An ISDN basic rate interface (BRI) service that maintains an always-on connection to an ISP server, or perhaps a corporate intranet server, using only the D channel. The D channel maintains the always on logical link between the client and the server systems, enabling the transfer of packet data, such as e-mail, at rates of up to 9.6 kbps over an X.25 switched virtual circuit (SVC).The always-on nature of the connection avoids the call set-up time required for a circuit-switched connection. From the end user terminal adapter (TA) to the central office (CO), the multilink point-topoint protocol (MPPP) is employed over the D channel.As the signaling and control requirements of BRI are relatively light, there is sufficient capacity on the D channel to reliably support AODI and the packet data transfers it occasions, as long as they do not exceed 9.6 kbps. See also always on, BRI, circuit switching, client, CO, D channel, intranet, ISDN, ISP, MPPP, server, SVC, TA, and X.25.