AIN is the acronym for Advanced Intelligent Network. It is the SS7 TCAP service that allows custom applications to be developed that use the SS7 network. Any of the other TCAP services could be duplicated with this, or you could create your own way of using it. The total subjectivity of its use makes it impossible to give a description of how it operates. The specifics depend solely on how the service in question was designed to work.