scope:

This first part of ETS 300 770 specifies the stage three of the Closed User Group (CUG) supplementary service for the pan-European Broadband Integrated Services Digital Network (B-ISDN) as provided by European public telecommunications operators at the TB reference point or coincident SB and TB reference point (as defined in ITU-T Recommendation I.413 [1]) by means of the Digital Subscriber Signalling System No. two (DSS2) protocol. Stage three identifies the protocol and procedures and switching functions needed to support a telecommunication service (see ITU-T Recommendation I.130 [2]).

In addition, this ETS specifies the protocol requirements at the TB reference point where the service is provided to the user via a private B-ISDN.

This ETS does not specify the additional protocol requirements where the service is provided to the user via a telecommunications network that is not a B-ISDN.

The CUG supplementary service enables users to form groups, to and from which access is restricted. A specific user may be a member of one or more closed user groups. Members of a specific closed user group can communicate among themselves but not, in general, with users outside the group. Specific CUG members can have additional capabilities that allow them to originate calls to destinations outside the group, and/or to receive calls from outside the group. Specific CUG members can have additional restrictions that prevent them from originating calls to other members of the CUG, or from receiving calls from other members of the CUG.

The CUG supplementary service is applicable to all telecommunication services.

Further parts of this ETS specify the method of testing required to identify conformance to this ETS.

This ETS is applicable to equipment, supporting the CUG supplementary service, to be attached at either side of a TB reference point or coincident SB and TB reference point when used as an access to the public B-ISDN.

The main part of this ETS specifies the protocol to support the CUG supplementary service in association with point-to-point connections using the procedures described in ITU-T Recommendation Q.2931 as modified by ETS 300 443-1 [19]. To support CUG also in other connection configurations, the required modifications are specified in annex B of this ETS (i.e., this annex specifies the CUG protocol for point-to-multipoint connections using the procedures of ITU-T Recommendation Q.2971 as modified by ETS 300 771-1 [21]).

The protocol specified in this ETS distinguishes two different types of CUG calls:

- Type 1: considers CUG calls with emulated N-ISDN services. These services can be identified by the presence of the N-BC information element in the SETUP message.

NOTE 1: The exact basic telecommunication service is specified by the content of the N-BC and, if present, of the N-HLC information elements (see ETS 300 443-1 [19]).

For these services, the control of the CUG supplementary service depends on the ISDN numbers (calling and called party) and on the basic telecommunication service involved in the call. This service dependency is required in order to guarantee CUG integrity in the same manner as in N-ISDN.

Type 2: considers CUG calls with B-ISDN applications for which CUG is provided without basic service dependence. No emulated N-ISDN services are involved, and no B-ISDN service type has been specified for these applications. These calls can be identified by the absence of the N-BC information element (see ETS 300 443-1 [19]) and by the absence of a coding specifying a B-ISDN service type in the SETUP message (see the paragraph below). For these applications, the control of the CUG supplementary service depends only on the ISDN numbers (calling and called party) involved in the call.

NOTE 2: In the future, there may be a necessity to distinguish a third type of CUG call. This type covers connections with B-ISDN basic services for which a service type is defined (see ITU-T Recommendation I.371 [12]). This type of CUG call is not supported by the current version of this ETS.