What the standard defines
1000BASE-T is gigabit Ethernet over balanced twisted-pair copper. It was published as IEEE 802.3ab and has since been folded into the base 802.3 standard, where it lives as one of the physical layers a port may implement.
Two things about how it works explain most of its behaviour. First, it uses all four pairs, and it uses them in both directions at the same time — each transceiver is sending and receiving on the same conductors, and cancels its own outgoing signal from what it hears. Second, it is specified over Category 5e cabling or better, across the same hundred-metre channel the cabling standards are built around.
Ports negotiate. Auto-negotiation is the exchange in which two ports advertise what they can do, settle on the highest rate and duplex mode they share, and agree which end supplies the timing reference. It happens before any traffic passes, which is why a mismatch shows up as a link at an unexpected rate rather than as a failure.
The figure the standard fixes is a gigabit per second of signalling in each direction.
What it does not promise
It does not promise a gigabit of file transfer. The rate on the wire is the outside of a stack: framing and protocol overhead take their share, and then the transfer is limited by whatever is slowest at the two ends — a drive, a processor, an encrypted share, a USB adapter.
It does not promise anything about the connection to the outside world.
It does not lift a device that has a slower port. A gigabit switch between a gigabit computer and a hundred-megabit printer produces a hundred-megabit path to the printer.
And it does not tell you the cabling is sound. A channel with a fault on one pair will still link — at a lower rate, without an error message. A port reporting a hundred megabits where a gigabit was expected is a cabling report, and one worth reading as such.
What it works alongside
It negotiates down to the hundred-megabit and ten-megabit physical layers, so it interoperates with anything older on the same cabling. It runs over Category 5e, 6 and 6A channels alike. The Power over Ethernet types are carried across the same pairs, so a gigabit link and a powered device are not alternatives. Above it, two-and-a-half and five-gigabit operation were specified to run on the same installed cabling, which is what makes an upgrade a matter of changing equipment rather than pulling cable.
The questions that come up before an order
Why does a link sometimes come up at a hundred megabits on a gigabit switch?
Gigabit operation over copper needs all four pairs in both directions. Where one pair is broken, miswired or badly terminated, auto-negotiation settles on a hundred-megabit link, which needs only two — the connection works, quietly, at a tenth of the rate.
Does a gigabit switch make the internet connection faster?
No. The port rate governs traffic inside the building. The connection arriving from the line runs at whatever the line and the provider deliver, and that figure is set elsewhere entirely.
Last reviewed 9 September 2026