CAPI Listener Utility


This tool analyzes message traffic of ISDN controllers with drivers conforming to the CAPI specification. The utility is an ISDN study tool. It lays the foundation for development of telematic resources, which the author produces in order to promote networked music education.

About the CAPI

The Common ISDN API (CAPI) is a standard application programming interface used to access ISDN equipment, without having to adjust to the variety of different hardware implementations. The CAPI specification is maintained and developed by a group consisting of many of the most important companies and organizations active in the ISDN field: The CAPI Association.

Specifications for basic call control

Most of the CAPI messages are accompanied by parameter structures which contain elaborate information on the basic call control functions. Many of them become meaningful only through studies in the ISDN, User-network interface layer 3 standard, which specifies the procedures for the establishing, maintaining and clearing of network connections. The pan-European version of this material, the ETS 300 102-1 is available at the Publications Download Area of the ETSI (European Telecommunications Standards Institute).


The application monitors ISDN communication by reading messages submitted by the installed CAPI driver. The program currently focuses on the mechanisms of basic call control. The CAPI Listener analyzes the controller, physical and logical connection IDs, error info, compatibility information, number and subaddress of both the calling and called party. It also describes the contents of the most common ISDN messages and information elements. In addition, CAPI version and controller capability information can be browsed.

In a multi-controller system the application may monitor any of the available controllers. Additional controllers may be listened to by running multiple instances of the CAPI Listener. The user may also filter the message stream by selecting relevant signalling protocol events, bearer capabilities and incoming subscriber numbers.

Response patterns

The output may be studied in different settings, which are governed by the response pattern selection. Currently three processes may be viewed:

Please note, that audio responses are generated only upon audio, i.e. non-data calls made to anyone of the selected multiple subscriber numbers (MSNs).

Additional functionality

The program is equipped with standard text editing, find, font selection, printing and email capabilities enabled by a slightly tuned MFC template.


Sample view of the application

Sample view of CAPI Listener