How works the system of frequency for aircraft commercial Airline Hello  I have to present a weather radar technology but i m block on  Air convention of communication.

I need to know how they exactly attribute licence to an aircraft and how all this aircraft can communicate without create collision between their communication .

Question one : Who attribute frequency for an Aicraft and how ? 
- How are they attribute telecommunication frequency for commercial Aircraft .Each time they do a new fly someone give them a new frequency band ? They have a frequency band  for a specif numbers of fly ( E.g after 10 fly ) ? 


Which authority give frequency band each countries ? but that mean if it's not a international authority you can use the frequency band only in this countries 
Which authority decide into international zone what frequency band do you use ? 


Question two : To much aircraft for number of channel
You have more than 20 000 aircraft use in the world however  I read you have 118–136.975 MHz Airband and only  potentially permitting 2,280 channels
so that mean a channel is use by more than one aircraft so what happen if two commercial aircraft have to use the same range of frequencies how their is not interference ?

My source : 
 (https://en.wikipedia.org/wiki/Airband ) 
http://www.caa.co.uk/Commercial-industry/Airspace/Communication-navigation-and-surveillance/Radio-licensing/Radio-licence-categories/

Thanks you in advance for your supports.
Best regards,
CANAL Bastien
I think you have a misunderstanding on how individual radio frequencies are used.  

Individual radio frequencies are not assigned to aircraft.  They are assigned to Air Traffic Control facilities like Control Towers or Area Control centers.  

Each aircraft will have it's own "radio licence" but they can tune in any frequency that is needed for both transmitting and receiving. All aircraft in a specific area will use the same frequency as assigned by ATC. You may have 20 or 30 aircraft on the same frequency, but only one aircraft should be transmitting at any time. 

There is a lot of congestion on the radios in a busy sector and "CPDLC" (Controller–pilot data link communications) is gradually being introduced in many areas. This allows ATC and the pilot to communicate directly via data-link text messages. 

In some areas where there is no ATC control, aircraft can monitor or communicate on a common "air to air" frequency.  123.45 is used in some areas.