ISDN is a type of digital telephone connection that allows voice, data, and video to be transmitted over standard telephone lines at higher speeds than traditional analog connections. It was widely used by businesses because it could transfer data files, including high-resolution images, at roughly twice the speed of regular phone lines.
For photographers and studios, ISDN was historically valuable for sending large image files quickly to clients, printers, or media outlets. While modern broadband and fiber-optic connections have largely replaced ISDN, understanding the technology is useful for historical context and for managing legacy workflows in professional photography or digital media.