The present document specifies the stage three of the Subaddressing (SUB) supplementary service for the pan-European
Integrated Services Digital Network (ISDN) as provided by the European public telecommunications operators by
means of the Signalling System No.7 protocol for the ISDN User Part (ISUP). Stage three identifies the protocol
procedures and switching functions needed to support a telecommunication service (see ITU-T
Recommendation I.130 [1]).
The present document does not specify the additional protocol requirements where the service is provided to the user
via a telecommunications network that is not an ISDN.
The present document does not specify the additional protocol requirements for the national signalling interface.
Although the present document applies only to the international interconnection, the specification of functions, formats
and codes of messages and signals, and actions performed at originating and destination local exchanges are retained.
Formats, codes and procedures marked for national use are included for informative purposes for the international
interface specification. If these items so marked are supported within a national network and operators network, then it
is proposed that they shall be supported in this manner.
NOTE: In the case where a national signalling system behaves differently, the international gateway exchange is
to support both the concerned national and international network.
The SUB supplementary service allows the called (served) user to expand the called (served) users addressing capacity
beyond the one given by the ISDN number.