In a communication system, an upper-level carrier has communication functions for both first and second sessions. A first communication unit communicates with the upper-level carrier through a first communication session established therewith. A second communication unit communicates with the upper-level carrier through a second communication session established therewith. A communication control unit controls the first and second communication units. The communication control unit disconnects the first communication session and directs a signal intended for the first communication session to the second communication session, so that the signal is delivered to the upper-level carrier by using the second communication session as a detour.