Communication channel is used to establish communication between a $TX^r$ and a $RX^r$
Wired communication : communication takes place with help of wires or cables, all types of signals can be sent, i.e. analog or digital. s/gs are in the form of electrical voltages. wired CN could be categorized as:
Co- axial cables.
Twisted pair wire.
The cables have electrical cores such as copper. fiber optic communication is also a type of wired CN. The CN s/g is in the form of light pulses, the core is made up of glass rather than metal. Higher data rate and larger repeater distance is possible with optical fibers.
Wireless channel : wireless communication takes place through air or atmosphere, it is also referred as radio communication. The transmitter and the receiver is an antenna that converts the electrical s/g from electro magnetic waves. It could be further classified as
Ground wave - Terrestrial CN
Sky wave - Long distance CN
Space wave - Very high CN
Satellite CN, mobile CN, TV, Radio, etc. all require wireless CN.
Space wave propagation : Space wave propagation is a type of radio wave propagation having very high frequencies. These waves have the ability to propagate through atmosphere from $RX^r$ to $RX^r$
They can travel directly or can travel after reflecting from earth surface to the troposphere surface of earth. Basically the technique of space wave propagation is used in bands having very high frequency.
Example : VHF & UHF, etc
Space wave propagation is also termed as line-of-sight propagation.
space wave propagation has few drawbacks which are listed as below:
1] Only LOS propagation is possible.
2] Limited by curvature of earth.