What kind of network does QSIG primarily support?

QSIG is a signaling protocol primarily designed to enable communication between private branch exchange (PBX) systems. It facilitates the interconnection of voice and data communication in a telephony environment, specifically among different manufacturers' PBXs. This protocol allows for features such as call transfer, call forwarding, and conference calls, ensuring seamless interaction across different systems.

The selection of the private branch exchange network as the correct answer emphasizes the role of QSIG in supporting internal business telephone networks, rather than public communication systems or data networks. Other types of networks, such as public switched telephone networks, cellular mobile networks, or data communications networks, utilize different signaling protocols and do not directly implement the specific functionalities that QSIG aims to provide within a PBX environment.