Fast Ethernet (100BASE SFP) Defined

By our current industry standards, all Fast Ethernet (100BASE SFP) physical layers carry traffic at a set nominal rate of 100 Mbit/second. The previous Ethernet speed was for 10 Mbit/second. Out of the Fast Ethernet physical layers, 100BASE-TX is by far the most common.

First introduced in 1995 as the IEEE 802.3u standard, the Fast Ethernet (100BASE SFP) is was the fastest version of Ethernet for 3 years before the eventual introduction of Gigabit Ethernet. The industry acronym “GE/FE” is occasionally used for devices that support both of these established standards.

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100BASE-T Defined

100BASE-T is defined as any of the several Fast Ethernet standards established for twisted pair cables, including: 100BASE-TX (100 Mbit/second over a 2-pair Cat5 or better cable), 100BASE-T4 (100 Mbit/second over 4-pair Cat3 or better cable, defunct), 100BASE-T2 (100 Mbit/second over 2-pair Cat3 or better cable, also defunct). The segment length for a 100BASE-T cable is limited to 100 metres (328 feet) (the same limit as 10BASE-T and gigabit Ethernet). All are, or were of standards under IEEE 802.3 (approved 1995). Close to all 100BASE-T installations are in 100BASE-TX.

100BASE-TX Defined

The 100BASE-TX is the predominant form of Fast Ethernet, and runs over 2 wire-pairs inside a category 5 or above cable set up. For each network segment, it can have a 100 meters (328 feet) maximum cable distance. One pair is used for each direction, which provides full-duplex operation with 100 Mbit/second of throughput for each given direction.

Like the 10BASE-T, the active pairs in a standard connection are terminated on pins 1, 2, 3 and 6. Since a typical category 5 cable will have 4 pairs, it can support a capacity of two 100BASE-TX links with a wiring adaptor. Cabling is usually conventionally wired to the TIA/EIA-568-B termination standard, T568A or T568B. This will place the active pairs on the orange and green pairs (ie: the canonical second and third pairs).

In terms of configuration, the 100BASE-TX networks are much like the 10BASE-T. When used in building a local area network, the devices on the network (such as the computers and printers) are typically connected to a hub or switch, which creates a star network. On the other hand, it is possible to connect 2 devices directly using a crossover cable. With present-day equipment standards, crossover cables are usually not needed, as most equipment supports auto-negotiation along with auto MDI-X to select and the match speed, duplex and pairing set up.

With 100BASE-TX hardware, the raw bits, presented in 4 bits wide clocked at 25 MHz at the MII, go through 4B5B the binary encoding process to generate a series of 0 and 1 symbols clocked at a 125 MHz symbol rate. The 4B5B encoding will provide DC equalization and spectrum shaping. As in the 100BASE-FX case, the bits are then transferred to the physical medium attachment layer through NRZI encoding. However, 100BASE-TX introduces an additional, medium dependent sublayer, which employs MLT-3 as a final encoding of the data stream before its transmission. This results in a maximum fundamental frequency of 31.25 MHz. This procedure is referenced from the ANSI X3.263 FDDI specifications, albeit with minor changes.