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The Ethernet physical layer is the physical layer component of the Ethernet family of computer network standards which defines the electrical or optical properties of the physical connection between a device and the network or between network devices.
Contents
- Physical layers
- Early implementations
- Fast Ethernet
- 1 Gbits
- 25 and 5 Gbits
- 10 Gbits
- 25 Gbits
- 40 and 100 Gbits
- 50 Gbits 100 Gbits 200 Gbits
- 200 Gbits 400 Gbits 1 Tbits and beyond
- First mile
- Sublayers
- Twisted pair cable
- Minimum cable lengths
- Related standards
- References
The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed. The speed ranges from 1 Mbit/s to 100 Gbit/s, while the physical medium can range from bulky coaxial cable to twisted pair and optical fiber. In general, network protocol stack software will work similarly on all physical layers.
10 Gigabit Ethernet was already used in both enterprise and carrier networks by 2007, with 40 Gbit/s and 100 Gigabit Ethernet ratified. Higher speeds are under development. Robert Metcalfe, one of the co-inventors of Ethernet, in 2008 said he believed commercial applications using Terabit Ethernet may occur by 2015, though it might require new Ethernet standards.
Many Ethernet adapters and switch ports support multiple speeds, using autonegotiation to set the speed and duplex for the best values supported by both connected devices. While this can practically be taken for granted for ports supporting twisted-pair cabling, only few optical-fiber ports support multiple speeds. If auto-negotiation fails, some multiple-speed devices sense the speed used by their partner, but will assume half-duplex. A 10/100 Ethernet port supports 10BASE-T and 100BASE-TX. A 10/100/1000 Ethernet port supports 10BASE-T, 100BASE-TX, and 1000BASE-T.
Physical layers
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 following sections provide a brief summary of official Ethernet media types (section numbers from the IEEE 802.3 standard are parenthesized). 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.
1 Gbit/s
All Gigabit Ethernet variants use a star topology. Initially, half-duplex mode was included in the standard but has been abandoned since. 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 fibre (long haul), multi-mode fibre (up to 400 m), copper backplane (up to 1 m) and copper twisted pair (up to 100 m). All 10-gigabit standards were consolidated into IEEE Std 802.3-2008. As of 2009, 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 and 100 Gbit/s
This version of Ethernet specified two speeds and was standardized in June 2010 as IEEE 802.3ba, with one addition in March 2011 as IEEE 802.3bg. The nomenclature is as follows:
50 Gbit/s, 100 Gbit/s, 200 Gbit/s
The IEEE 802.3cd Task Force is currently developing 50 and next-generation 100 and 200 Gbit/s standards using one, two, or four 50 Gbit/s lanes respectively. 200 Gbit/s over single-mode fiber is developed alongside 400 Gbit/s by task force 802.3bs.
200 Gbit/s, 400 Gbit/s, 1 Tbit/s, and beyond
The standards body the Institute of Electrical and Electronic Engineers (IEEE) wants to define a new Ethernet standard capable of 200 and 400 Gbit/s, and possibly 1 Tbit/s.
Initially, some believed that Terabit Ethernet may make a debut as early as 2015, and 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:
Sublayers
From Fast Ethernet on, the physical layer specifications are divided into three sublayers in order to simplify design and interoperability:
Twisted-pair cable
Several varieties of Ethernet were specifically designed to run over 4-pair copper structured cabling already installed in many locations. ANSI recommends using Category 6 cable for new installations.
Combining 10Base-T (or 100BASE-TX) with "IEEE 802.3af mode A" allows a hub 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 "IEEE 802.3af Mode B" supply power over the "spare" pairs not used by 10BASE-T and 100BASE-TX.
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.
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.