1. A method of processing an interactive service at a receiver, the method comprising: receiving uncompressed audio contents or uncompressed video contents from an external decoding unit; extracting an identifier of a frame from the received content; sending a request containing the identifier to a server; and receiving a trigger for the content from the server based on the request, wherein the trigger indicates the current time of the contents and references a particular interactive event in an application parameter table or signals that the event is to be executed now or at a specified future time, wherein the application parameter table includes information about at least one of applications.