The physical layer is perhaps the easiest layer to grasp and involves the actual physical medium used to transfer the messages.
It is the most basic network layer and provides the means for transferring the raw bits of the data using a physical signal. The actual signal used will vary for different types of mediums such as electrical modulation, radio waves or optical signals.
In theory, any medium can be used at the physical layer - RFC 1149 even purported to be on the "Standard for the transmission of IP datagrams on avian carriers" (carrier pigeons!) - not used needless to say!
Some Physical Mediums Used
Next: Legacy Protocols