1.1. ISDN Features and Benefits

The Integrated Services Digital Network (ISDN) is a digital communications network capable of carrying all forms (voice, computer and facsimile) of digitized data between switched end points. This network is a digital-switched system that makes a connection only when requested.

Control over switched connections is provided by a protocol of messages that pass between the two ends of the digital link. Any type of equipment can be connected to an ISDN, provided the equipment is capable of generating a digital bit stream that conforms to ISDN standards.

ISDN technology offers the benefits inherent in digital connectivity such as fast connection (setup and tear-down), fast Direct Dialing In service (DDI), and fast Automatic Number Identification (ANI) acquisition. In addition, ISDN Primary Rate Interface (PRI) applications can take advantage of the following features, if offered by the network (see Appendix B, "ISDN Call Scenarios", for details):

• Two B Channel Transfer (TBCT) - Enables a user to request the switch to connect together two independent calls on the user's interface. The user who made the request is released from the calls and the other two users are directly connected. This feature is supported for the 5ESS and 4ESS protocols; see Appendix B, "ISDN Call Scenarios" for details. The feature is also supported by the Q.SIG protocol.
• Non-Call Associated Signaling (NCAS) - Allows users to communicate via user-to-user signaling without setting up a circuit-switched connection (this signaling does not occupy B channel bandwidth). A temporary signaling connection is established (and cleared) in a manner similar to the control of a circuit-switched connection. This feature is supported for the 5ESS protocol. For details, see Appendix B, "ISDN Call Scenarios".
• Vari-A-Bill - A flexible billing option enabling a customer to modify the charge for a call while the call is in a stable state (for example, between answer and disconnect). This feature is available from the AT&T network only.
• ANI-on-demand (AT&T only) - Allows the user to request a caller ID number to identify the origin of the call, when necessary.
• NFAS - Non-Facility Associated Signaling that provides support for multiple ISDN spans from a single D channel. See the Release Catalog for your operating system for the products that support the NFAS D channel.
• DDI (Direct Dialing In) - DDI service, also called Dialed Number Identification Service (DNIS), allows an outside caller to dial an extension within a company without requiring an operator's assistance to transfer the call.
• User-to-user information - An information element (IE) that may be included in setup, connect, or disconnect messages.
• Call-by-Call service selection - This feature allows the user to access different services, such as an 800 line or a WATS line, on a per call basis.
• LAP-D Layer 2 access - Known as the data link layer, this feature provides reliable transfer of data across the physical link and sends blocks of frames with the necessary synchronization, error control, and flow control.

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