"Local Area Network" or LAN is a term meaning "private network." The dominant set of standards for the physical properties of LANs is called Ethernet. The Ethernet standards are published by the Institute of Electrical and Electronics Engineers and periodically updated to create better performing networks. The "100" LAN refers to one of these standards.
Ethernet was originally a proprietary networking system owned by Xerox. The early Xerox standards recommended the use of coaxial cable. In 1983, the responsibility for managing the standards was handed over to the IEEE, and Ethernet became an open standard. An open standard is available to all, either free of charge, or for a subscription fee. The IEEE has since produced a number of amendments to the Ethernet standards; each carries the code 802.3, followed by one or two letters to indicate a series.
Although the IEEE uses the 802.3 code for all its Ethernet standards, the complete LAN systems it defines are given a different code. This naming system has three elements. The first is the data throughput speed. Originally, this was expressed in megabits per second, but later systems are given a code based on gigabits per second. The next part of the name gives its transmission method, which is either baseband or broadband. The final part is a code for the cable type of the network.
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The 100 Megabits per second version of Ethernet was released by the IEEE in 1995 with the publication of 802.3u. The standard defined three different cabling systems, each achieving the same performance levels. These networks types are called 100BASE-T4 and 100BASE-TX (which together are known as 100BASE-T) and 100BASE-FX. The "T" in the first two standards refers to twisted pair cable. The "F" of 100BASE-FX indicates that the standard used two strands of multi-mode fiber optic cable.
Twisted pair cable for networking comes in two forms: Unshielded Twisted Pair, or UTP, and Shielded Twisted Pair, or STP. Of the two, UTP is more widely implemented. Both types of cable contain eight wires configured as pairs, with the two wires of each pair twisted around each other. The twisting forms a protection against magnetic interference, making shielding unnecessary, although STP has additional shielding. Usually, only four of the eight wires inside the cable are used. UTP cable is categorized into grades and higher grades have better capabilities. The 100BASE-TX uses Cat-5 UTP cable, but that can replaced by the equivalent STP cable. The 100BASE-T4 allows for the employment of lower-grade UTP cable, which are called "Cat-3" and "Cat-4." These implementations use all the wires inside the cable, not just two pairs.