Troubleshooting a Sudden Drop in Internet Speed: A Case Study of 900 Mbps Downplan Deterioration
Maintaining optimal internet performance is essential for both productivity and entertainment. However, fluctuations and unexpected reductions in speed can be frustrating and perplexing. Recently, a high-capacity fiber-optic subscriber in the UK experienced a significant decrease in download speeds, prompting a detailed troubleshooting process. This article explores the situation, investigative steps, and practical recommendations to address such issues.
Background
Since 2022, the user has subscribed to a fiber-optic internet plan with a rated speed of 900 Mbps downstream and 450 Mbps upstream, supplied through OFNL infrastructure in the UK. For almost three years, the connection has delivered consistent performance across all wired devices. However, over the past five days, observed download speeds have plummeted to between 200 Mbps and 600 Mbps, well below the expected threshold.
Network Setup Overview
Understanding the network architecture is key:
- ONT (Optical Network Terminal)
- Main router: ASUS AX88U Pro (configured with AiMesh)
- Internal cabling: Cat5e Ethernet wiring
- Connected devices: Multiple computers, including a dedicated PC connected via wired Ethernet, with internal cabling bypassing the router in some cases.
The user conducted speed tests both through ASUS’s internal tools and across multiple devices wired directly to the network.
Troubleshooting Steps Undertaken
1. Rule Out Local Equipment Issues
The user tested the ISP-provided router with the same setup. Even with the ISP’s router assigned as the primary device, the download speeds remained limited (~600 Mbps max), indicating the issue is unlikely related to the customer’s hardware or internal wiring.
2. Replace and Test Wired Connections
A new, dedicated computer was used to connect directly to the ISP router using freshly tested Ethernet cables. Despite this, speeds persisted below nominal levels, suggesting the problem isn’t isolated to particular devices.
3. Verify the Router and Network Configuration
The ASUS routers, configured with AiMesh, were isolated to test for any configuration-related issues. Whether using the ASUS AX88U Pro or the older AC86U as the primary router, speed limitations persisted.
4. Engage the ISP’s Technical Support and Engineer Visit
The ISP’s technician tested the connection directly at the ONT, reporting speeds of up to 1 Gbps—consistent with the plan. The ON
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