The Unshielded Twisted Pair (UTP) cable provides a low cost Ethernet technology, that supports 10BT. The UTP cabling system uses a RJ-45 connector and 100 Ohm unshielded twisted pair cabling. This connects the computer directly (i.e. using a point to point link) to a wiring hub which acts as a media repeater. The maximum distance of a 10BT link is 100 m.
100BASE-T uses the same cabling to provide 100 Mbit/s Ethernet in either a half-duplex (using CSMA/CD) or full-duplex form (switched). 100BASE-TX runs over two pairs of wires in Category 5 unshielded twisted pair cable. Like 10BASE-T, the normal pairs are coloured orange and green pairs (using pins 1, 2, 3 and 6 of the RJ-45 connector). This cable has a maximum bandwidth of 100-125 MHz. A Manchester encoded waveform would require 200-400 MHz of bandwidth, far in excess of that offered by this type of cable.
A scheme using 4B5B binary encoding therefore generates a series of 0 and 1 bits clocked at 125 MHz; the 4B5B encoding provides DC equalisation and spectrum shaping. 4B5B encoding works by mapping each group of four bits (one 1/2 of a byte) to one group of 5 bits. Since there are (2^5) 32 possible combinations of 5 bits, and there are only (2^4) 16 combinations of 4 bits one half the patterns are unused. The chosen set of 16 5-bit patterns are those with the most transitions, this ensures clocking information is present in the signal (for locking the receiver DPLL). This results in a bandwidth increased of 25%. For consistency with previous versions of Ethernet, the least significant 4 bits of each byte is sent first.
Cross-Talk requirements (i.e. to limit RF Emission) led to the need for a scrambler, this randomises the bit stream to prevent the same set of byte values generating a completely repetitive pattern, which would have stong signal components at some characteristic frequencies.
The data is finally sent as a 3-level physical waveform known as MLT-3. MLT-3 cycles through a set of voltage levels {-1, 0, +1}, to indicate a 1-bit. The signal stays the same when transmitting a 0 bit. It takes four 1 bits to generate a complete cycle, this the maximum fundamental frequency is reduced to one fourth of the baud rate.
This scheme of 45/5b with MLT-3 encoding leads to a waveform of 31.25 MHz, well within the specification for Unshielded Twisted Pair Cabling.
Information about Unshielded Twisted Pair (UTP) cabling