InnoWave Telecommunications

Placement of the Protocol Entities for HLR Interrogation

Depending on the capabilities of the associated switches and the called target (national or international MSISDN), there are different routing procedures. In general, the local switching center analyzes the MSISDN. Due to the NDC, this analysis of the MSISDN allows the separation of the mobile traffic from other traffic. The case that mobile call numbers are integrated into the numbering plan of the fixed network is currently not provided.

In the case of a national number, the local exchange recognizes from the NDC that the number is a mobile ISDN number. The fixed network and home PLMN of the called subscriber reside in the same country. In the ideal case, the local switch can interrogate the HLR responsible for this MSISDN (HLR in the home PLMN of the subscriber) and obtain the routing information (Figure 8.5a). The connection can then be switched through via fixed connections of the ISDN directly to the MSC.

If the local exchange does not have the required protocol intelligence for the interrogation of the HLR, the connection can be passed on preliminarily to a transit exchange, which then assumes the HLR interrogation and routing determination to the current MSC (Figure 8.5b). If the fixed network is not at all capable of performing an HLR interrogation, the connection has to be directed through a GMSC. This GMSC connects through to the current MSC (Figure 8.5c). For all three cases, the mobile station could also reside in a foreign PLMN (roaming); the connection is then made through international lines to the current MSC after interrogating the HLR of the home PLMN.

In the case of an international call number, the local exchange recognizes only the international CC and directs the call to an International Switching Center (ISC). Then the ISC can recognize the NDC of the mobile network and process the call accordingly. Figures 8.6 and 8.7 show examples for the processing of routing information. An inter-

Figure 8.5: Routing variants for national MSISDN

national call to a mobile subscriber involves at least three networks: the country from which the call originates; the country with the home PLMN of the subscriber, Home PLMN (H-PLMN); and the country in which the mobile subscriber is currently roaming, Visited PLMN (V-PLMN). The traffic between countries is routed through ISCs. Depending on the capabilities of the ISC, there are several routing variants for international calls to mobile subscribers. The difference is determined by the entity that performs the HLR interrogation, resulting in differently occupied line capacities.

LE Local Exchange

ISC International Switching Center

Figure 8.6: Routing for international MSISDN (HLR interrogation from ISC)

LE Local Exchange

ISC International Switching Center

Figure 8.6: Routing for international MSISDN (HLR interrogation from ISC)

If the ISC performs the HLR interrogation, the routing to the current MSC is performed either by the ISC of the originating call or by the ISC of the mobile subscriber's H-PLMN (Figure 8.6). If no ISC can process the routing, again a GMSC has to get involved, either a GMSC in the country where the call originates or the GMSC of the H-PLMN (Figure 8.7).

Figure 8.7: Routing through GMSC for international MSISDN

For the routing procedures explained here, it does not matter which kind of subscriber is calling, i.e. the subscriber may be in the fixed network or in the mobile network. However, for calls from mobile subscribers, the HLR interrogation is usually performed at the local exchange (MSC).