CAPI (Common Application Programming Interface) is an international standard interface that application s can use to communicate directly with ISDN equipment. Using CAPI, an application program can be written to initiate and terminate phone calls in computers equipped for ISDN. Computer telephony ( CTI ) applications can be written for ISDN users. Officially, CAPI is referred to as Common-ISDN-API and is embodied in ETS 300 838 ("Integrated Service Digital Network (SDN); Harmonized Programmable Communication Interface (HPCI) for ISDN." ETS refers to standards from the European Telecommunication Standards Institute (ETSI). The standard is internationalized by recommendation T.200 "Programmable communication interface for terminal equipment connected to ISDN" from the International Telecommunications Union (ITU).
CAPI can be compared with the Intel-Microsoft "standard" programming interface, the Telephony Application Program Interface ( TAPI ). CAPI includes signaling and data exchange protocol s not included in TAPI. TAPI services are also provided by CAPI and a TAPI application can be mapped to CAPI functions.
Because ISDN is widely used in Germany, the Netherlands, and Scandinavia, users there are accustomed to receiving a CAPI software program or driver along with their ISDN computer card . Not all CAPI driver versions support all functions. CAPI provides functions that are independent from physical signaling protocols that vary among different countries. CAPI supports these protocols: HDLC , HDLC inverted, SDLC , LAPD, X.75, Voice (PCM), Fax group 3 (T.30), V.110/V.120, and compression ( V.xx ).