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In computer networking, Fast Ethernet is a collective term for a number of Ethernet standards that carry traffic at the nominal rate of 100 Mbit/s, against the original Ethernet speed of 10 Mbit/s. Of the 100 megabit Ethernet standards 100baseTX (T="Twisted" Pair Copper) is by far the most common and is supported by the vast majority of Ethernet hardware currently produced. Full duplex fast Ethernet is sometimes referred to as "200 Mbit/s" though this is somewhat misleading as that level of improvement will only be achieved if traffic patterns are symmetrical. Fast Ethernet was introduced in 1995[1] and remained the fastest version of Ethernet for three years before being superseded by gigabit Ethernet.[2]


General Design

A fast Ethernet adapter can be logically divided into a Media Access Controller (MAC) which deals with the higher level issues of medium availability and a Physical Layer Interface (PHY). The MAC may be linked to the PHY by a 4 bit 25 MHz synchronous parallel interface known as a Media Independent Interface (MII) or a 2 bit 50 MHz variant Reduced Media Independent Interface (RMII). Repeaters (hubs) are also allowed and connect to multiple PHYs for their different interfaces.

The MII may (rarely) be an external connection but is usually a connection between ICs in a network adapter or even within a single IC. The specs are written based on the assumption that the interface between MAC and PHY will be a MII but they do not require it.

The MII fixes the theoretical maximum data bit rate for all versions of fast Ethernet to 100 Mbit/s. The data signaling rate actually observed on real networks is less than the theoretical maximum, due to the necessary header and trailer (addressing and error-detection bits) on every frame, the occasional "lost frame" due to noise, and time waiting after each sent frame for other devices on the network to finish transmitting.


3Com 3c905-TX 100BASE-TX PCI network interface card

100BASE-T is any of several Fast Ethernet standards for twisted pair cables, including: 100BASE-TX (100 Mbit/s over two-pair Cat5 or better cable), 100BASE-T4 (100 Mbit/s over four-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbit/s over two-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 metres (328 ft) (as with 10BASE-T and gigabit Ethernet). All are or were standards under IEEE 802.3 (approved 1995).

In the early days of Fast Ethernet, much vendor advertising centered on claims by competing standards that "ours will work better with existing cables than theirs." In practice, it was quickly discovered that few existing networks actually met the assumed standards, because 10-megabit Ethernet was very tolerant of minor deviations from specified electrical characteristics and few installers ever bothered to make exact measurements of cable and connection quality; if Ethernet worked over a cable, it was deemed acceptable. Thus most networks had to be rewired for 100-megabit speed whether or not there had supposedly been CAT3 or CAT5 cable runs. The vast majority of common implementations or installations of 100BASE-T are done with 100BASE-TX.



RJ-45 Wiring (TIA/EIA-568-B T568B)
Pin Pair Wire Color
1 2 1 Pair 2 Wire 1 white/orange
2 2 2 Pair 2 Wire 2 orange
3 3 1 Pair 3 Wire 1 white/green
4 1 2 Pair 1 Wire 2 blue
5 1 1 Pair 1 Wire 1 white/blue
6 3 2 Pair 3 Wire 2 green
7 4 1 Pair 4 Wire 1 white/brown
8 4 2 Pair 4 Wire 2 brown

100BASE-TX is the predominant form of Fast Ethernet, and runs over two wire-pairs inside a category 5 or above cable (a typical category 5 cable contains 4 pairs and can therefore support two 100BASE-TX links). Like 10BASE-T, the proper pairs are the orange and green pairs (canonical second and third pairs) in TIA/EIA-568-B's termination standards, T568A or T568B. These pairs use pins 1, 2, 3 and 6.

In T568A and T568B, wires are in the order 1, 2, 3, 6, 4, 5, 7, 8 on the modular jack at each end. The color-order would be green/white, green, orange/white, blue, blue/white, orange, brown/white, brown for T568A, and orange/white, orange, green/white, blue, blue/white, green, brown/white, brown for T568B.

Each network segment can have a maximum distance of 100 metres (328 ft). In its typical configuration, 100BASE-TX uses one pair of twisted wires in each direction, providing 100 Mbit/s of throughput in each direction (full-duplex). See IEEE 802.3 for more details.

The configuration of 100BASE-TX networks is very similar to 10BASE-T. When used to build a local area network, the devices on the network (computers, printers etc.) are typically connected to a hub or switch, creating a star network. Alternatively it is possible to connect two devices directly using a crossover cable.

With 100BASE-TX hardware, the raw bits (4 bits wide clocked at 25 MHz at the MII) go through 4B5B binary encoding to generate a series of 0 and 1 symbols clocked at 125 MHz symbol rate. The 4B5B encoding provides DC equalization and spectrum shaping (see the standard for details). Just as in the 100BASE-FX case, the bits are then transferred to the physical medium attachment layer using NRZI encoding. However, 100BASE-TX introduces an additional, medium dependent sublayer, which employs MLT-3 as a final encoding of the data stream before transmission, resulting in a maximum "fundamental frequency" of 31.25 MHz. The procedure is borrowed from the ANSI X3.263 FDDI specifications, with minor discrepancies.[3]


100BASE-T4 was an early implementation of Fast Ethernet. It requires four twisted copper pairs, but those pairs were only required to be category 3 rather than the category 5 required by TX. One pair is reserved for transmit, one for receive, and the remaining two will switch direction as negotiated. A very unusual 8B6T code is used to convert 8 data bits into 6 base-3 digits (the signal shaping is possible as there are three times as many 6-digit base-3 numbers as there are 8-digit base-2 numbers). The two resulting 3-digit base-3 symbols are sent in parallel over 3 pairs using 3-level pulse-amplitude modulation (PAM-3). This standard can be implemented with CAT 3, 4, 5 UTP cables, or STP if needed against interference. Maximum distance is limited to 100 meters and most likely to be found with old UTP media.


Symbol Line signal level
000 0
001 +1
010 -1
011 -2
100(ESC) +2

In 100BASE-T2, the data is transmitted over two copper pairs, 4 bits per symbol. First, a 4 bit symbol is expanded into two 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register; see the standard for details. This is needed to flatten the bandwidth and emission spectrum of the signal, as well as to match transmission line properties. The mapping of the original bits to the symbol codes is not constant in time and has a fairly large period (appearing as a pseudo-random sequence). The final mapping from symbols to PAM-5 line modulation levels obeys the table on the right.



100BASE-FX is a version of Fast Ethernet over optical fiber. It uses a 1300 nm near-infrared (NIR) light wavelength transmitted via two strands of optical fiber, one for receive(RX) and the other for transmit(TX). Maximum length is 400 metres (1,310 ft) for half-duplex connections (to ensure collisions are detected), 2 kilometres (6,600 ft) for full-duplex over multimode optical fiber, or 10,000 meters (32,808 feet) for full-duplex single mode optical fiber. 100BASE-FX uses the same 4B5B encoding and NRZI line code that 100BASE-TX does. 100BASE-FX should use SC, ST, or MIC connectors with SC being the preferred option.[4]

100BASE-FX is not compatible with 10BASE-FL, the 10 MBit/s version over optical fiber.


100BASE-SX is a version of Fast Ethernet over optical fiber. It uses two strands of multi-mode optical fiber for receive and transmit. It is a lower cost alternative to using 100BASE-FX, because it uses short wavelength optics which are significantly less expensive than the long wavelength optics used in 100BASE-FX. 100BASE-SX can operate at distances up to 550 metres (1,800 ft).

100BASE-SX uses the same wavelength as 10BASE-FL, the 10 MBit/s version over optical fiber. Unlike 100BASE-FX, this allows 100BASE-SX to be backwards-compatible with 10BASE-FL.

Because of the shorter wavelength used (850 nm) and the shorter distance it can support, 100BASE-SX uses less expensive optical components (LEDs instead of lasers) which makes it an attractive option for those upgrading from 10BASE-FL and those who do not require long distances.


100BASE-BX is a version of Fast Ethernet over a single strand of optical fiber (unlike 100BASE-FX, which uses a pair of fibers). Single-mode fiber is used, along with a special multiplexer which splits the signal into transmit and receive wavelengths. The two wavelengths used for transmit and receive are either 1310/1550nm or 1310/1490nm (in this case 1550nm can be used for CATV downstream). The terminals on each side of the fibre are not equal, as the one transmitting "downstream" (from the center of the network to the outside) uses the 1550nm or 1490 nm wavelength, and the one transmitting "upstream" uses the 1310 nm wavelength. Distances can be 10, 20 or 40 km.


100BASE-LX10 is a version of Fast Ethernet over two single-mode optical fibers. It has a nominal reach of 10km and a nominal wavelength of 1310nm. It is described in IEEE 802.3-2005 Section 5 chapter 58.

See also


  1. ^ IEEE 802.3u-1995
  2. ^ [1] The 802.3z Gigabit Ethernet Standard was published
  3. ^ "The 100BASE-TX PMD (and MDI) is specified by incorporating the FDDI TP-PMD standard, ANSI X3.263: 1995 (TP-PMD), by reference, with the modifications noted below." (section 25.2 of IEEE802.3-2002).
  4. ^ 802.3-2008 section 26.4.1

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

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