, which is too high for attenuation and the crosstalk charac- teristics of twisted pairs. To make the signal spectrum suitable for different quality cables, the following coding schemes are specified: The 100BaseTX uses 4B5B line coding in which four bits are encoded into a five-bit symbol to the line and the spectrum lies below 125 MHz, requiring category five cable pairs. For timing 5-bit sym- bols are defined in such a way that there are always transitions on the line signal for receiver synchronization. The 100BaseT4 uses 8B6T line coding (8 bits encoded into a sym- bol containing 6 three-level pulses) and it divides data between three pairs in each direction to manage with voice-grade pairs. The 100BaseT2 uses PAM5 encoding (five-level pulses) to reduce the spectral width and make it suitable for voice-grade cable pairs. The segment length in all 100-Mbps networks is limited to a maxi- mum of 100m to ensure that round-trip timing specifications are met. The fast Ethernet standard also specifies optical fiber connections that allow longer distances than a twisted pair. Just as in the case of the 10BaseT system, we can extend or improve the performance of the 100BaseT network by using switches instead of repeaters (or hubs). Switches also allow full-duplex transmission in ports that use one of the full-duplex technologies shown in Table 6.2. The fast Ethernet specifications include a mechanism for autonegotia- tion of the medium speed. This makes it possible for vendors to provide multiple-speed Ethernet interfaces that can be installed and run 10 Mbps, 100 Mbps, or 1 Gbps automatically. With the help of switches that support multiple data rates, we can gradually update the network and increase the data rate only where it is required. As a next step, the Gigabit Ethernet that we introduce later in this chapter further increases the capacity of the Ether- net networks. It provides a smooth path to gradually increasing the perform- ance of Ethernet LANs. 6.5.9 Autonegotiation The Ethernet specifications include 