03.043GPPSignalling Requirements Relating to Routing of Calls to Mobile SubscribersTS
The number dialled by the calling subscriber contains no indication of the location of the called MS. In order to route the call to the MS the network must be aware of the location of the MS and the routeing address to be used. The routeing address is the Mobile Station Roaming Number (MSRN); the only network entity which can provide the MSRN is the Home Location Register (HLR) of the HPLMN, hence in order to route the call to the visited MSC (VMSC) where the called MS is located it is necessary to interrogate the HLR.
The preferred arrangement for interrogation of the HLR is to direct the call to a Gateway MSC (GMSC) which is part of the HPLMN. This simplifies the process of charging the called subscriber correctly if the call has to be re-routed because the called MS has roamed to a different PLMN or if the call has to be forwarded.
The consequence of this is that the preferred signalling procedure is as follows:
i) When a subscriber wants to call an MS he dials the MSISDN of the called MS.
ii) The originating local exchange (LE), or a transit exchange, analyses the dialled number and recognises the NDC which identifies the HPLMN. If the calling subscriber is in a different country from the called MS, this analysis will in general not be performed by the originating LE; when the international prefix is recognised, the call is routed to an outgoing International Switching Centre (ISC) without any further analysis, and the incoming ISC in the destination country recognises the NDC of the HPLMN.
iii) The call is directed to a GMSC in the HPLMN. The GMSC interrogates the HLR, which returns the MSRN for the called MS. The HLR obtains the MSRN by interrogating the VLR where the called MS is currently registered. This procedure uses the Mobile Application Part of Signalling System CCITT No 7 – see Technical Specification GSM 09.02.
iv) The GMSC uses the MSRN to route the call to the VMSC.
Clause 5 describes the routeing of a call from a subscriber served by the HPLMN.
Clause 6 describes the routeing of a call from a subscriber in the same country as the HPLMN, but not served by the HPLMN.
Clause 7 describes the routeing of a call from a subscriber in a different country from the HPLMN.