This concise paper reports the result of a study leading to the design for a subscriber's loop signaling system in a data switching network. The data switching network is intended to have improved quality and versatility, and be less expensive than the existing data communication systems. Therefore, various requirements are imposed on the subscriber's loop signaling system. The three-level subscriber's loop signaling system described in this concise paper is considered to satisfy these requirements. In this system, signals are defined by three logical states in the subscriber's loop, i.e., +, -, andN. This system is applicable to both asynchronous and synchronous terminals. The data terminal equipment-data circuit terminating equipment (DTEDCE) interface and the subscriber's loop transmission system, suitable for this signaling system, are also described.