With the development of mobile communication technology and the evolution of communication networks, a variety of mobile communication systems have been developed. In order to solve the problem of global roaming between the mobile communication systems, the International Mobile Telecommunication (IMT)-2000 systems have been developed. The IMT-2000 systems are classified into a Code Division Multiple Access (CDMA) 2000-based synchronous system and a Wideband Code Division Multiple Access (WCDMA)-based asynchronous system.
Furthermore, in order to support global roaming between the mobile communication systems, a mobile communication terminal capable of being used for both a synchronous type system and an asynchronous type system, that is, a Dual Band Dual Mode (DBDM) mobile communication terminal, has been developed. Using the DBDM mobile communication terminal, different types of services can be used in an asynchronous system area and a synchronous system area.
Currently, the asynchronous mobile communication network is being constructed in regions having high demand for service and, therefore, the synchronous mobile communication network is evolving such that the service area thereof includes the service areas of an asynchronous system. In such a situation, handover between the systems is necessary so as to provide continuous service in the case where a user moves between the asynchronous mobile communication system and the synchronous mobile communication system.
Conventionally, at the time of handover between the asynchronous and synchronous mobile communication systems, handover is performed in such a manner that communication with a mobile communication system with which a connection exists is interrupted and connection with a different mobile communication system is made. This case is problematic in that a considerable handover time (several hundred msec) is required, resulting in an unacceptably long interruption, therefore the quality of service is degraded.