Editing Gigabit Ethernet (section)

===1000BASE-T===
[[File:Supermicro AOC-SGP-I2 Gigabit Ethernet NIC, PCI-Express x4 card.jpg|thumb|[[Supermicro]] AOC-SGP-I2 dual-port Gigabit Ethernet [[Network interface controller|NIC]], a [[PCI Express]] ×4 card]]

1000BASE-T (also known as IEEE 802.3ab) is a standard for Gigabit Ethernet over twisted-pair wiring.

Each 1000BASE-T network segment is recommended to be a maximum length of {{convert|100|m|ft|abbr=off|sp=us}},<ref>{{cite video |last1=Barrera |first1=Dan |title=Interview: Dan Barrera With Ideal Networks About TIA 42 Cabling Standards & Testing Processes |url=https://www.youtube.com/watch?v=kNa_IdfivKs | archive-url=https://ghostarchive.org/varchive/youtube/20211117/kNa_IdfivKs| archive-date=November 17, 2021 | url-status=live|website=Youtube |access-date=April 8, 2020 |time=11:49}}{{cbignore}}</ref>{{efn|In ISO the length is purely informative. The length is not a pass/fail criterion as testing the conformance to EN 50173 series standards.}} and must use [[Category 5 cable]] or better (including [[Cat 5e]] and [[Cat 6]]).

[[Autonegotiation]] is a requirement for using 1000BASE-T<ref>{{Cite web|url=http://standards.ieee.org/reading/ieee/interp/IEEE802.3af-2003interp-6.pdf |archive-url=https://web.archive.org/web/20060130134410/http://standards.ieee.org/reading/ieee/interp/IEEE802.3af-2003interp-6.pdf |url-status=dead |archive-date=January 30, 2006 |title=Auto-Negotiation; 802.3-2002 |work=IEEE Standards Interpretations |publisher=IEEE |access-date=November 5, 2007}}</ref> according to ''Section 28D.5 Extensions required for Clause40 (1000BASE-T)''.<ref>{{Cite web|author=IEEE |url=https://standards.ieee.org/wp-content/uploads/import/download/802.3-2008_downloads.zip |title=Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method and Physical Layer specifications |publisher=SECTION TWO: This section includes Clause21 through Clause 33 and Annex 22A through Annex 33E.|access-date=February 18, 2010}}</ref> At least the clock source has to be negotiated, as one endpoint must be master and the other endpoint must be slave.

In a departure from both [[10BASE-T]] and [[100BASE-TX]], 1000BASE-T uses four lanes over all four cable pairs for simultaneous transmission in both directions through the use of [[echo cancellation]] with [[adaptive equalization]] called hybrid circuits<ref>{{Cite web |author=IEEE |url=http://standards.ieee.org/getieee802/download/802.3-2008_section1.pdf |title=1.4 Definitions 1.4.187 hybrid |access-date=August 9, 2017 |archive-url=https://web.archive.org/web/20101206030247/http://standards.ieee.org/getieee802/download/802.3-2008_section1.pdf |archive-date=December 6, 2010 |url-status=dead }}</ref> (this is like [[telephone hybrid]]) and five-level [[pulse-amplitude modulation]] (PAM-5). The symbol rate is identical to that of 100BASE-TX (125&nbsp;[[megabaud]] (MBd)) and the noise immunity of the five-level signaling is also identical to that of the three-level signaling in 100BASE-TX, since 1000BASE-T uses four-dimensional [[trellis coded modulation]] (TCM) to achieve a 6&nbsp;[[Decibel|dB]] [[coding gain]] across the four pairs.

Since negotiation takes place on only two pairs, if two GbE interfaces are connected through a cable with only two pairs, the interfaces will successfully choose 'gigabit' as the highest common denominator (HCD),{{clarify|date=February 2024}} but the link will never come up. Most GbE physical devices have a specific register to diagnose this behavior. Some drivers offer an "Ethernet@Wirespeed" option where this situation leads to a slower yet functional connection.<ref>{{Cite web|url=https://www.broadcom.com/application/ethernet_nic.php |title=Broadcom Ethernet NIC FAQs |access-date=April 25, 2016}}</ref>

The data is transmitted over four copper pairs, eight [[bit]]s at a time. First, eight bits of data are expanded into four three-bit symbols through a non-trivial scrambling procedure based on a [[linear-feedback shift register]]; this is similar to what is done in [[100BASE-T2]], but uses different parameters. The three-bit symbols are then mapped to voltage levels which vary continuously during transmission. An example mapping is as follows:

{| class="wikitable" style="text-align: center;"
! Symbol 
| 000 || 001 || 010 || 011 || 100 || 101 || 110 || 111
|-
! Line signal level
| 0 || +1 || +2 || −1 || 0 || +1 || −2 || −1
|}

''[[Auto-MDIX|Automatic MDI/MDI-X Configuration]]'' is specified as an optional feature in the 1000BASE-T standard,<ref>Clause 40.4.4 in [[IEEE 802.3-2008]]</ref> meaning that straight-through cables will often work between two GbE-capable network node interfaces (both MDI) and between two switch or hub interfaces (both MDI-X). This feature eliminates the need for [[Ethernet crossover cable|crossover cables]], making obsolete the uplink vs normal port choices and manual selector switches found on many older hubs and switches and greatly reduces installation errors.

In order to extend and maximize the use of existing Cat-5e and Cat-6 cabling, the newer standards [[2.5GBASE-T and 5GBASE-T]] operate at 2.5 and {{nowrap|5.0 Gbit/s}}, respectively, on existing copper infrastructure designed for use with 1000BASE-T. They are based on [[10GBASE-T]] but use lower signaling frequencies.