Fast Ethernet

3Com 3C905B-TX 100BASE-TX PCI network interface card

100BASE-TX is the predominant form of Fast Ethernet, and runs over two wire-pairs inside a category 5 or above cable. Each network segment can have a maximum cabling distance of 100 metres (328 ft). One pair is used for each direction, providing full-duplex operation with 100 Mbit/s of throughput in each direction.

Like 10BASE-T, the active pairs in a standard connection are terminated on pins 1, 2, 3 and 6. Since a typical category 5 cable contains 4 pairs, it can support two 100BASE-TX links with a wiring adaptor.[5] Cabling is conventional wired to TIA/EIA-568-B's termination standards, T568A or T568B. This places the active pairs on the orange and green pairs (canonical second and third pairs).

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 today's equipment, crossover cables are generally not needed as most equipment support auto-negotiation along with auto MDI-X to select and match speed, duplex and pairing.

With 100BASE-TX hardware, the raw bits, presented 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 a 125 MHz symbol rate. The 4B5B encoding provides DC equalization and spectrum shaping. 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 changes.[6]

In 100BASE-T1[7] the data is transmitted over a single copper pair, 3 bits per symbol, each transmitted as code pair using PAM3. It supports only full-duplex, transmitting in both directions simultaneously. The twisted-pair cable is required to support 66 MHz, with a maximum length of 15 m. No specific connector is defined. The standard is intended for automotive applications or when Fast Ethernet is to be integrated into another application. It was developed as BroadR-Reach before IEEE standardization.[8]

In 100BASE-T2, standardized in IEEE 802.3y, the data is transmitted over two copper pairs, but these pairs are only required to be category 3 rather than the category 5 required by 100BASE-TX. Data is transmitted and received on both pairs simultaneously[9] thus allowing full-duplex operation. Transmission uses 4 bits per symbol. The 4-bit symbol is expanded into two 3-bit symbols through a non-trivial scrambling procedure based on a linear feedback shift register.[10] 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-T2 was not widely adopted but the technology developed for it is used in 1000BASE-T.[11]

100BASE-T4 was an early implementation of Fast Ethernet. It requires four twisted copper pairs of voice grade twisted pair, a lower performing cable compared to category 5 cable used by 100BASE-TX. Maximum distance is limited to 100 meters. One pair is reserved for transmit, one for receive, and the remaining two switch direction. The fact that 3 pairs are used to transmit in each direction makes 100BASE-T4 inherently half-duplex.

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 nearly 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).

100BASE-T4 was not widely adopted but some of the technology developed for it is used in 1000BASE-T.[11]

Proposed and marketed by Hewlett Packard, 100BaseVG was an alternative design using category 3 cabling and a token concept instead of CSMA/CD. It was slated for standardization as IEEE 802.12 but it quickly vanished when switched 100BASE-TX became popular.

Fast Ethernet speed is not available on all SFP ports,[18] but supported by some devices.[19][20] An SFP port for Gigabit Ethernet should not be assumed to be backwards compatible with Fast Ethernet.

To have interoperable there is some criteria that have to be meet:[21]

• Line encoding
• Wavelength[lower-alpha 1]
• Duplex mode
• Media count
• Media type and dimension

100BASE-X Ethernet is not backward compatible with 10BASE-F and is not forward compatible with 1000BASE-X.

100BASE-FX is a version of Fast Ethernet over optical fiber. The 100BASE-FX Physical Medium Dependent (PMD) sublayer is defined by FDDI's PMD,[23] so 100BASE-FX is not compatible with 10BASE-FL, the 10 Mbit/s version over optical fiber.

100BASE-FX is still used for existing installation of multimode fiber where more speed is not required, like industrial automation plants.[13]

100BASE-LFX is a non-standard term to refer to Fast Ethernet transmission. It is very similar to 100BASE-FX but achieves longer distances up to 4-5 km over a pair of multi-mode fibers due to higher quality optics than a FX, running on 1310 nm wavelength lasers. It is easily confused with 100BASE-LX10 or 100BASE-FX because the use of -FX, -LX and -LX10 is ambiguous between vendors. The range is achieved with use of Fabry Perot laser transmitter.[24]

When operating at 1300 nm, the signal attenuation per km is about half the loss of 850nm.[25][26] Reach over multi-mode fiber may be extended by using 1300 nm wavelength.

100BASE-SX is a version of Fast Ethernet over optical fiber standardized in TIA/EIA-785-1-2002. 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 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-EX is a non-standard but industry accepted[16] term to refer to Fast Ethernet transmission. It is very similar to 100BASE-LX10 but achieves longer distances up to 40 km over a pair of single-mode fibers due to higher quality optics than a LX10, running on 1310 nm wavelength lasers. It is sometimes referred to as LH (Long Haul), and is easily confused with 100BASE-LX10 or 100BASE-ZX because the use of -LX(10), -LH, -EX, and -ZX is ambiguous between vendors.

100BASE-ZX is a non-standard but multi-vendor[17] term to refer to Fast Ethernet transmission using 1,550 nm wavelength to achieve distances of at least 70km over single-mode fiber. Some vendors specify distances up to 160km over single-mode fiber, sometimes called 100BASE-EZX. Ranges beyond 80 km are highly dependent upon the path loss of the fiber in use, specifically the attenuation figure in dB per km, the number and quality of connectors/patch panels and splices located between transceivers.[27]