causes “mo-data”, “mo-SMS”, “mo-signalling”, “mo-VideoCall”, and “mo-VoiceCall” in TS 38.331 or “mo-data”, “mo-signalling”, and “mo-VoiceCall” in TS 36.331), or
• “Allow RRC connection establishment only for emergency sessions and mobile terminated services” (i.e., allow only traffic corresponding to TS 38.331 or the RRC causes "emergency" and "mt-Access" in TS 36.331), or
• "RRC connection establishment is only allowed for high-priority sessions and mobile terminated services" (i.e., only traffic corresponding to the RRC causes "highPriorityAccess," "mps-Priority Access," "mcs-PriorityAccess," and "mt-Access" in TS 38.331 or "highPriorityAccess," "mo-ExceptionData," and "mt-Access" in TS 36.331 is allowed).

3. Overload Handling: The NG-RAN handles the situation as follows:

• If the OVERLOAD START message contains the AMF Traffic Load Reduction Indication IE, the signaling traffic is reduced by the indicated percentage; otherwise, only signaling traffic not indicated as rejected is sent to the AMF.
• If the Overload Start NSSAI List IE is included in the OVERLOAD START message, the NG-RAN node shall:
• If the Slice Traffic Load Reduction Indication IE is present, reduce the UE's signaling traffic by the indicated percentage, provided that the IE is present and the requested NSSAI contains only the S-NSSAI contained in the Overload Start NSSAI List IE and the signaling traffic reduction indicated by the Overload Action IE in the Slice Overload Response IE; otherwise, ensure that only signaling traffic from the UE (if the requested NSSAI matches, only signaling traffic from the UE's requested NSSAI containing S-NSSAIs other than the S-NSSAI contained in the Overload Start NSSAI List IE) or signaling traffic not reduced as indicated by the Overload Action IE in the Slice Overload Response IE) is sent to the AMF.
• If overload control is in progress and the NG-RAN node receives another OVERLOAD START message, the NG-RAN node shall replace the previously received message content with the new content.