1000BASE-T, or IEEE 802.3ab, is a Gigabit Ethernet wiring standard for Local Area Networks (LAN) that uses copper-based networking hardware as its medium. It is one of the most widely used standards of Ethernet over twisted pair.

The Institute of Electrical and Electronics Engineers Standards Association (IEEE-SA) Standards Board approved it as a standard on June 28, 1999, after two and a half years of effort within the IEEE 802.3 Working Group.

The "1000" in its identifier, assigned by the IEEE, denotes that it transmits Ethernet frames at a speed of 1000 megabits, or 1 gigabyte, per second. "BASE" indicates baseband signalling. This means that the medium exclusively transmits Ethernet signals. "T" means that it uses twisted pair cables as its medium.

1000BASE-T requires four pairs of CAT-5, CAT-5e or CAT-6 Unshielded Twisted Pair (UTP) cables with full-duplex transmission on each pair for a maximum distance of 100 meters for each network segment. These UTP cables are needed to simultaneously transmit signals in both directions.

This is made possible by the 4-data line, 5-level pulse amplitude modulation (4D-PAM5) block-encoding scheme. PAM is a technique that encodes message information in the amplitude of a series of signal pulses. Its five levels give it 625 code points. PAM5 signaling increases the amount of information transmitted with each code point.

For noise immunity, 1000BASE-T uses a four-dimensional Trellis Coded Modulation, giving it a 6-decibel coding gain across the four cables. DSP-based adaptive filtering also cancels the effects of echo, crosstalk and noise. To avoid collision in full-duplex operation, transmitted pulses between two devices must not coincide in time. There are two techniques to help prevent this: Carrier Sense Multiple Access, and Time Division Multiple Access.

Autonegotiation or NWay, the procedure of selecting common transmission parameters for two connected devices, is also a requirement for this standard. 1000BASE-T allows autonegotiation between 100Mbps and 1000Mbps.

Transmission of data is done eight bits at a time. Much like the tap sequence used for a linear feedback shift register, a non-trivial scrambling procedure expands the data into four 3-bit symbols, which are then assigned to different voltage levels.

For example, the "000" 3-bit symbol may correspond to a 0 voltage level, "001" corresponds to a +1 voltage level, "002" corresponds to a +2 voltage level, and so on.

1000BASE-T may be used for broadband applications on the desktop computer, for server switching at data centers, and for uplinks from desktop computer switches, as well as other networking services.

A simplified version of 1000BASE-T, known as 1000BASE-TX or TIA/EIA-854, was developed by the Telecommunications Industry Association (TIA). The design was proposed as a cheaper alternative to 1000BASE-T that was also easier to install.

It used 3-level pulse amplitude modulation (PAM3) instead of PAM5 as its block-encoding scheme, and required two cables rather than four, which reduced the cost. However, both pairs had to be Category 6 cable, which was significantly more costly than Category 5 cable. This was the most probable cause of the commercial failure of 1000BASE-TX.