Gigabit Ethernet utilises five levels and 8b/10b encoding, to provide more efficient use of limited cable bandwidth, sending 1 Gbps within approx 100-125 MHz of bandwidth (i.e. the capacity of a UTP Cat5e cable). In comparison, 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. 8b/10b encoding assigns each byte of data to a 10 bit code. The byte is split up into the 3 most significant bits and the 5 least significant bits. It is then represented as two decimal numbers with the least significant bits first e.g. for the octet 101 00110 the result is the decimal 6.5. 10 bits are used to create this code and the naming convention follows the format /D6.5/. There are also 12 special codes which follow the naming convention /Kx.y/.
The 10 bit code must contain either five ones and five zeros, or four ones and six zeros, or six ones and four zeros. This prevents a sequence of too many consecutive ones and zeros, assisting clock synchronisation. Two 'commas' are used to aid in bit synchronisation, these 'commas' are the 7 bit patterns 0011111 (+comma) and 1100000 (-comma).
To maintain a DC balance, a calculation called the Running Disparity calculation is used to keep the number of '0's transmitted the same as the number of '1's transmitted.
The interface encodes the 10 bit code after it is scrambled and converts it into a physical layer signal by mapping pairs of bits 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}.
A 1Gbps the line speed results in a transmission rate of 10/8 x 1 = 1.25Gbps. In Gigabit Ethernet this rate is then reduced using PAM-5 a 5-level code (achieving less bandwidth than possible with a 3 level code).
Information about Unshielded Twisted Pair (UTP) cabling