In order to define what is meant by a physical medium let us reflect on the brief life of a bit. Consider a bit travelling from one end system through a series of links and router to another end systems. The source end system first transmits the bit and shortly thereafter the first router in the series receive the bit the first router then transmits the bit and shortly thereafter a second router the bit and so on. Thus the bit when travelling from sources to destination passes through a series of transmitter receiver pairs. For each transmitter receiver pair the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium.
Physical media can be divided in to two categories: Guided and unguided media guided media include twisted pair cable coaxial cable and fiber optic cable. Unguided media is free space. Unguided media, which are those that provide a conduit from one device to another include twisted pair cable coaxial cable and fiber poetic cable. A signal travelling along any of these media is directed and contained by the physical limits of the medium.
Unguided Media Transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free and thus available to anyone who has a device capable of receiving them.
figure classes of transmission media