5G System Learning - System Overload (Stop)

In mobile communication networks, "system overload" occurs when excessive service traffic or too many devices simultaneously attempt to connect overwhelm network resources, leading to congestion, slow speeds, or connection failures. System protection mechanisms are activated to address these overloads. Specific strategies include network operators releasing more licensed spectrum, allocating resources through network slicing, implementing throttling within core network functional units, and enabling mechanisms such as backoff timers and overload messages to effectively control and manage the volume of users.

1. Overload Activation: In a 5G (NR) network, the Access and Mobility Management Function (AMF) sends an "Overload Activation" message to other relevant network elements (such as gNBs) based on its processing capacity (configuration) thresholds, indicating an overload condition. This triggers congestion control measures (such as rejecting connection requests from some user equipment (UEs)) to protect the network from failure. Overload Activation involves the AMF sending an NGAP Overload Activation message to the NG-RAN (Radio Access Network) node, requesting it to limit certain types of traffic and redirect or reject requests to maintain network stability during periods of high demand.

1.1 Overload Control Involves

• Congestion Detection: The AMF or other network elements, such as the User Plane Function (UPF), monitor network load and identify when predefined congestion thresholds are exceeded.
• Overload Control Message: Upon detecting an overload, the AMF sends an NGAP Overload Control message to the connected NG-RAN node.
• Congestion Control Actions: Upon receiving the message, the NG-RAN node initiates control actions to manage the overload. These actions include:
• Rejecting Certain Connections: The NG-RAN may reject connection requests from user equipment (UE) for non-emergency or high-priority services.
• Limiting Uplink Signaling: The NG-RAN can limit the transmission of uplink NAS (Non-Access Stratum) signaling to the AMF, further reducing the load on the network core.
• Traffic Limiting: The network may limit or reduce the amount of traffic it handles to prevent system failure.

1.2 Overload Control has three objectives:

• Maintaining Network Stability: The primary goal is to prevent complete network failure during periods of extreme traffic or unexpected load spikes.
• Ensuring Service Continuity: By managing load, the network can continue to provide essential services even if less critical services are temporarily limited.
• Protecting Resources: Overload control protects resources such as UDM bandwidth and other critical network functions from being overwhelmed by excessive control plane signaling.

2.the Overload Stop procedure signals the NG-RAN node to which the AMF is connected that the overload situation has ended and that normal operations should resume. The Overload Stop procedure uses non-UE-associated signaling. A successful Overload Stop operation is shown in Figure 8.7.8.2-1 below, where:

An NG-RAN node that receives the "OVERLOAD STOP" message should assume that the overload situation for the receiving AMF has ended and should resume normal operations for traffic applicable to the AMF.