Many online sites are there, but you have to look for one where you can also learn the Pre Basics of GDS before learning the Amadeus GDS so that you can understand the Travel Basics then Amadeus GDS.
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Travel Online Classes is a platform where you can learn already existing courses, but you can also book an individual live session with the trainer to learn how to start your travel career, travel agency, travel business, hotel business, car rental business, tourism business, etc