Case Study: Verifying the N2 Interface Signaling between gNB and AMF

Scenario Overview:

In this case study, we analyze a real-world issue where a 5G network operator experiences problems during UE registration due to issues with the N2 interface between the gNB (Next Generation Node B) and AMF (Access and Mobility Management Function). Specifically, the UEs (User Equipment) are failing to register, and there is a noticeable performance degradation in the network, affecting a large portion of users in a certain region. This issue has a direct impact on the user experience, especially for customers trying to access the 5G network services.

The operator wants to diagnose whether the N2 interface signaling is faulty, causing delayed or dropped registration attempts.

Step 1: Identifying the Problem

Problem Symptoms:

• User Complaints: Users in a specific region (let’s assume a city area) are reporting difficulties registering for 5G, with frequent service unavailability during peak hours.
• Network Monitoring Alert: The network monitoring system shows an abnormally high registration failure rate of 15% during the last 24 hours.
• Impact on Users: 15% of users in the city area are experiencing registration issues, affecting 15,000+ customers daily (assuming 100,000 active users in that region). Additionally, the operator has noticed a rise in call drop rates and poor data throughput.

Step 2: Analyzing the N2 Interface Signaling

Network Elements Involved:

• gNB (Radio Access Network): Responsible for managing UE access to the radio network.
• AMF (Access and Mobility Management Function): Handles the control-plane signaling for UE mobility, registration, and session management.
• UE (User Equipment): Mobile devices that attempt to connect to the 5G network.

N2 Interface Role:

The N2 interface is critical for carrying control plane signaling between the gNB and AMF. This signaling involves:

• Registration Request: UE initiates a connection request to the network.
• Session Management: Sessions for data transfer are established or modified.
• Mobility Management: Handles handovers, tracking, and updating of UE’s location.

Step 3: Capturing and Analyzing the N2 Signaling

Tool Setup:

• Wireshark and Tshark for capturing packet data on the N2 interface.
• gNB logs for capturing any N2 signaling errors, dropped packets, or timeouts.
• AMF logs to trace the handling of the registration request.

a) Capturing N2 Packets

The network engineer starts by capturing traffic on both the gNB and AMF for N2 interface communications.

gNB Command:

tcpdump -i eth0 port 36412 -w n2_signaling.pcap

Reason: This command filters and captures packets between gNB and AMF over the N2 interface (port 36412).

AMF Command:

bash        

tcpdump -i eth0 port 36412 -w n2_amf_signaling.pcap

• Reason: Ensures that packets from AMF to gNB are captured for analysis.

b) Packet Capture Review

Upon analyzing the captured packets, several key issues are identified in the signaling:

1. Missing InitialUEMessage: In several instances, the InitialUEMessage from gNB is either delayed or missing entirely, which means the AMF is not receiving registration requests. This suggests an issue with the N2 connection or a misconfigured path.
2. N2 Signaling Delays: Some InitialUEMessage packets are received at the AMF, but the response times are significantly delayed, resulting in timeouts.
3. Response Not Sent by AMF: In cases where the InitialUEMessage is received by the AMF, the response, such as IdentityRequest, is not generated within the expected time, leading to a lack of feedback for UE registration.

Error Indicators in Packet Analysis:

• Missing packets: Sometimes, the InitialUEMessage from the gNB is lost in transit, often due to network congestion or interface issues.
• Timeouts: Delays are visible in the packet analysis, with round-trip times (RTT) of over 500ms in some cases, indicating network congestion or misconfiguration.

Step 4: Log Analysis for Additional Insights

gNB Logs:

• Error Message: "Failed to send InitialUEMessage to AMF."This suggests that the gNB is not able to reach the AMF in certain conditions, possibly due to network issues on the N2 interface.
• Warning Message: "N2 interface timeout: No response from AMF after sending InitialUEMessage."This implies that there are delays or congestion on the N2 interface, leading to timeouts while waiting for an AMF response.

AMF Logs:

• Error Message: "Unable to process InitialUEMessage, no response received from gNB."This confirms that while the AMF is not receiving the initial registration request, it’s expecting it from the gNB.
• Warning Message: "AMF not able to communicate with N2 interface – possible misconfiguration."This suggests a misconfiguration in either the gNB or the AMF, leading to failure in signaling across the N2 interface.

Step 5: Network Configuration and Issues Identification

Based on the analysis of packet captures and logs, several network-related issues are identified:

1. N2 Interface Configuration: A routing issue between the gNB and AMF is preventing the InitialUEMessage from reaching the AMF. Specifically, N2 interface IP addresses were incorrectly configured, leading to routing issues during UE registration.
2. Network Congestion: During peak hours (e.g., evening rush), the N2 interface suffers from network congestion, leading to delays in packet delivery. This congestion can be attributed to overloaded transport links or misconfigured Quality of Service (QoS) parameters in the backhaul network.
3. Firewall Blocking N2 Signaling: In certain instances, firewall rules were identified to be blocking specific N2 signaling packets. Port 36412 (used for N2 signaling) was restricted in some access control lists (ACLs), preventing proper communication.

Step 6: Resolving the Issue

Fixes Applied:

1. N2 Interface Configuration Correction:
2. Network Congestion Mitigation:
3. Firewall Rule Adjustment:

Verification:

After implementing the fixes, the operator conducted a retest by monitoring the UE registration process and verifying that the N2 signaling was successfully reaching the AMF without delays or packet loss. The registration success rate improved from 85% to 98% within 24 hours.

Impact and Performance Improvement

The resolution of this issue had a direct and measurable impact on the network performance and user experience:

• UE Registration Success Rate: The registration success rate increased from 85% to 98% within 24 hours.
• Impact on Users: Approximately 15,000 users (from a total of 100,000 active users) who were previously facing registration failures reported no further issues. This improved customer satisfaction by 20% in the affected region.
• Improved Throughput: With the reduction in call drops and successful sessions, average data throughput for affected users increased by 12%.
• Operational Efficiency: Network troubleshooting time was reduced, and the time spent by engineers resolving similar issues decreased by 30% due to improved N2 interface monitoring and better configuration management.

Conclusion

This case study highlights how N2 interface signaling issues between the gNB and AMF can severely affect the UE registration process, leading to service degradation, network congestion, and poor user experience. Through detailed packet capture analysis, log inspection, and targeted network configuration corrections, the issue was identified and resolved. As a result, the operator saw a 13% improvement in UE registration success, and user satisfaction in the affected region increased by 20%. This case demonstrates the critical importance of interface management and real-time monitoring in maintaining a robust and efficient 5G network.