| Full title | IEEE Standard for Information Technology. Local and Metropolitan Area Networks. Part 3: CSMA/CD Access Method and Physical Layer Specifications. Physical Layer Parameters and Specifications for 1000 Mb/s Operation over 4-Pair of Category 5 Balanced Copper Cabling, Type 1000BASE-T |
| Publisher | IEEE Standards Association, IEEE 802.3 Ethernet Working Group |
| Approved | 26 June 1999 by the IEEE Standards Board. Published 26 July 1999 |
| Current home | Clause 40 of IEEE 802.3. The current base edition is IEEE 802.3-2022 |
| Status | IEEE 802.3ab-1999 is superseded. It was incorporated into IEEE Std 802.3, 2000 Edition, and IEEE lists it as superseded by IEEE 802.3-2002 |
| Related documents | ISO/IEC 11801-1, ANSI/TIA-568.2, TIA/EIA TSB-95, IEEE 802.3bz |
What it covers
IEEE 802.3ab is the amendment to IEEE 802.3 that added 1000BASE-T, Gigabit Ethernet over balanced twisted-pair copper cabling. It defines the 1000BASE-T Physical Coding Sublayer (PCS), Physical Medium Attachment (PMA) sublayer and Medium Dependent Interface (MDI), and specifies the cabling medium.
The project’s stated purpose was to give 1000 Mb/s operation over installed Category 5 cabling, then the main horizontal cabling in buildings. The scope referred to four pairs of 100 ohm Category 5 cabling as defined by EIA/TIA-568-A, or the equivalent built from material to ISO/IEC 11801:1995.
Amendments are folded into the next revision of IEEE 802.3. 1000BASE-T has been part of the base standard since IEEE Std 802.3, 2000 Edition, and is Clause 40 of the current edition, IEEE 802.3-2022.
Key technical points
| Item | 1000BASE-T |
|---|---|
| Data rate | 1000 Mb/s |
| Pairs used | All four pairs |
| Line code | PAM-5 (five signal levels), 2 bits per symbol |
| Signalling rate | 125 Mbaud on each pair |
| Cabling | 4-pair, 100 ohm, Category 5 balanced cabling |
| Link length | Category 5 link of at least 100 m |
| Auto-negotiation | Clause 28, used to advertise 1000BASE-T and duplex mode |
| Clock | One PHY acts as MASTER and the other as SLAVE, recovering timing from the received signal |
Because each pair carries traffic in both directions at once, the receiver uses echo and crosstalk cancellation and equalisation. The PMA also provides four test modes used in conformance testing.
Why Category 5e followed
Full four-pair, two-way transmission made parameters such as return loss, ELFEXT and delay skew important. These were not fully defined for field testing of installed Category 5 cabling. TIA published TSB-95, additional transmission performance guidelines for 4-pair Category 5 cabling. Its limits are reported to have been taken into 802.3ab. Category 5e, with tighter limits, was approved in January 2000 as Addendum 5 (ANSI/TIA/EIA-568-A-5).
How it relates to other standards
| Document | Relationship |
|---|---|
| ISO/IEC 11801-1 and BS EN 50173-1 | Define generic balanced cabling classes, including Class D channel performance |
| ANSI/TIA-568.2 | Defines Category 5e, 6, 6A and 8 balanced twisted-pair cabling. Category 5 is no longer a recognised category for new installations |
| IEEE 802.3bz | 2.5GBASE-T and 5GBASE-T, intended for higher speeds over the same installed Category 5e and Category 6 cabling |
| IEEE 802.3af/at and 802.3bt | Power over Ethernet delivered over the same cabling |
What it means in practice
- 1000BASE-T needs all four pairs. Two-pair cabling or cords that split pairs will not support it.
- For new installations specify at least Category 5e / Class D. Category 5 has been superseded.
- Older Category 5 links can be field tested against 1000BASE-T or TSB-95 limits. This is a channel test, so retest if the user cords change.
- 1000BASE-T was specified for links of up to 100 m. Longer runs need a different medium, such as optical fibre.
Related reading: Cat5e vs Cat6 vs Cat6A · How to read a cable test certificate.