Half Duplex

What is half duplex?

Every Ethernet port can operate in one of two modes. Full duplex means the port sends and receives data at the same time, like a two-lane road with traffic flowing in both directions. Half duplex means it can only do one at a time, like a single-lane bridge where cars take turns.

On a modern network, every wired connection should be full duplex. If a port falls back to half duplex, it is almost always a sign that something is wrong with the cable or the connection.

Why it matters

Half duplex effectively cuts your usable throughput in half, and it gets worse under load. When a port is half duplex, your device has to wait for a gap before it can send data. If two devices try to transmit at the same time, a "collision" occurs and both have to back off and retry. This creates noticeable slowdowns during video calls, file transfers, and anything else that sends and receives data simultaneously.

The frustrating part is that the connection still works, so most people never realize their port is in half duplex. They just experience slower speeds and occasional hiccups without knowing why.

What you can do

• Replace the Ethernet cable; a damaged or low-quality cable is the most common cause of half-duplex fallback
• Check that the cable is fully seated in the port on both ends
• Inspect the cable connectors for bent, corroded, or missing pins
• If the cable runs a long distance or through walls, test with a short patch cable to isolate the problem
• Avoid forcing port settings to a fixed speed or duplex in your router or switch configuration; autonegotiation handles this correctly when the cable is good
• If replacing the cable does not fix it, try a different port on your switch or router; the port hardware itself may be damaged

What Network Weather shows you

Network Weather reads the duplex setting from your router or managed switch and flags any port operating in half duplex.