Baset Ethernet

The 100BASE-T Ethernet LAN is also known as Fast Ethenet. As indicated by the designation, 100BASE-T Ethernet operates at a speed of 100 Mbps using twisted-pair wire. The computers are connected to a hub or a switch in a star topology, and the distance of the twisted pairs is limited to 100 meters. Operating 100 Mbps on UTP is challenging, and so three options for doing so were developed, one for category 3 UTP, one for shielded twisted pair, and one for category 5 UTP. One problem with extending the 10BASE-T transmission format is that Manchester line coding is inefficient in its use of bandwidth. Recall from the section on line coding that Manchester coding pulses vary at twice the information rate, so the use of Manchester coding would have required operation at 200 Mpulses/second. Another problem is that higher pulse rates result in more electromagnetic interference. For this reason, new and more efficient line codes were used in the new standards.

In the 100BASE-T4 format, four category 3 twisted-pair wires are used. At any given time three pairs are used to jointly provide 100 Mbps in a given direction; that is, each pair provides 33 1/3 Mbps. The fourth pair is used for collision detection. The transmission uses ternary signaling in which the transmitted pulses can take on three levels, +A, 0, or —A. The line code maps a group of eight bits into a corresponding group of six ternary symbols that are transmitted over the three parallel channels over two pulse intervals, or equivalently four bits into three ternary symbols/pulse interval. This mapping is possible because 24 = 16 < 33 = 27. The transmitter on each pair sends 25 Mpulses/ second, which gives a bit rate of 25 Mp/s x 4 bits/3 pulses = 33 1/3 Mbps as required. As an option, four category 5 twisted pairs can be used instead category 3 twisted pairs.

In the 100BASE-TX format, two category 5 twisted pairs are used to connect to the hub. Transmission is full duplex with each pair transmitting in one of the directions at a pulse rate of 125 Mpulses/second. The line code used takes a group of four bits and maps it into five binary pulses, giving a bit rate of 125

Mpulses/second x 4 bits/pulse = 100 Mbps. An option allows two pairs of shielded twisted wire to be used instead of the category 5 pairs.

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