Ethernet physical layer


The Ethernet physical layer is the physical layer functionality of the Ethernet family of computer network standards. The physical layer defines the electrical or optical properties 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.
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 1 Mbit/s to 400 Gbit/s. The physical medium ranges from bulky coaxial cable to twisted pair and optical fiber with a standardized reach of up to 40 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 40 Gbit/s and 100 Gigabit Ethernet ratified. In 2017, the fastest additions to the Ethernet family were 200 and 400 Gbit/s.

Naming conventions

Generally, layers are named by their specifications:
For 10 Mbit/s, 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

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

Fast Ethernet

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

1 Gbit/s

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 5 Gbit/s

2.5GBASE-T and 5GBASE-T are scaled-down variants of 10GBASE-T. These physical layers support twisted pair copper cabling only.

10 Gbit/s

10 Gigabit Ethernet defines a version of Ethernet with a nominal data rate of 10 Gbit/s, ten times as fast as Gigabit Ethernet. In 2002, the first 10 Gigabit Ethernet standard was published as IEEE Std 802.3ae-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 is predominantly deployed in carrier networks, where 10GBASE-LR and 10GBASE-ER enjoy significant market shares.

25 Gbit/s

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

40 Gbit/s

This class of Ethernet was standardized in June 2010 as IEEE 802.3ba along with the first 100 Gbit/s generation, with an addition in March 2011 as IEEE 802.3bg, and the fastest yet twisted-pair standard in IEEE 802.3bq-2016.
The nomenclature is as follows:

50 Gbit/s

The IEEE 802.3cd Task Force has developed 50 Gbit/s along with next-generation 100 and 200 Gbit/s standards using 50 Gbit/s lanes-

100 Gbit/s

The first generation of 100G Ethernet using 10 and 25 Gbit/s lanes was standardized in June 2010 as IEEE 802.3ba alongside 40 Gbit/s. The second generation using 50 Gbit/s lanes has been developed by the IEEE 802.3cd Task Force along with 50 and 200 Gbit/s standards. The third generation using a single 100 Gbit/s lane is currently being developed by the IEEE 802.3ck Task Force along with 200 and 400 Gbit/s PHYs and attachment unit interfaces using 100 Gbit/s lanes.

200 Gbit/s

First generation 200 Gbit/s 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 200 Gbit/s standards using one, two, or four 50 Gbit/s lanes respectively. The next generation using 100 Gbit/s lanes is currently being developed by the IEEE 802.3ck Task Force along with 100 and 400 Gbit/s PHYs and attachment unit interfaces using 100 Gbit/s lanes.

400 Gbit/s

The Institute of Electrical and Electronics Engineers has defined a new Ethernet standard capable of 200 and 400 Gbit/s in IEEE 802.3bs-2017. 1 Tbit/s may be a further goal.
In May 2018, IEEE 802.3 started the 802.3ck Task Force to develop standards for 100, 200, and 400 Gbit/s PHYs and attachment unit interfaces using 100 Gbit/s lanes.
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. It is worth noting that these were theoretical predictions of technological ability, rather than estimates of when such speeds would actually become available at a practical price point.

First mile

For providing Internet access service directly from providers to homes and small businesses:
NameStandard Description
10BaseSProprietaryEthernet 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		 10 Gbit/s passive optical network with 1 or 10 Gbit/s uplink for 10 or 20 km range

Sublayers

Starting with Fast Ethernet, the physical layer specifications are divided into three sublayers in order to simplify design and interoperability:
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 transmit low electrical power along with the data. This is called Power over Ethernet and there are several, incremental 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.
PinPairColortelephone10BASE-T
100BASE-TX		1000BASE-T
onwards		PoE mode APoE mode B
13 white/greenTX+BI_DA+48 V out
23 greenTX−BI_DA–48 V out
32 white/orangeRX+BI_DB+48 V return
41 blueringunusedBI_DC+48 V out
51 white/bluetipunusedBI_DC–48 V out
62 orangeRX−BI_DB–48 V return
74 white/brownunusedBI_DD+48 V return
84 brownunusedBI_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.

Minimum cable lengths

Fiber connections have minimum cable lengths due to level requirements on received signals. Fiber ports designed for long-haul wavelengths require a signal attenuator if used within a building.
10BASE2 installations, running on RG-58 coaxial cable, require a minimum of 0.5 m between stations tapped into the network cable, this is to minimize reflections.
10BASE-T, 100BASE-T, and 1000BASE-T installations running on twisted pair cable use a star topology. No minimum cable length is required for these networks.

Related standards

Some networking standards are not part of the IEEE 802.3 Ethernet standard, but support the Ethernet frame format, and are capable of interoperating with it.
Other networking standards do not use the Ethernet frame format but can still be connected to Ethernet using MAC-based bridging.
Other special-purpose physical layers include Avionics Full-Duplex Switched Ethernet and TTEthernet — Time-Triggered Ethernet for embedded systems.