I pay for 900 down, all of a sudden I am getting 500 – 600 down.

Troubleshooting Sudden Drop in Internet Speed: A Case Study of a UK Fiber Subscriber

Many internet users expect consistent high-speed connectivity, especially when subscribing to premium plans. However, experiencing a sudden decline in internet performance can be frustrating and puzzling. This article explores a real-world scenario encountered by a UK-based fiber internet subscriber, detailing their troubleshooting process and seeking professional insights.

Background

Since 2022, the user has been on an OFNL fiber plan offering 900 Mbps download and 450 Mbps upload speeds. For nearly three years, this level of service was achieved reliably across all wired devices without issues. However, over the past five days, the user noticed their download speeds plummeting to a range between 200 Mbps and 600 Mbps, inconsistent with their paid plan.

Network Setup Overview

The subscriber’s network configuration includes:

Optical Network Terminal (ONT)
AX88U Pro router
Internal Cat5e Ethernet cabling
ASUS AC86U router
Multiple devices connected via wired and wireless connections

They also have a secondary computer connected directly through Ethernet that bypasses the ASUS routers, aiming to isolate potential causes.

Troubleshooting Steps Taken

Testing with ISP-Provided Equipment
The ISP was initially quick to attribute the issue to the user’s equipment, as they were not using their own router. To verify, the user connected the ISP’s router directly to the ONT with a new Ethernet cable, testing the speed on a dedicated computer with WiFi disabled.
Result: The computer achieved around 600 Mbps download speed—less than the expected 900 Mbps but consistent across tests, suggesting the problem wasn’t solely with the user’s hardware.
Replacing Ethernet Cables and Hardware
To eliminate wiring problems, the user replaced the Ethernet cables from ONT to the router and tested various configurations:
Connecting the ASUS AC86U router as the main gateway
Using the AX88U Pro alone
Connecting devices directly to the ONT
Engineer Visit and Equipment Replacement
An engineer visited and confirmed that the speed directly from the ONT was up to 1 Gbps. They replaced the ONT to rule out hardware fault but the speed issues persisted.