Physical Media

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