1.3. ISDN Connections

ISDN messages can be thought of as a digital equivalent to the analog signaling used to communicate status and connection information across an analog network. Establishing ISDN connections can be related to establishing analog connections as described in Table 1:

Table 1. ISDN vs. Analog Connections 

Step	ISDN Connection	Analog Connection
	The calling party decides to make a call. (See Note below.)	The calling party goes "off-hook."
	The calling party sends digital address information to the local Central Office (CO). Note: Steps 1 and 2 are the equivalent of the ISDN setup message.	The calling party "dials" the called party's phone number.
	The CO accepts the digital address and interconnects local and long-distance circuits, on demand, to reach the called party.	The CO receives the dialed digits and attempts to connect to the called party.
	The called party receives this address information and responds by sending the calling party an Alerting or Progress message.	The calling party receives either "ringback" or "busy" signal.
	If the called party accepts the call, a Connect message is sent to the calling party and the parties are connected.	The called party "goes off-hook" to answer the call and the parties are connected.

Many ISDN calls are digital from end-to-end, but a majority are still analog at the ends of the connections. That is, one end or the other connects to a Plain Old analog Telephone Service (POTS). In addition, the call may be routed over both digital and analog links. In these cases, in-band signaling techniques can be used in addition to ISDN signaling so that an application can obtain good feedback from the network regardless of the type of intermediate connections.

Call progress using audio tones is generally not used for digital protocols. The called party's condition is reported using signaling instead of call progress tones. However, call progress tone detection is desirable for digital circuits for protocols that do not have the capability to report call progress using signaling and when the connection traverses analog lines. For example:

• When a CO is in the telephone path and it cannot transmit the called party's condition, the busy tone is the only way to recognize a busy condition.
• For telephone circuits that include analog links, the local line may not have access to all of the digital signaling information.

To use call progress in this manner, use the call progress feature in the voice library after issuing the gc_MakeCall( ) function. See also Section 1.5, Configuration and Resource Association.

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