Ethernet physical layer

The physical-layer specifications of the Ethernet family of computer network standards are published by the Institute of Electrical and Electronics Engineers, which defines the electrical or optical properties and the transfer speed of the physical connection between a device and the network or between network devices. It is complemented by the MAC layer and the logical link layer. An implementation of a specific physical layer is commonly referred to as PHY.
The Ethernet physical layer has evolved over its existence starting in 1980 and encompasses multiple physical media interfaces and several orders of magnitude of speed from to. The physical medium ranges from bulky coaxial cable to twisted pair and optical fiber with a standardized reach of up to 80 km. In general, network protocol stack software will work similarly on all physical layers.
Many Ethernet adapters and switch ports support multiple speeds by using autonegotiation to set the speed and duplex for the best values supported by both connected devices. If autonegotiation fails, some multiple-speed devices sense the speed used by their partner, but this may result in a duplex mismatch. With rare exceptions, a 100BASE-TX port also supports 10BASE-T while a 1000BASE-T port also supports 10BASE-T and 100BASE-TX. Most 10GBASE-T ports also support 1000BASE-T, some even 100BASE-TX or 10BASE-T. While autonegotiation can practically be relied on for Ethernet over twisted pair, few optical-fiber ports support multiple speeds. In any case, even multi-rate fiber interfaces only support a single wavelength.
10 Gigabit Ethernet was already used in both enterprise and carrier networks by 2007, with and 100 Gigabit Ethernet ratified. In 2024, the fastest additions to the Ethernet family were variants.

Naming conventions

Generally, layers are named by their specifications:

10, 100, 1000, 10G,... – the nominal, usable speed at the top of the physical layer, excluding line codes but including other physical layer overhead ; some WAN PHYs run at slightly reduced bitrates for compatibility reasons; encoded PHY sublayers usually run at higher bitrates
BASE, BROAD, PASS – indicates baseband, broadband, or passband signaling respectively
-T, -T1, -S, -L, -E, -Z, -C, -K, -H... – medium : T = twisted pair, -T1 = single-pair twisted pair, S = 850 nm short wavelength, L = 1300 nm long wavelength, E or Z = 1500 nm extra long wavelength, B = bidirectional fiber using WDM, P = passive optical, C = copper/twinax, K = backplane, 2 or 5 or 36 = coax with 185/500/3600 m reach, F = fiber, various wavelengths, H = plastic optical fiber
X, R – [|PCS] encoding method : X for 8b/10b block encoding, R for large block encoding
1, 2, 4, 10 – for LAN PHYs indicates number of lanes used per link; for WAN PHYs indicates reach in kilometers

For, no encoding is indicated as all variants use Manchester code. Most twisted pair layers use unique encoding, so most often just -T is used.
The reach, especially for optical connections, is defined as the maximum achievable link length that is guaranteed to work when all channel parameters are met. With better channel parameters, often a longer, stable link length can be achieved. Vice versa, a link with worse channel parameters can also work but only over a shorter distance. Reach and maximum distance have the same meaning.

Physical layers

The following sections provide a brief summary of official Ethernet media types. In addition to these official standards, many vendors have implemented proprietary media types for various reasons—often to support longer distances over fiber optic cabling.

Early implementations and

Early Ethernet standards used Manchester coding so that the signal was self-clocking and not adversely affected by high-pass filters.

All Fast Ethernet variants use a star topology and generally use 4B5B line coding.

All Gigabit Ethernet variants use a star topology. 1000BASE-X variants use 8b/10b PCS encoding. Initially, half-duplex mode was included in the standard but has since been abandoned. Very few devices support gigabit speed in half-duplex.

2.5 and

2.5GBASE-T and 5GBASE-T are scaled-down variants of 10GBASE-T and provide longer reach over pre-Cat 6A cabling. These physical layers support twisted-pair copper cabling and backplanes only.

10 Gigabit Ethernet is a version of Ethernet with a nominal data rate of, ten times as fast as Gigabit Ethernet. The first 10 Gigabit Ethernet standard, IEEE Std 802.3ae-2002, was published in 2002. Subsequent standards encompass media types for single-mode fiber, multi-mode fiber, copper backplane and copper twisted pair. All 10-gigabit standards were consolidated into IEEE Std 802.3-2008. Most 10-gigabit variants use 64b/66b PCS code. 10 Gigabit Ethernet, specifically 10GBASE-LR and 10GBASE-ER, enjoys significant market shares in carrier networks.

Single-lane 25-gigabit Ethernet is based on one 25.78125 GBd lane of the four from the 100 Gigabit Ethernet standard developed by the P802.3by task force. 25GBASE-T over twisted pair was approved alongside 40GBASE-T within IEEE 802.3bq.

This class of Ethernet was standardized in June 2010 as IEEE 802.3ba. The work also included the first generation, published in March 2011 as IEEE 802.3bg. A twisted-pair standard was published in 2016 as IEEE 802.3bq-2016.

The IEEE 802.3cd task force developed along with next-generation 100 and standards using lanes.

The first generation of 100 Gigabit Ethernet using 10 and lanes was standardized in June 2010 as IEEE 802.3ba alongside 40 Gigabit Ethernet. The second generation using lanes was developed by the IEEE 802.3cd task force along with 50 and standards. The third generation using a single lane was standardized in September 2022 as IEEE 802.3ck along with 200 and Ethernet.

First generation have been defined by the IEEE 802.3bs task force and standardized in 802.3bs-2017. The IEEE 802.3cd task force has developed 50 and next-generation 100 and standards using one, two, or four lanes respectively. The next generation using lanes was standardized in September 2022 as IEEE 802.3ck along with 100 and PHYs and attachment unit interfaces using lanes.

An Ethernet standard capable of 200 and is defined in IEEE 802.3bs-2017. may be a further goal.
In May 2018, IEEE 802.3 started the 802.3ck task force to develop standards for 100, 200, and PHYs and attachment unit interfaces using lanes. The new standards were approved in September 2022.
In 2008, Robert Metcalfe, one of the co-inventors of Ethernet, said he believed commercial applications using Terabit Ethernet may occur by 2015, though it might require new Ethernet standards. It was predicted this would be followed rapidly by a scaling to 100 Terabit, possibly as early as 2020. These were theoretical predictions of technological ability, rather than estimates of when such speeds would actually become available at a practical price point.

The Ethernet Technology Consortium proposed an Ethernet PCS variant based on tightly bundled 400GBASE-R in April 2020.
In February 2024, the IEEE 802.3df Task Force defined variants for Ethernet over twinaxial copper, electrical backplanes, single-mode and multi-mode optical fiber along with new 200 and variants using lanes.

1.6 Tbit/s

In December 2022, IEEE started the P802.3dj Task Force to define variants for 200, 400, 800 and over twinaxial copper, electrical backplanes, single-mode and multi-mode optical fiber along with new variants using lanes.

First mile

provides Internet access service directly from providers to homes and small businesses.

Name	Standard	Description
10BaseS	Proprietary	Ethernet over VDSL, used in Long Reach Ethernet products; uses passband instead of the indicated baseband
2BASE-TL		Over telephone wires
10PASS-TS		Over telephone wires
100BASE-LX10		Single-mode fiber-optics
100BASE-BX10		Single-mode fiber-optics
1000BASE-LX10		Single-mode fiber-optics
1000BASE-BX10		Single-mode fiber-optics
1000BASE-PX10		Passive optical network
1000BASE-PX20		Passive optical network
10GBASE-PR 10/1GBASE-PRX		passive optical network with 1 or uplink for 10 or 20 km range
25GBASE-PR 50GBASE-PR		25 and passive optical network

Sublayers

Starting with Fast Ethernet, the physical layer specifications are divided into three sublayers in order to simplify design and interoperability:

PCS - This sublayer performs auto-negotiation and basic encoding, lane separation and recombination. For Ethernet, the bit rate at the top of the PCS is the nominal bit rate, e.g. for classic Ethernet or for Gigabit Ethernet.
PMA - This sublayer performs PMA framing, octet synchronization/detection, and polynomial scrambling/descrambling.
[|PMD] - This sublayer consists of a transceiver for the physical medium.
Twisted-pair cable

Several varieties of Ethernet were specifically designed to run over 4-pair copper structured cabling already installed in many locations. In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T and above use all four cable pairs for simultaneous transmission in both directions through the use of echo cancellation.
Using point-to-point copper cabling provides the opportunity to deliver electrical power along with the data. This is called power over Ethernet and there are several variations defined in IEEE 802.3 standards. Combining 10BASE-T with Mode A allows a hub or a switch to transmit both power and data over only two pairs. This was designed to leave the other two pairs free for analog telephone signals. The pins used in Mode B supply power over the spare pairs not used by 10BASE-T and 100BASE-TX. 4PPoE defined in IEEE 802.3bt can use all four pairs to supply up to 100 W.

Pin	Pair	Color	Telephone	,	onwards	PoE mode A	PoE mode B
1	3	Image:Wire white green stripe.svg\|60px\|Pair 3 Wire 1 white/green		TX+	BI_DA+	48 V out
2	3	Image:Wire green.svg\|60px\|Pair 3 Wire 2 green		TX−	BI_DA–	48 V out
3	2	Image:Wire white orange stripe.svg\|60px\|Pair 2 Wire 1 white/orange		RX+	BI_DB+	48 V return
4	1	Image:Wire blue.svg\|60px\|Pair 1 Wire 2 blue	ring	unused	BI_DC+		48 V out
5	1	Image:Wire white blue stripe.svg\|60px\|Pair 1 Wire 1 white/blue	tip	unused	BI_DC–		48 V out
6	2	Image:Wire orange.svg\|60px\|Pair 2 Wire 2 orange		RX−	BI_DB–	48 V return
7	4	Image:Wire white brown stripe.svg\|60px\|Pair 4 Wire 1 white/brown		unused	BI_DD+		48 V return
8	4	Image:Wire brown.svg\|60px\|Pair 4 Wire 2 brown		unused	BI_DD–		48 V return

The cable requirements depend on the transmission speed and the employed encoding method. Generally, faster speeds require both higher-grade cables and more sophisticated encoding.