Integrated Services Digital Network (ISDN); Completion of Calls to Busy Subscriber (CCBS) Supplementary Service Functional Capabilities and Information Flows
"This European Telecommunication Standard (ETS) defines the stage two of the Completion of Calls to Busy Subscriber (CCBS) supplementary service for the pan-European Integrated Services Digital Network (ISDN) as provided by European public telecommunications operators. Stage two identifies the functional capabilities and the information flows needed to support the service as described in stage one. The stage two description also identifies user operations not directly associated with a call (see CCITT Recommendation I.130 [3]). This ETS is specified according to the methodology specified in CCITT Recommendation Q.65 [6]. This ETS does not formally describe the relationship between this supplementary service and the basic call but@ where possible@ the information is included for guidance. In addition@ this ETS does not specify the requirements where the service is provided to the user via a private ISDN. This ETS does not specify the requirements for the allocation of defined functional entities within a private ISDN; it does however define which functional entities may be allocated to a private ISDN. This ETS does not specify the additional requirements where the service is provided to the user via a telecommunication network that is not an ISDN. The CCBS supplementary service enables user A@ encountering a busy destination B@ to have the call completed without having to make a new call attempt when the destination B becomes not busy. The CCBS supplementary service is applicable to all circuit-switched telecommunication services. This ETS is applicable to the stage three standards for the ISDN CCBS supplementary service. The term ""stage three"" is also defined in CCITT Recommendation I.130 [3]. Where the text indicates the status of a requirement@ i.e. as a strict command or prohibition@ as authorization leaving freedom@ as a capability or possibility@ this shall be reflected in the text of the relevant stage two and stage three standards. Furthermore@ conformance to this ETS is met by conforming to the stage three standards with the field of application appropriate to the equipment being implemented. Therefore@ no method of testing is provided for this ETS."