(Always On/Dynamic ISDN) A connection between an ISDN customer and an information service provider using the X.25-based D channel, which is a signaling channel that is always active. AO/DI uses 9.6 Kbps of the 16 Kbps D channel and switches to a circuit-switched B channel when more bandwidth is required. It enables low-bandwidth traffic, such as e-mail, newsfeeds, automatic data collection and credit card verifications, to be handled quickly and cost effectively. See ISDN.