Ethernet physical layer

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Ethernet physical layer
A standard 8P8C (often called RJ45) connector used most commonly on cat5 cable, one of the types of cabling used in Ethernet networks
Standard IEEE 802.3 (2002 onwards)
Physical media Coaxial cable, twisted pair, optical fiber
Network topology Point-to-point, star, bus
Major variants 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 1000BASE-T, 1000BASE-SX, 1000BASE-LX10
Maximum distance 100 metres (328 ft) over twisted pair, up to 100 km over optical fiber
Mode of operation Differential (Balanced)
Maximum bit rate 1 Mbit/s to 100 Gbit/s
Voltage levels ± 2.5V (over twisted pair)
Available signals Tx+, Tx−, Rx+, Rx−
Common connector types 8P8C, LC, SC, ST

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.

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[1][2] and 100 Gigabit Ethernet[3] ratified.[4] Higher speeds are under development.[5] 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.[6]

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,[7] 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[edit]

Generally, layers are named by their specifications:[8]

  • 10, 100, 1000, 10G, ... – the nominal, usable speed for the MAC layer (no suffix = megabit/s, G = gigabit/s), encoded PHY sublayers usually run higher bitrates
  • BASE, BROAD, PASS – indicates baseband, broadband, or passband signaling
  • -T, -S, -L, -C, -K, ... – medium: T = twisted pair, S = 850 nm short wavelength (multi-mode fiber), L = 1300 nm long wavelength (mostly single-mode fiber), E/Z = 1500 nm extra long wavelength (single-mode), B = WDM bidirectional fiber (mostly single-mode), P = passive optical (PON), C = copper/twinax, K = backplane, 2/5/36 = coax with 185/500/3600 m reach, F = fiber, various wavelengths
  • X, RPCS encoding method (varying with the generation), e.g. X for 8b/10b block encoding (4B5B for Fast Ethernet), R for large blocks (64b/66b)
  • 1, 2, 4, 10 – number of lanes used per link or reach for WAN PHYs

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[edit]

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

Name Standard (Clause) Common connectors Description
Xerox experimental Ethernet Proprietary The original 2.94 Mbit/s Ethernet implementation had eight bit addresses and other differences in frame format and was carried on 50 Ω coaxial cable.[9]
10BASE5 802.3 (8) AUI Original standard uses a single coaxial cable into which you literally tap a connection by drilling into the cable to connect to the core and screen. Largely obsolete, though due to its widespread deployment in the early 1980s, some systems may still be in use.[10] Was known also as Thick-Ethernet. 10 Mbit/s over copper RG-8X (expensive) coaxial cabling, bus topology with collision detection.
10BASE2 802.3 (10) BNC 50 Ω coaxial cable connects machines together, each machine using a T-connector to connect to its NIC. Requires terminators at each end. For many years during the mid to late 1980, this was the dominant Ethernet standard. Also called Thin Ethernet, ThinNet or Cheapernet. 10 Mbit/s over RG-58 coaxial cabling, bus topology with collision detection.
10BROAD36 802.3 (11) F An early standard supporting Ethernet over longer distances. It utilized broadband modulation techniques, similar to those employed in cable modem systems, and operated over coaxial cable. 10 Mbit/s, scrambled NRZ signaling modulated (PSK) over high frequency carrier, broad bandwidth coaxial cabling, bus topology with collision detection.
1BASE5 802.3 (12) 8P8C Operated at 1 Mbit/s over twisted pair to an active hub. Although a commercial failure, 1BASE5 defined the architecture for all subsequent Ethernet evolution. Also called StarLAN. 1 Mbit/s over copper twisted pair cabling, star topology.
StarLAN 10 Proprietary 8P8C 10 Mbit/s over copper twisted pair cabling, star topology - evolved into 10BASE-T
LattisNet UTP Proprietary 8P8C 10 Mbit/s over copper twisted pair cabling, star topology - evolved into 10BASE-T
10BASE-T 802.3 (14) 8P8C Runs over four wires (two twisted pairs) on a Category 3 or Category 5 cable. A repeater hub or switch sits in the middle and has a port for each node. This is also the configuration used for 100BASE-T and gigabit Ethernet. Copper twisted pair cabling, star topology – direct evolution of 1BASE-5.
FOIRL 802.3 (9.9) ST Fiber-optic inter-repeater link; the original standard for Ethernet over fiber
10BASE-F 802.3 (15) A generic term for the family of 10 Mbit/s Ethernet standards using fiber optic cable: 10BASE-FL, 10BASE-FB and 10BASE-FP. Of these only 10BASE-FL is in widespread use. 10 Mbit/s over fiber pair
10BASE‑FL 802.3 (15&18) ST An updated version of the FOIRL standard
10BASE‑FB 802.3 (15&17) Intended for backbones connecting a number of hubs or switches; it is now obsolete.[11]
10BASE‑FP 802.3 (15&16) A passive star network that required no repeater, it was never implemented.[11]

Fast Ethernet[edit]

Main article: Fast Ethernet

All Fast Ethernet variants use a star topology.

Name Standard (Clause) Common connectors Description
100BASE‑T 802.3 (21) A term for any of the three standards for 100 Mbit/s Ethernet over twisted pair cable. Includes 100BASE-TX, 100BASE-T4 and 100BASE-T2. As of 2009, 100BASE-TX has totally dominated the market, and is often considered to be synonymous with 100BASE-T in informal usage. All of them use a star topology.
100BASE-TX 802.3 (24&25) 8P8C 4B5B MLT-3 coded signaling, CAT5 copper cabling with two twisted pairs.
100BASE-T4 802.3 (23) 8P8C 8B6T PAM-3 coded signaling, CAT3 copper cabling (as used for 10BASE-T installations) with four twisted pairs (uses all four pairs in the cable). Now obsolete, as CAT5 cabling is the norm. Limited to half-duplex.
100BASE-T2 802.3 (32) No products exist.[citation needed] PAM-5 coded signaling, CAT3 copper cabling with two twisted pairs, star topology. Supports full-duplex. It is functionally equivalent to 100BASE-TX, but supports old telephone cable. However, special sophisticated digital signal processors are required to handle encoding schemes required, making this option fairly expensive at the time. It arrived well after 100BASE-TX was established in the market. The technology developed for 100BASE-T2 was the foundation for 1000BASE-T.
100BASE-T1 802.3 (96) none specified Uses PAM-3 modulation over a single, bi-directional twisted pair of up to 15 m using a PMA similar to 1000BASE-T at 66.7 MBd. It is intended for automotive applications.
100BASE‑FX 802.3 (24&26) ST, SC 4B5B NRZI coded signaling, two strands of multi-mode optical fiber. Maximum length is 400 meters for half-duplex connections (to ensure collisions are detected) or 2 kilometers for full-duplex. The specifications are largely borrowed from FDDI.
100BASE‑SX TIA-785 ST, SC 100 Mbit/s Ethernet over multi-mode fiber. Maximum length is 300 meters. 100BASE-SX used short wavelength (850 nm) optics that was sharable with 10BASE-FL, thus making it possible to have an auto-negotiation scheme and have 10/100 fiber adapters.
100BASE‑BX10 802.3 (58) ST, SC, LC 100 Mbit/s Ethernet bidirectionally over a single strand of single-mode optical fiber. An optical multiplexer is used to split transmit and receive signals into different wavelengths allowing them to share the same fiber. Supports up to 10 km.
100BASE-LX10 802.3 (58) ST, SC, LC 100 Mbit/s Ethernet up to 10 km over a pair of single-mode fibers.
100Base‑VG 802.12 8P8C Standardized by a different IEEE 802 subgroup, 802.12, because it used a different, more centralized form of media access ("Demand Priority"). Championed by only HP, 100VG-AnyLAN (as was the marketing name) was the earliest in the market. It needed four pairs in a Cat-3 cable. Now obsolete (802.12 has been "inactive" since 1997) the standard has been withdrawn.

1 Gbit/s[edit]

Main article: Gigabit Ethernet

All Gigabit Ethernet variants use a star topology. Initially, half-duplex mode was included in the standard but has been abandoned since.[12] Very few devices support gigabit speed in half-duplex.

Name Standard (Clause) Common connectors Description
1000BASE‑T 802.3 (40) 8P8C PAM-5 coded signaling, at least Category 5 cable, with Category 5e strongly recommended copper cabling with four twisted pairs. Each pair is used in both directions simultaneously.
1000BASE-T1 802.3bp-2016 uses a single, bi-directional twisted pair in full duplex mode only; cables specified for 15 m or 40 m reach, intended for automotive and industrial applications
1000BASE‑TX TIA-854 Only over Cat-6 copper cabling. Unimplemented.
1000BASE‑SX 802.3 (38) ST, SC, LC 8B10B NRZ coded signaling on 850 nm carrier, short-range multi-mode fiber (up to 550 m).
1000BASE‑LX 802.3 (38) SC, LC 8B10B NRZ coded signaling on 1310 nm carrier, multi-mode fiber (up to 550 m) or single-mode fiber of up to 5 km
1000BASE‑CX 802.3 (39) CX4/SFF-8470 8B10B NRZ coded signaling, balanced shielded twisted pair (up to 25 m) over special copper cable. Predates 1000BASE-T and rarely used.
1000BASE‑BX10 802.3 (59) SC, LC up to 10 km on 1490 and 1390 nm carriers; bidirectional over single strand of single-mode fibre.
1000BASE‑LX10 802.3 (59) SC, LC identical with 1000BASE-LX but increased power/sensitivity for up to 10 km over a pair of single-mode fibres; vendor-specific extensions exist for up to 40 km reach
1000BASE‑PX10‑D 802.3 (60) SC, LC downstream (from head-end to tail-ends) over single-mode fiber using point-to-multipoint topology (supports at least 10 km).
1000BASE‑PX10‑U 802.3 (60) upstream (from a tail-end to the head-end) over single-mode fiber using point-to-multipoint topology (supports at least 10 km).
1000BASE‑PX20‑D 802.3 (60) downstream (from head-end to tail-ends) over single-mode fiber using point-to-multipoint topology (supports at least 20 km).
1000BASE‑PX20‑U 802.3 (60) upstream (from a tail-end to the head-end) over single-mode fiber using point-to-multipoint topology (supports at least 20 km).
multi-vendor SC, LC up to 40 or 100 km over single-mode fibre on 1550 nm carrier[13]
1000BASE‑KX 802.3 (70) 1 m over backplane

2.5 and 5 Gbit/s[edit]

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

Name Standard (Clause) Common connectors Description
2.5GBASE-T 802.3bz 8P8C 100 m of Cat 5e
5GBASE-T 802.3bz 8P8C 100 m of Cat 6

10 Gbit/s[edit]

Main article: 10 Gigabit Ethernet

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.

Name Standard (Clause) Common connectors Description
10GBASE‑SR 802.3 (52) SC, LC designed to support short distances over deployed multi-mode fiber cabling, it has a range of between 26 m and 400 m depending on cable type (modal bandwidth:reach: 160 MHz·km:26 m, 200 MHz·km:33 m, 400 MHz·km:66 m, 500 MHz·km:82 m, 2000 MHz·km:300 m, 4700 MHz·km:400 m)[14] using 850 nm wavelength
10GBASE‑LX4 802.3 (53) SC, LC uses wavelength division multiplexing (1275, 1300, 1325, and 1350 nm) over deployed multi-mode cabling to support ranges of between 240 m and 300 m (400/500 MHz·km modal bandwidth). Also supports 10 km over single-mode fiber.
10GBASE‑LR 802.3 (52) SC, LC supports 10 km over single-mode fiber using 1,310 nm wavelength
10GBASE‑ER 802.3 (52) SC, LC supports 40 km over single-mode fiber using 1,550 nm wavelength
10GBASE‑SW 802.3 (52) A variation of 10GBASE-SR with 9.58464 Gbit/s, designed to be mapped directly as OC-192 / STM-64 SONET/SDH streams (850 nm wavelength)
10GBASE‑LW 802.3 (52) A variation of 10GBASE-LR with 9.58464 Gbit/s, designed to be mapped directly OC-192 / STM-64 SONET/SDH streams (1,310 nm wavelength)
10GBASE‑EW 802.3 (52) A variation of 10GBASE-ER with 9.58464 Gbit/s, designed to be mapped directly OC-192 / STM-64 SONET/SDH streams (1,550 nm wavelength)
10GBASE‑CX4 802.3 (54) CX4/SFF-8470 Designed to support short distances over copper cabling, it uses InfiniBand 4x connectors and CX4 cabling and allows a cable length of up to 15 m. Was specified by the IEEE Std 802.3an-2006 which has been incorporated into the IEEE Std 802.3-2008.
10GBASE‑T 802.3 (55) 8P8C Uses Cat 6A twisted-pair wiring, four lanes at 800 MBd each, PAM-16 with "DSQ128" line code
10GBASE‑LRM 802.3 (68) SC, LC Extend to 220 m over deployed 500 MHz·km multimode fiber (1,310 nm wavelength)
10GBASE‑KX4 802.3 (71) 1 m over 4 lanes of backplane
10GBASE‑KR 802.3 (72) 1 m over a single lane of backplane
SFP+ Direct Attach SFF-8431 SFP+ up to 15 m over twinaxial cabling

25 Gbit/s[edit]

Main article: 25 Gigabit Ethernet

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.[15] 25GBASE-T over twisted pair was approved alongside 40GBASE-T within IEEE 802.3bq.[16][17]

Name Standard (Clause) Common connectors Description
25GBASE-T 802.3bq scaled-down version of 40GBASE-T
25GBASE-KR 802.3by for printed-circuit backplane, derived from 100GBASE-KR4
25GBASE-CR-S/-L 802.3by CX4 over twinaxial cabling with 3 m (-CR-S) and 5 m (-CR-L) reach
25GBASE-SR 802.3by LC/SC over multi-mode cabling with 100 m (OM4) or 70 m (OM3) reach

40 and 100 Gbit/s[edit]

Main article: 100 Gigabit Ethernet

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.[18][19] The nomenclature is as follows:[20]

40 gigabits/second 100 gigabits/second Standard (Clause) Common connectors Description
40GBASE-KR4 802.3 (84) at least 1 m over a backplane
40GBASE-CR4 100GBASE-CR10 802.3 (85) QSFP+/CX4 approximately 7 m over copper cable assembly
40GBASE-SR4 100GBASE-SR10 802.3 (86) MPO at least 100 m over 2000 MHz·km multi-mode fiber (OM3)
at least 150 m over new 4700 MHz·km multi-mode fiber (OM4)
40GBASE-LR4 802.3 (87) SC, LC at least 10 km over single-mode fiber
100GBASE-LR4 802.3 (88) SC, LC
100GBASE-ER4 802.3 (88) SC, LC at least 40 km over single-mode fiber
40GBASE-FR 802.3 (89) SC, LC Single-mode fiber over 2 km

50 Gbit/s, 100 Gbit/s, 200 Gbit/s[edit]

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.[21] 200 Gbit/s over single-mode fiber is developed alongside 400 Gbit/s by task force 802.3bs.

Name Standard (Clause) Common connectors Description
802.3cd CX4/CXP over twinaxial cable with 3 m reach
over printed-circuit backplane, consistent with 802.3bs Clause 124
LC/SC/MPO over multi-mode fiber with 100 m reach
50GBASE-LR/10 LC/SC over single-mode fiber with 2 and 10 km reach

200 Gbit/s, 400 Gbit/s, 1 Tbit/s, and beyond[edit]

Main article: Terabit Ethernet

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.[22][23][24]

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.[25]

"First mile"[edit]

For providing Internet access service directly from providers to homes and small businesses:

Name Standard (Clause) Description
10BaseS Proprietary[26] Ethernet over VDSL, used in Long Reach Ethernet products[27]
2BASE-TL 802.3 (63) Over telephone wires
10PASS-TS 802.3 (62)
100BASE-LX10 802.3 (58) Single-mode fiber-optics
1000BASE-LX10 802.3 (59)
1000BASE-PX10 802.3 (60) Passive optical network


From Fast Ethernet on, the physical layer specifications are divided into three sublayers in order to simplify design and interoperability:[28]

  • PCS (Physical Coding Sublayer) - This sublayer performs auto-negotiation and encoding such as 8b/10b
  • PMA (Physical Medium Attachment sublayer) - This sublayer performs PMA framing, octet synchronization/detection, and scrambling/descrambling
  • PMD (Physical Medium Dependent sublayer) - This sublayer consists of a transceiver for the physical medium

Twisted-pair cable[edit]

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.[citation needed]

8P8C wiring (TIA/EIA-568A)
Pin Pair Color telephone 10BASE-T 100BASE-TX 1000BASE-T PoE mode A PoE mode B
1 3 Pair 3 Wire 1 white/green TX+ z bidi 48 V out
2 3 Pair 3 Wire 2 green TX− z bidi 48 V out
3 2 Pair 2 Wire 1 white/orange RX+ z bidi 48 V return
4 1 Pair 1 Wire 2 blue ring bidi 48 V out
5 1 Pair 1 Wire 1 white/blue tip bidi 48 V out
6 2 Pair 2 Wire 2 orange RX− z bidi 48 V return
7 4 Pair 4 Wire 1 white/brown bidi 48 V return
8 4 Pair 4 Wire 2 brown bidi 48 V return

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.[29][not in citation given] 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.

Comparison of twisted pair based ethernet technologies

Minimum cable lengths[edit]

Fiber connections have minimum cable lengths due to level requirements on received signals.[30] 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.[31]

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.[32][33]

Related standards[edit]

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.

  • LattisNet—A SynOptics pre-standard twisted-pair 10 Mbit/s variant.
  • 100BaseVG—An early contender for 100 Mbit/s Ethernet. It runs over Category 3 cabling. Uses four pairs. Commercial failure.
  • TIA 100BASE-SX—Promoted by the Telecommunications Industry Association. 100BASE-SX is an alternative implementation of 100 Mbit/s Ethernet over fiber; it is incompatible with the official 100BASE-FX standard. Its main feature is interoperability with 10BASE-FL, supporting autonegotiation between 10 Mbit/s and 100 Mbit/s operation – a feature lacking in the official standards due to the use of differing LED wavelengths. It is targeted at the installed base of 10 Mbit/s fiber network installations.
  • TIA 1000BASE-TX—Promoted by the Telecommunications Industry Association, it was a commercial failure, and no products exist. 1000BASE-TX uses a simpler protocol than the official 1000BASE-T standard so the electronics can be cheaper, but requires Category 6 cabling.
  •—A standard developed by ITU-T and promoted by HomeGrid Forum for high-speed (up to 1 Gbit/s) local area networks over existing home wiring (coaxial cables, power lines and phone lines). defines an Application Protocol Convergence (APC) layer that accepts Ethernet frames and encapsulates them into MSDUs.

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.


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External links[edit]