The Difference
| Mode | Meaning | Simple Analogy | Modern Example |
|---|---|---|---|
| Full-duplex | Send and receive at the same time | A phone call where both sides can speak simultaneously | Switched Ethernet |
| Half-duplex | Send or receive, but not both at once | A walkie-talkie where people take turns | Most Wi-Fi airtime, legacy hubs |
How Full-Duplex Works at the Wire Level
A standard Cat5e or Cat6 Ethernet cable contains four twisted pairs. In a 1000BASE-T gigabit link, all four pairs carry data simultaneously, with each pair using both directions at once through a technique called hybrid cancellation. The transmit and receive signals are electrically separated at each end, allowing data to flow in both directions across the same wires at the same time without interference. There is no turn-taking, no waiting, and no collision domain — the link is always available in both directions simultaneously.
This is why a full-duplex gigabit link can sustain simultaneous 1 Gbps download and 1 Gbps upload in a local transfer, for a theoretical aggregate of 2 Gbps of data movement — though your internet plan caps the external portion of that to its purchased rate.
CSMA/CD: How Half-Duplex Ethernet Worked
Before switches became standard, Ethernet used hubs that placed every connected device in a single shared collision domain. Devices used Carrier Sense Multiple Access with Collision Detection (CSMA/CD): before transmitting, a device would listen to the wire to check if it was idle. If two devices transmitted simultaneously, their signals collided and both detected the electrical noise. Each device then sent a jam signal, waited for a random backoff period drawn from an exponential distribution, and retried. Under heavy load, collisions multiplied and effective throughput dropped well below the rated link speed. Half-duplex 10BASE-T under moderate contention often delivered only 30–50% of its rated bandwidth.
Why Switches Made Full-Duplex Normal
Modern switches create a dedicated point-to-point link between each device and the switch port. Because there is no shared medium, there is no collision domain — each port is its own isolated segment. The switch and the connected device can send and receive simultaneously using the separate wire pairs, enabling full-duplex operation. CSMA/CD is disabled entirely on full-duplex links because there is nothing to contend with. This is why replacing a hub with a switch immediately and dramatically improved network performance even at the same rated speed.
Wi-Fi Is Half-Duplex
Wi-Fi is shared radio airtime. The 2.4 GHz or 5 GHz radio channel is a broadcast medium — only one device can successfully transmit on a channel at a time in normal 802.11 operation. Devices use CSMA/CA (Collision Avoidance): they listen for a clear channel, wait a random backoff period, and then transmit. If a collision occurs anyway (both devices sensed the channel clear simultaneously), both retransmit after another random wait.
Even when your phone shows a 600 Mbps Wi-Fi link rate, neighboring devices on the same channel, same band, or even adjacent channels compete for the same airtime. This is why a wired full-duplex Ethernet link almost always delivers more consistent throughput and lower latency than Wi-Fi at a similar advertised speed. Wi-Fi 6 introduced OFDMA (Orthogonal Frequency Division Multiple Access), which begins to address this by allowing an access point to schedule transmissions from multiple devices simultaneously within a channel — but it is not true full-duplex and the shared-medium constraint still applies.
Duplex Mismatch in Detail
A duplex mismatch occurs when one side of an Ethernet link negotiates full-duplex and the other side is forced or stuck at half-duplex. The full-duplex side transmits freely in both directions simultaneously. The half-duplex side interprets the other side's transmissions while it is also transmitting as collisions, and invokes CSMA/CD backoff. The result is a cascade of late collisions, retransmissions, and FCS errors. Throughput can collapse to 10–20% of rated speed even on a link that shows as "connected" and "1 Gbps" in the operating system.
Mismatches most commonly occur when one side has its speed or duplex forced manually while the other auto-negotiates, when older managed switches have port settings that override autonegotiation, or when a damaged cable causes autonegotiation to fail and one side falls back to half-duplex as a safe default.
| Indicator | How to Check | What It Suggests |
|---|---|---|
| Late collision counter rising | ethtool -S eth0 or switch port statistics | Classic duplex mismatch signature |
| Link shows 1 Gbps but throughput is 100 Mbps or less | iperf3 test between two wired hosts | Mismatch, bad cable, or faulty adapter |
| High FCS or CRC error count | Switch port counters or ip -s link show eth0 | Cable, adapter, or negotiation problem |
Detecting and Fixing a Mismatch
On Linux, ethtool eth0 shows the negotiated speed and duplex. Look for "Duplex: Full" — if it says "Half," investigate why. Check the switch port configuration for any forced settings. On Windows, open Device Manager, find the network adapter, go to Properties → Advanced, and look for Speed & Duplex — it should be set to Auto Negotiation unless there is a specific reason to force a value. If forcing is required on one side, it must be forced on both sides to the identical speed and duplex. Forcing one side and leaving the other on auto-negotiate is the most common cause of mismatches.
Before changing software settings, replace the Ethernet cable and try a different switch port. Physical cable faults are a more common cause of negotiation failures than configuration errors.
Why Full-Duplex Does Not Double Your Internet Plan
A 1 Gbps full-duplex Ethernet link can carry traffic in both directions at once on the local segment. Your internet plan is still limited by the ISP service rate, modem capabilities, router WAN port speed, and the upstream and downstream capacity sold in your plan. Full-duplex helps the local link avoid direction conflicts; it does not override the purchased internet speed or change what the ISP delivers.
Frequently Asked Questions
Is modern Ethernet full-duplex?
Yes. Normal switched Ethernet links operate in full-duplex when both sides successfully autonegotiate. CSMA/CD collision handling is disabled on full-duplex links.
Is Wi-Fi full-duplex?
No. Wi-Fi is a shared half-duplex medium using CSMA/CA. Devices take turns transmitting on a shared radio channel. Wi-Fi 6 OFDMA improves multi-user efficiency but does not make Wi-Fi full-duplex.
What is a duplex mismatch?
It happens when the two sides of an Ethernet link operate in different duplex modes. The half-duplex side sees the full-duplex side's transmissions as collisions, causing a cascade of retransmissions, late collisions, and throughput collapse to a fraction of rated speed.