The first truly open computer telephony (CT) development standard for link-level protocols. CSTA was developed by the European Computer Manufacturers Association (ECMA) and subsequently was improved and formally standardized by the ITU-T, incorporating the Switch-to-Computer Applications Interface (SCAI). CSTA is a full protocol stack that requires an open system interface to a PBX, automatic call distributor (ACD), or Centrex central office (CO). See also ACD, Centrex, computer telephony, ECMA, ITU-T, PBX, and SCAI.