There are two sets of routing scenarios for connecting a GSM PLMN to a PSPDN/ISDN, namely:
‑ via a transit ISDN or PSTN; or
‑ direct connection.
These are identified in GSM 03.70.
What concerns the numbering please see GSM 03.03, GSM 03.70, and ETS 300 099.
7.2 Interworking through transit networks
"In this scenario an PH or an PSPDN port is considered to be part of the ISDN or the PSPDN, as shown in figures 1 and 2/GSM 09.06."
Figure 1/GSM 09.06 shows the access to a conventional X.32 PSPDN port of the PSPDN via transit networks ISDN and/or PSTN. The information transfer capability attribute value indicated in the GSM PLMN is "3.1 kHz audio, ex PLMN". In this case, modems are used.
Figure 2/GSM 09.06 shows the access to the packet mode service provided according to the concept of the ETSI Packet Handler (PH) with AU functionality as specified in ETS 300 099 clause 5 for Case A. The information transfer capability attribute value is "unrestricted digital information". The application of this scenario requires an arrangement with the ISDN operator for granting access to the PH for GSM PLMN subscribers.
"PHs supporting only X.31 case B can also be used. For these PHs agreements should exist between the PLMN and the fixed network operators to signal the case B between PLMN and ISDN. The GSM HPLMN shall take care of the X.31 case A/B conversion in order to proceed the call. The location of this conversion within the HPLMN is network dependent." The procedures used for mobile originated and mobile terminated calls to establish the circuit connection are defined in clauses 8 and 9 for the basic and dedicated packet services, respectively.
Figure 1/GSM 09.06: Interworking through ISDN and PSTN (3,1 kHz exPLMN)
Figure 2/GSM 09.06: Interworking through ISDN (unrestricted digital information)
NOTE: The packet handler logically is a part of the ISDN.
7.3 Direct Interworking
In this scenario the PH logically is part of the GSM PLMN (figure 3/GSM 09.06) and may be physically located in the GSM PLMN, the PSPDN or the ISDN. In the latter two cases, an arrangement with the PSPDN or ISDN operators is required in order to arrange for GSM PLMN particularities for dedicated access to cope with the "loop detection" issue.
The interface between the MSC and the PH shall be the PH interface (PHI) as specified in ETS 300 099 for Case A. In cases where the available PH only supports the ISDN X.31 Case B, the PH is required to perform a GSM specific function, i.e. to substitute in the X.25 "call request" packet the PH port address in place of a foreign subscribers calling address. The GSM PLMN shall take care of the X.31 Case A/B conversion in order to proceed the call.
Figure 3/GSM 09.06: Direct interworking
Mobile originated and mobile terminated call procedures are defined in clauses 8 and 9.