What does ISDN define?

ISDN, or Integrated Services Digital Network, is primarily concerned with standards for digital transmission over traditional telephone networks. It allows for simultaneous transmission of voice, video, and data over the same digital line, providing a more efficient and higher-quality communication method compared to analog systems. This capability is specifically reflected in the two-way data transmission aspect, which enables multiple types of communication services to be used concurrently, enhancing the overall utility of telecommunication systems.

The focus of ISDN is on establishing a digital connection to facilitate these diverse forms of communication, making it integral to the modern digital landscape, particularly in applications requiring robust, high-quality connections for both voice and data services. The other options do not accurately describe ISDN's primary role or function, as they pertain to different areas of technology and communication.