Always On/Dynamic ISDN (AO/DI) is a service where the ISDN subscriber is always online and can have a higher bandwidth if required. The AO/DI technology uses the D channel of
ISDN for packet-switched transmission. The data rate of the D channel of 16 kbit/s is limited to only 9.6 kbit/s by the protocols
used, such as the Bandwidth Allocation Protocol (BAP), X.25 or Q.931. The user is constantly online via the packet-switched D channel. If the data rate of the D-channel is not sufficient, the two B-channels
can be switched on if necessary with Dynamic ISDN (DI) for a broaderband transmission. TheAO/DI concept was developed by the Vendors ISDN Association (VIA
) and can be operated on existing technologies without hardware changes in the network infrastructure. The AO/DI technology is characterized by high availability and cost-effectiveness and offers the user always-on operation to the Internet. Network operators and providers offer this service with flat rates.