Engadget explains how to check Ethernet link speed and trace a slow wired connection

According to the report, a wired PC on a gigabit connection that reports about 90 Mbps or 100 Mbps does not automatically need a new Ethernet cable. An online speed test can measure more than the cable, so Engadget said the first step is to check the negotiated Ethernet link speed rather than buy a tester. On Windows 11, users can open Settings > Network & internet > Ethernet and read the Link speed (Receive/Transmit) field. On Windows 10 and Windows 11, pressing Windows + R, typing ncpa.cpl, right-clicking the Ethernet adapter and selecting Status shows the connection rate in the Speed field. On a Mac, System Settings > Network > Ethernet, or the USB Ethernet adapter, then Details > Hardware displays the Speed field. Engadget advised leaving the configuration on Automatic because manual speed settings can cause connection problems.

The link-speed figure describes the connection between the computer and the router or network switch, not the internet speed or the highest speed the cable can handle, the report said. The reading alone also cannot confirm whether a cable is faulty. Engadget said users should check the cable jacket for markings such as Cat5, Cat5e or Cat6 because the connectors look virtually identical. Cat5e and Cat6 both support gigabit Ethernet, and Cat6 is sufficient for 1,000 Mbps. The category is not an absolute speed ceiling: older cables can sometimes support very high speeds over short runs, but a successful short-run test does not guarantee the same performance over longer distances, according to the report.

If a computer reports 100 Mbps when 1 Gbps is expected, Engadget said users should verify the hardware at both ends. A 100 Mbps LAN port on a router or a similarly limited USB Ethernet adapter will cap the connection regardless of cable quality. Even with gigabit-capable ports at both ends, damaged wire or a loose connector can disrupt the connection or force a fallback to slower speeds. Gigabit Ethernet uses all four twisted pairs inside the cable, while conventional 100 Mbps Ethernet uses two, the report said.

Engadget recommended reseating the connectors first, then swapping in a short, known-good cable using the same ports. If the reading changes from 100 Mbps to 1 Gbps, the original cable or its connectors are strong suspects; if nothing changes, users should check port specifications and adapter settings before spending money on a replacement. For a more thorough check, iPerf3 can measure real throughput between two wired computers without involving the ISP. Users install it on both computers, turn off Wi-Fi, run iperf3 -s on one computer and iperf3 -c SERVER_IP on the other, replacing SERVER_IP with the first computer's local IP address, then compare results after swapping cables while keeping the rest of the setup the same.

A multimeter or simple cable tester can determine some wiring faults but cannot determine the maximum data rate a cable supports, according to Engadget. That requires specialized cable qualification equipment, and certification testers verify compliance with cabling standards.

Editor's Summary

Engadget's guidance centers on checking negotiated link speed before replacing an Ethernet cable. It notes that 100 Mbps readings can stem from router ports, USB adapters or damaged connectors as well as the cable, and that full throughput testing requires tools such as iPerf3 or specialized qualification equipment.