The invention relates to a solution for determining an allowable round trip time for a communication between a base station and a terminal device served by the base station in an asynchronous communication system, At least some aspects of the solution relate to a method performed by a controller, the method comprises: determining round trip times of terminal devices served by the base station; selecting a maximum round trip time among the determined round trip times of the terminal devices served by the base station as the allowable round trip time; and delivering the selected allowable round trip time to the base station. The solution also relates to applying the determined round trip time by a base station and a terminal device as well as to a system comprising the mentioned entities and to computer program products.