Originally, Ethernet was used with coaxial cable. Today, for 10BaseT and 100BaseT Ethernet, Category 5 Unshielded Twisted Pair (UTP) cabling is most commonly used. This is commonly referred to as Cat 5 UTP, or simply Cat5. Cat5 consists of 4 twisted pairs, or 8 wires. The eight pins of an RJ-45 connector are used at the ends of the cabling.
For 10BaseT, pins 1 and 2 are used to transmit and pins 3 and 6
are used to receive. The other four pins are not used. When
connecting an Ethernet NIC in a computer to a network hub,
switch, or other network device, the cable should be wired
straight through. In the case of a computer to computer
connection, or network device to network device connection,
the cable should be "rolled" as shown below:
Ethernet uses Manchester signaling, where a logic 0 is represented by a downward transition in voltage, and a logic 1 is represented by an upward transition in voltage, each at the middle of the clock period.
Below is an example of Manchester signaling:
Manchester signaling has the advantage of transitioning every bit, which helps maintain a clock signal at all time. Additionally, in the first 7 bytes of every Ethernet frame, an alternating pattern of 1's and 0's is transmitted. For 10Mbps Ethernet (which uses a 20 MHz clock), this produces a 10 MHz square wave that is used to syncronize the transmitter and receiver.