Use
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to search your arriving international flight to check to see if the flight has regular arrival delays. Suvarnabhumi only has one terminal and two sections - international and domestic - both are connected to the main terminal with domestic check in and security to the left of the terminal (from the street entrance). However you will need to clear customs and immigration and collect any hold luggage, before then checking into a domestic flight. Most domestic flights have to be checked in 45 minutes before departure and hold luggage will need to be checked. Domestic security is normally quite quick and easy. All domestic airlines fly from the same domestic section of the airport, there is no advantage of one airline over another, although Bangkok Airways does have a very nice free airport lounge with free food and drink that everyone flying with them can use, so if you give yourself more time you can then use that lounge for free food and drink and relaxing. Thai Smile and Bangkok Airways also include 20kg of hold luggage in the ticket price, and Air Asia ticket prices do not work out much difference in price after adding hold luggage.