In order to transport digital data over a medium (copper or fiber) there needs to be encoding technology. When talking about Ethernet you will often hear reference to the bit transmission speed in Mbps and the signal frequency in MHz, however, for a particular cable type or network technology the encoding method is more important than simply defining the signal frequency.
What is used to encode the data is determined by the bit transmission rate and the physical layer of the network. For example, Fast Ethernet uses the 4B/5B encoding method which groups bits into 4-bit nibbles and then applies a 5-bit code to each of these groups. This encoding method produces a signal frequency twice that of the underlying copper or fiber cable used for the transmission.
The underlying copper cable can be either twisted pair or coaxial cable. Twisted pair is the most commonly used in office networks. The physical layer of twisted pair cables uses Manchester encoding to convert the binary digits into electrical signals, the first of which is non-return-to-zero. This encoding system uses two different voltage levels for the binary digits; positive voltage is used to send a logic '1' and negative voltage for '0'.
Fiber optics are increasingly being used in business networks because they can provide lightning-fast speeds and more bandwidth than traditional copper cables. They are also immune to electromagnetic interference and can transmit data over longer distances without losing the integrity of the signal. This capacity for long-distance transmission makes them ideal for connecting servers and storage in large data centers.