Call Routing Examples

INTRA-PLMN CALL ROUTING

When an MS calls another MS within the same PLMN it first carries out the mobile-initiated
call procedure with its Home MSC (HMSC).
The HMSC must first identify the location of the MS in order to route the call correctly. As
this is not deducible from the dialled number (the destination MS’s MSISDN1 number), the
HMSC must obtain the destination MS’s Mobile Station Roaming Number (MSRN).
An MS’s MSRN is stored in its local VLR, in this case the VLR of the destination MS (Visited
VLR or VVLR). However, the HMSC cannot interrogate the VVLR directly as it does not
know its location. It must therefore request this information from the HLR.
The HMSC requests this information from the HLR using the MSISDN number. The HLR
interrogates the VVLR and extracts the MSRN of the destination MS. This is passed back to
the HMSC which than then forward the call to the VMSC.
The VMSC carries out paging procedures to initiate a mobile terminated call at the distant
end.

INTER-PLMN CALL ROUTING

If subscriber within the HPLMN calls a subscriber external to the HPLMN, the dialled
MSISDN number is not related to the current location of the destination MS. The call is
therefore first routed to a GMSC in the HPLMN.
The GMSC recognises the destination Network Dialling Code (NDC) from the dialled number
and therefore known which VPLMN to access. It therefore forwards the call to a GMSC in the
VPLMN.
The VPLMN GMSC must then obtain the MSRN for the destination mobile. This procedure
follows that of the Intra-PLMN call routing with the GMSC taking the place of the HMSC
described in the previous example
This simplifies the process of charging the called subscriber correctly if the call has to be rerouted
because the called MS has roamed to a different PLMN or if the call has to be
forwarded.