What is ISDN?
What is Always On / Dynamic ISDN (AO/DI)?
AO/DI is only available if you have subscribed to it, your ISDN service provider supports it, and you are using Multilink PPP. AO/DI uses the ISDN D-channel X-25 packet service to maintain an “always on” connection between you and your Service Provider. Low bandwidth requirements can be met using this constant connection, such as sending and receiving e-mail, credit card verification, etc.
If additional bandwidth is required (to download a large file, for example), AO/DI, in conjunction with Bandwidth on Demand (BOD), automatically adds B-channels of 64 Kbps each, for a total of 128 Kbps when both B-channels are in use. When the additional bandwidth is no longer required, one or both B-channels are dropped, leaving the D-channel connection in place.
AO/DI enhances ISDN use in a number of ways:
•AO/DI can quickly and automatically adjust to the voice and data needs of end-users. For example, if both B-channels are bonded in a data connection, and an incoming call is received, one of the B-channels is automatically freed up for the telephone call. If a fax call also comes in, the remaining B-channel may be allocated to the fax call. The data connection continues on the D-channel until a B-channel is available, if needed.
•As B-channels are invoked only when additional bandwidth is needed, connection costs are significantly reduced.
DIVA T/A ISDN Modem Reference Guide ◆ Page 12