Extension-dependent call forwarding in an exchange

For performing call forwarding from terminal stations of a dependent switch which is configured as second switch connected at the exchange (e.g. from extensions of a private branch exchange PBX), an exchange sets up a call forwarding list in which a call address is in each case allocated to a terminal station code as forwarding destination for one or more terminal stations of the dependent switch. When a connection request relating to a subscriber at a dependent switch of the exchange arrives, the code of the desired terminal station is used for checking whether a call address is allocated to this code in an associated call forwarding list. If this is so, the connection is forwarded to this call address.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The exemplary embodiment relates to a private branch exchange which is connected to an exchange of a public network. It should be noted at this point that the exemplary embodiment is not restrictive. In particular, the invention can be generally used for implementing external call forwarding between a switching system and a second switch which is allocated to it as a dependent switch, the dependent switch being of the same level as or of a lower level than the first switching system. In general, the invention can be applied to switching systems of any type, including switching systems of public and private networks. In addition, the two switching systems do not necessarily have to be connected to one another directly but can also be connected indirectly—e.g. via other switching facilities. Referring to FIG. 1, a private branch exchange BX 1 is connected to an exchange VSS of a telecommunication network, a public telephone network PTN in the example considered here, via three access lines AL 1 , AL 2 , AL 3 . The private branch exchange BX 1 serves a private branch network BN 1 to which a number of extension users Bn&equals;B 1 , B 2 , B 3 , . . . belongs. The exchange VSS is, e.g. an EWSD system by the applicant, in which the connections are connected to a central switching network SN via line trunk groups LTG. In addition to the central control processor CP of the exchange VSS, so-called group processors GP which take the load of line trunk group-specific switching tasks of the switching operation off the central processor CP are also provided in the line trunk groups LTG. A group number cx 1 is allocated to the private branch exchange BX 1 or, respectively, the private branch network BN 1 in the telephone network PTN. In the private branch network, a direct dialing number cn&equals;c 1 , c 2 , c 3 , . . . , for example in each case a two-digit number c 1 &equals;“22”, c 2 &equals;“23”, etc., is in each case allocated to each extension Bn. To implement the extension-specific call forwarding according to the invention, screening for the direct dialing number cn dialed after the group number is performed in the exchange VSS in the case when a connection request for the directory number cx 1 arrives at the private branch exchange BX 1 . The invention thus provides that the extension code is not passed transparently to the private branch exchange BX 1 as in previously used methods but is used by the exchange VSS for influencing the switching sequence. For this purpose, a call forwarding list RRL allocated to the private branch exchange BX 1 is provided in the database of the exchange VSS. This list includes e.g. a number of entries RLE which, e.g., in each case include two components. The first component RRC specifies an extension code for which a call forwarding according to the invention is to be performed. The second component RRN specifies, in each case, the associated forwarding destination. For example, as shown in FIGS. 1 and 2 , the extension B 1 , which can be reached via the number c 1 , is forwarded to the subscriber PNA having the directory number rna and similarly the extension having the code c 4 is forwarded to the subscriber rnb corresponding to the directory number rnb, which, for example, is connected to another exchange VSB. Naturally, the call addresses mentioned as forwarding destinations are not restricted to subscribers who are connected to the exchange VSS but can be any subscribers belonging to the network PTN or available via this network. In addition, the forwarding destination mentioned can be an extension of another private branch exchange or of the same one as shown in the example of extension B 2 —code c 2 —which is forwarded to code c 3 , or respectively, extension B 3 . The call forwarding according to the invention is administered by the network PTN, e.g. by the network operator, to the exchange VSS and relates to the administration (setting up, processing, etc.) of the list RRL. In addition, service-dependent administration of the call forwarding for the private branch exchange BX 1 is also possible in order to activate the call forwarding in a service-dependent manner—e.g. only for fax calls—for example, in the case of ISDN subscribers or an ISDN private branch exchange. For this purpose, an additional feature can be introduced in the call list RRL or the entries RLE which specifies the service for which the call forwarding is to be performed. It is also conceivable that the call forwarding according to the invention can be controlled from the private branch exchange BX 1 or an extension B 1 , . . . at the exchange VSS. This can be done e.g. in accordance with the principles of subscriber controlled input (SCI), in which the call forwarding for an extension is input from this extension, or of remote subscriber controlled input (RSCI) in which the call forwarding is input from another extension—for example protected by password—compare in this context ETSI standard EN 300 207-1, chapter 9, and ETSI standard EN 300 691. Naturally, the SCI-oriented method requires that the extension for which the call forwarding is set up actually exists whereas this is not mandatory in the case of an RSCI method. FIG. 3 shows an example of a switching sequence of a call forwarding according to the invention which takes place at the exchange VSS. The call forwarding is performed by the central processor CP with the aid of the group processor GP. When a connection request for a line cx 1 of the exchange arrives—it is possible that a code (e.g. direct dialing digits) of an extension is already supplied together with the call address cx 1 at this point—a check is first made as to whether the call address cx 1 specified in the connection request relates to a private branch exchange and whether extension-oriented call forwarding according to the invention is activated for this. If yes, a further check is made as to whether an associated call forwarding list RRL exists for the call address cx 1 . If this is not so (“negative” branches), the connection to the call address cx 1 is set up in a familiar manner without call forwarding. In the case of relevance here (“OK” branch), that the connection request relates to a private branch exchange—namely the private branch exchange BX 1 connected to the connection cx 1 , for which call forwarding and a call forwarding list RLL according to the invention are set up, a check is made for an extension code. For the example considered here, in which the code is implemented as suffix of the group number cx 1 (so-called “direct dialing”) in the form of selection digits, a check is made as to whether direct dialing digits have been dialed. If this is not so (“missing”), the direct dialing digits (or generally a code) are requested. If a code cn exists (“present”), the call forwarding list RLL is searched for cn in the column RRC. The search supplies one of the following three results: 1) The code cn is found in the list RRL. The connection is forwarded to the call address RRL (cn) mentioned in the RRL component. 2) The code cn is not found. The further connection setup is then carried out without call forwarding (“negative” branch), the code cn is forwarded to the private branch exchange as in the known method of the connection setup. 3) No unambiguous statement is possible—usually because additional direct dialing digits are necessary or the given extension code cn is not adequate for unambiguous selection of an entry for other reasons. In this case (“inadequate” branch) completion of the code—e.g. further direct dialing digits—is requested. The invention can be used with respect to analog and digital (like ISDN) private branch exchanges, preferably private branch exchanges with direct dialing. One aspect of the invention is that for it to be possible for the call forwarding according to the invention to be performed, it is sufficient if the private branch exchange is configured at the exchange regardless of the actual presence of the private branch exchange. For this reason, the invention can perform extension-specific call forwarding even if the relevant private branch exchange or the associated private branch network has been partially, or wholly removed (e.g. relocated). Furthermore, the call-forwarding according to the invention can also be cascaded in such a manner that the forwarding destination itself is a second exchange or private branch exchange for which an extension-specific call forwarding is set up.