The original 10 Mbps Ethernet used Manchester encoding, while Fast Ethernet (at 100 Mbps) used a 4b/5b MLT code. Gigabit Ethernet utilises five levels and 8b/10b encoding, that is scrambled and converted into a physical layer signal to provide even more efficient encoding of the clock information.
Gigabit Ethernet over twisted pair cabling (1000BTX) faces particular problems with the limited cable bandwidth, sending 1 Gbps within approx 100-125 MHz of bandwidth (i.e. the capacity of a UTP Cat5e cable).
The interface converts the data to a physical layer signal using a 5-level Pulse-Amplitude-Modulation (PAM). This sends a group of bits as one of five signal levels, i.e.one of ({+2, +1, 0, -1, -2}
Information about Unshielded Twisted Pair (UTP) cabling