This Technical Specification defines the stage two description of the Shared Inter Working Function (SIWF) on Global Syst em for Mobile communications (GSM). A stage one description does not exist because this service is not vis ible to the service subscribers and users. This stage two identifies the functional Capabilities and information flows needed to support th e service. The signalling system protocols and switching functions nee ded to implement this service are defined in the relevant specificatio ns. Interworking with other networks (e.g. PSTN) needs the presence of specific functions associated with the MSC, known as interworking functions (IWF). The IWFs depend on the type of network with which it is desired to interconnect and the type of service desired. A Shared Inter Working Function is a network function that may be used by any MSC in the same PLMN to provide interworking for a data/fax ca ll. Whereas an IWF can only be used by its MSC, the SIWF can be used by several other network nodes e.g. any MSC within the same PLMN. SIWF is applied to data services in GSM phase 2 and GSM phase 2+ (as defi ned in GSM 02.02, GSM 02.03 and GSM 02.34). The usage of a SIWF requires no additional manipulation at the MS. There is an interest to have a Shared Interworking Function (SIWF) for the following reasons: - possibility to fabricate/obtain the specific functionality needed for data services in areas where data traffic is expected to below, - handling of data calls in case of local overload of the data traffic , - quick introduction and roll-out of new data services.