Transmitted within the data stream. See also out of band . ISDN Integrated Services Digital Network , a set of standards for a new class of telephone services. For Telephony, ISDN's major significance is its ability to provide multiple channels on a single line. A switching system in which the phones have multiple buttons that permit the user to directly select incoming or outgoing lines.
Key systems can typically support fewer users than PBXs, and their features are more limited.
A computer may provide its applications with access to multiple lines, and each line may provide different capabilities. It is the choice of the service provider for a line to decide how to model its resources. Note that a line need not correspond to a physical connection from the switch to the computer. For example, the realization of a line may involve a LAN-based server and a shared control link to the switch. In essence, a line is any device that implements the line behavior defined by TAPI and by the SPI as the set of functions and messages for lines.