5.3. Function References: CRNs, CRVs, and Line Device Handles

Most functions ask for the line device handle and/or the call reference number (CRN), which together enable applications to be written independent of the hardware type or signaling protocol.

The line device handle is a unique logical number assigned to a specific device or device group by the ISDN API. The line device handle enables the API function to address any system resource using a single device identifier. The system architecture also permits more than one device to be addressed as a unit, as needed to process a call.

A call reference number (CRN) identifies a call on a specific line device. The CRN is created by the ISDN API library when a call is requested either by the application or the network. The relationship between the CRN and a line device is established when a call is requested and acknowledged by the other end. The valid lifespan of the CRN is the duration of the call. Afterward, the CRN can be reassigned.

A call reference value (CRV), which conforms to the Q.931 standard, is a network-assigned value that is used to identify a call on a specific line device. The CRV is transmitted over the network and maintained by the ISDN firmware. The ISDN firmware maintains a table to match the host-assigned CRN and the network-associated CRV. Use the cc_GetCRN( ) function to obtain the CRN. The CRV for a particular CRN can be obtained by using the cc_GetNetCRV( ) function.

The following list summarizes the use and assignment of CRNs and CRVs and offers some additional points to keep in mind when using Dialogic ISDN library functions:

• Each CRN is a unique number in the system.
• A call is not associated with a physical port.

New CRNs and CRVs are created either when CCEV_OFFERED occurs or when cc_MakeCall( ) successfully sends a setup message to the network.

• The CRN/CRV is no longer valid after cc_ReleaseCall( ), cc_ReleaseCallEx( ), or cc_Restart( ) is issued.
• After a CRN/CRV of a given value is released, it may be reassigned for subsequent calls.

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