Fast Ethernet

In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of. The prior Ethernet speed was. Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common.
Fast Ethernet was introduced in 1995 as the IEEE 802.3u standard and remained the fastest version of Ethernet for three years before the introduction of Gigabit Ethernet. The acronym GE/FE is sometimes used for devices supporting both standards.

Nomenclature

The 100 in the media type designation refers to the transmission speed of, while the BASE refers to baseband signaling. The letter following the dash refers to the physical medium that carries the signal, while the last character refers to the line code method used. Fast Ethernet is sometimes referred to as 100BASE-X, where X is a placeholder for the FX and TX variants.

General design

Fast Ethernet is an extension of the 10-megabit Ethernet standard. It runs on twisted pair or optical fiber cable in a star wired bus topology, similar to the IEEE standard 802.3i called 10BASE-T, itself an evolution of 10BASE5 and 10BASE2. Fast Ethernet devices are generally backward compatible with existing 10BASE-T systems, enabling plug-and-play upgrades from 10BASE-T. Most switches and other networking devices with ports capable of Fast Ethernet can perform autonegotiation, sensing a piece of 10BASE-T equipment and setting the port to 10BASE-T half duplex if the 10BASE-T equipment cannot perform autonegotiation itself. The standard specifies the use of CSMA/CD for media access control. A full-duplex mode is also specified and in practice, modern networks use Ethernet switches and operate in full-duplex mode, even as legacy devices that use half duplex still exist.
A Fast Ethernet adapter can be logically divided into a media access controller, which deals with the higher-level issues of medium availability, and a physical layer interface. The MAC is typically linked to the PHY by a four-bit 25 MHz synchronous parallel interface known as a media-independent interface, or by a two-bit 50 MHz variant called reduced media independent interface. In rare cases, the MII may be an external connection but is usually a connection between ICs in a network adapter or even two sections within a single IC. The specs are written based on the assumption that the interface between MAC and PHY will be an MII but they do not require it. Fast Ethernet or Ethernet hubs may use the MII to connect to multiple PHYs for their different interfaces.
The MII fixes the theoretical maximum data bit rate for all versions of Fast Ethernet to. The information rate actually observed on real networks is less than the theoretical maximum, due to the necessary header and trailer on every Ethernet frame, and the required interpacket gap between transmissions.

Copper

100BASE-T is any of several Fast Ethernet standards for twisted pair cables, including: 100BASE-TX, 100BASE-T4, 100BASE-T2. The segment length for a 100BASE-T cable is limited to . All are or were standards under IEEE 802.3. Almost all 100BASE-T installations are 100BASE-TX.

Name	Added in amendment	Status	Speed	Pairs required	Lanes per direction	Bits per hertz	Line code	Symbol rate per lane	Bandwidth	Max distance	Cable	Cable rating	Usage
			100	2	1	3.2	4B5B MLT-3 NRZI	125	31.25	100	Cat 5	100	LAN
			100	1	1	2.6	PAM-3 4B/3B	75	37.5	15	Cat 5e	66	Automotive, IoT, M2M
			100	2	2	4	LFSR PAM-5	25	12.5	100	Cat 3	16
			100	4	3	2.6	8B6T PAM-3 Half-duplex only	25	12.5	100	Cat 3	16
			100	4	4	1.6	5B6B Half-duplex only	30	15	100	Cat 3	16

Pin	Pair	Wire	Color
1	3	+/tip	white/green
2	3	−/ring	green
3	2	+/tip	white/orange
4	1	+/ring	blue
5	1	-/tip	white/blue
6	2	−/ring	orange
7	4	+/tip	white/brown
8	4	−/ring	brown

Pin	Pair	Wire	Color
1	2	+/tip	white/orange
2	2	−/ring	orange
3	3	+/tip	white/green
4	1	+/ring	blue
5	1	-/tip	white/blue
6	3	−/ring	green
7	4	+/tip	white/brown
8	4	−/ring	brown

100BASE-TX

100BASE-TX is the predominant form of Fast Ethernet, and runs over two pairs of wire inside a Category 5 or above cable. Cable distance between nodes can be up to. One pair is used for each direction, providing full-duplex operation at 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 four pairs and the performance requirements of 100BASE-TX do not exceed the capabilities of even the worst-performing pair, one typical cable can carry two 100BASE-TX links with a simple wiring adaptor on each end. Cabling is conventionally wired to one of ANSI/TIA-568's termination standards, T568A or T568B. 100BASE-TX uses pairs 2 and 3.
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 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 supports 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.

100BASE-T1

In 100BASE-T1 the data is transmitted over a single copper pair, 3 bits per symbol, each transmitted as code pair using PAM3. It supports full-duplex transmission. 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 Open Alliance BroadR-Reach before IEEE standardization.

100BASE-T2

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 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. 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. 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.

100BASE-T4

100BASE-T4 was an early implementation of Fast Ethernet. It required four twisted copper pairs of voice grade twisted pair, a lower-performing cable compared to Category 5 cable used by 100BASE-TX. Maximum distance was limited to 100 meters. One pair was reserved for transmit and one for receive, and the remaining two switched direction. The fact that three pairs were used to transmit in each direction made 100BASE-T4 inherently half-duplex. Using three cable pairs allowed it to reach while running at lower carrier frequencies, which allowed it to run on older cabling that many companies had recently installed for 10BASE-T networks.
A very unusual 8B6T code was used to convert 8 data bits into 6 base-3 digits. The two resulting 3-digit base-3 symbols were sent in parallel over three pairs using 3-level pulse-amplitude modulation.
100BASE-T4 was not widely adopted but some of the technology developed for it is used in 1000BASE-T. Very few hubs were released with 100BASE-T4 support. Some examples include the 3com 3C250-T4 Superstack II HUB 100, IBM 8225 Fast Ethernet Stackable Hub and Intel LinkBuilder FMS 100 T4. The same applies to network interface controllers. Bridging 100BASE-T4 with 100BASE-TX required additional network equipment.