Layer 1 (Physical): cables, ports & link

Acutis · plain-English network help · updated June 2026

Start here: Layer 1 is the cable and the link

Layer 1 — the Physical layer — is the actual electrical or wireless connection: the Ethernet cable, the port it plugs into, the network card (NIC) inside your device, the power feeding everything, and the Wi-Fi radio signal. It carries raw bits, nothing more. Layer 1 is where you always start troubleshooting, because it's the most common failure and the cheapest to fix. If there's no physical link, no amount of IP, DNS or app troubleshooting above it will ever work. The fastest tell is the link light: if the little LED next to the port is dark, you have a Layer 1 problem, full stop.

What breaks at Layer 1

Physical problems are mechanical and electrical. The usual suspects:

• Bad or loose cable. A cable that isn't clicked fully into the jack, has a broken plastic tab so it backs out, or has a damaged conductor inside. Cables are the number-one cause of Layer 1 faults.
• Dead port. A switch port, wall jack or router port that has failed or been disabled. The cable is fine; the hole it plugs into isn't.
• Failed NIC. The network adapter in your computer can fail, get disabled in software, or have an out-of-date driver that drops the link.
• Power. A switch, access point or media converter that lost power, or a PoE device not getting enough wattage from its injector.
• Weak Wi-Fi signal. On wireless, "physical" means radio strength. Too far from the access point, too many walls, or interference all degrade the link before any higher layer can run.
• Speed/duplex at the physical level. A worn cable or mismatched port can negotiate down to 10 Mbps half-duplex, or refuse to link at gigabit, even when a link light is showing.

Symptoms that point to Layer 1

• No link light. The LED by the NIC or switch port is off. The operating system reports "network cable unplugged" or "no connection."
• Intermittent drops. The connection works, then dies for a few seconds, then returns — classic of a partially seated or damaged cable, or a cable run near electrical interference.
• Link only at low speed. A gigabit connection that only links at 100 Mbps often means a bad pair in the cable.
• Wi-Fi shows one or two bars and constantly reconnects. The radio link is too weak to hold.

How to diagnose and fix it

Work through these in order — each one is quick and rules a cause in or out:

1. Look at the link lights. Check the LED on the NIC and on the switch/router port. A solid or blinking light means Layer 1 is up; a dark light means it isn't. This single check decides where you go next.
2. Reseat the cable. Unplug both ends and push each back in until it clicks. A surprising share of "outages" are just a cable that worked its way loose.
3. Swap the cable. Try a known-good cable. If the link comes up, the old cable was the fault — retire it.
4. Try another port. Move the cable to a different port on the switch or router. If it links there, the original port is dead or disabled.
5. Check power. Confirm the switch, access point or PoE injector has power and its status lights are normal. Power-cycle it if in doubt.
6. Test the NIC. Plug the same cable and port into a different device. If that device links fine, the original NIC or its driver is the problem — reinstall or update the driver, or re-enable the adapter.
7. For Wi-Fi, move closer. Stand next to the access point. If the link steadies, the fault is signal strength — reposition the router, reduce obstructions, or add a mesh node.

Once you have a steady link light at a sensible speed, Layer 1 is healthy. If the problem persists after that, the fault is higher up — climb to Layer 2.

🔧 Confirm the link with our free tools

Once you've reseated or swapped the cable — or moved closer to the access point — confirm the physical layer is actually solid again, right from your browser:

1. Open the ping test. A steady low latency with no packet loss means Layer 1 is solid again; spikes or dropped responses mean the link still isn't.
2. Run the speed test to confirm you're getting the throughput the link should carry — a healthy cable that only links slowly will show here.
3. Check what's my IP to confirm you're actually reaching the internet, not just the local switch.