et operates at 100 Mbps and is compatible with 10 Base-T standard Ethernet as it uses the same twisted-pair cabling and CSMA/CD method. The trade-off is with distance between repeaters, a maximum of 205 metres being achievable for 100 Base-T. Fast Ethernet can also use other types of wiring - 100 Base-TX, which is a higher-grade twisted-pair, or 100 Base-FX, which is a two strand fibre-optic cable. Faster speeds to 1 Gbps or 10 Gbps are also available. The PMD sub-layer is specified separately from the Ethernet standard, and for UTP cabling this is based on the Twisted Pair-Physical Medium Dependent (TP-PMD) specification developed by the ANSI X3T9.5 committee. The same frame formats and CSMA/CD technology are used in 100 Base-T as in standard 10 Base-T Ethernet, and the 10-fold increase in speed is achieved by increasing the clock speed from 10 MHz to 125 MHz, and reducing the interval between transmitted frames, known as the Inter-Packet Gap (IPG), from 9.6 us to 0.96 us. A 125 MHz clock speed is required to deliver a 100 Mbps effective data rate because of the 4B/5B encoding described below. Input bit stream 4B/5B encoding 4-bit 5-bit 4-bit 5-bit nibble symbol nibble symbol 11110 10010 01001 10011 Feedback Shift Register 10100 10110 10101 10111 01010 11010 01011 11011 01110 11100 MLT-3 coding Output 01111 11101 Figure 2-7: 100 Base-T Ethernet Data Encoding Scheme Chapter Two To overcome the inherent low-pass nature of the UTP physical medium, and to ensure that the level of RF emissions above 30 MHz comply with FCC regulations, the 100 Base-T data encoding scheme was designed to bring the peak power in the transmitted data signal down to 31.25 MHz (close to the FCC limit) and to reduce the power in high frequency harmonics at 62.5 MHz, 125 MHz and above. 4B/5B encoding is the first step in the encoding scheme (Figure 2-7). Each 4-bit nibble of input data has a 5th bit added to ensure there are sufficient transitions in the transmitted bit stream to allow the receiver to synchronise for reliable decoding. In th