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Chapter 6. Ethernet > 100BaseTX

6.4. 100BaseTX

100BaseTX and 100BaseFX form the subgroup of 100BaseX, which is a part of the 100-Mbps Ethernet group of 100BaseT. Physical layers of both 100BaseTX, which uses two pairs of Category 5 unshielded twisted pair cables, and 100BaseFX, which uses two multimode fibers, are borrowed from existing FDDI standards to save the technology development time. Basics of physical layer 100BaseTX are defined in ANSI X3.263, FDDI Token Ring Twisted Pair Physical Layer, with modifications specified in Ethernet standards 100BaseX and 100BaseTX. The ANSI X3.263 standards define the MLT-3 line code, line voltages, and waveforms, as well as channel and crosstalk models. The 100BaseX standards specify the 4B5B encoding, while the 100BaseTX standards clarify transmit and receive pair connections on the RJ45 connector.

With the 4B5B encoding, every 4 bits of information are converted into a 5-bit code resulting in an effective bit rate of 125 Mbps over the transmission media. 100BaseTX uses a higher grade unshielded twisted pair cable of Category 5 instead of the Category 3 cable used by 10BaseT and 100BaseT4. The Category 5 cable is rated for transmission at frequencies up to 100 MHz, while the Category 3 cable is rated for only 16 MHz. In other words, Category 5 cable has less attenuation and crosstalk. On the other hand, signaling at a rate of 125 Mbps requires adaptive channel equalization to compensate for intersymbol interferences caused by amplitude and phase distortions of twisted pair cables of different lengths. The adaptive channel equalization for 100BaseTX transceiver can be implemented in either analog or digital circuits. The digital adaptive channel equalizer performs better but demands a high-speed Analog-to-Digital Convertor (ADC) and digital circuits.


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