Communication Between a VPLMN VLR and an HPLMN HLR (Roaming in a GSM Network)

When a roamer switches ON a mobile station (MS) for the first time in a VPLMN, the VLR initiates the update location procedure with the roamer’s HLR. The only information available to the VPLMN VLR at this time is the IMSI of the roamer. The VPLMN VLR uses this to derive routing information (SCCP addressing) for communicating with the HPLMN HLR. The derived address is known as the mobile global title (MGT) or E.214 address.

When responding to the VPLMN VLR, the HPLMN HLR inserts its own E.164 address in the CgPA of the SCCP message. The E.164 part, as defined in the ITU-T E.164 Recommendation, is used to identify the country and PLMN or PLMN and HLR, where the roamer is registered.

On receiving an initial response from the HPLMN HLR, a VPLMN VLR then derives the routing information for subsequent communication with the HPLMN HLR from the calling party address in the received response.

This means that the VPLMN VLR is able to address the HPLMN HLR using an E.214 MGT that has been originally derived from the roamer’s IMSI and an E.164 HLR address.

An E.214 MGT translation is done either at the application level or at the SCCP level in the VLR using a routing table.

As shown in Figure 6.5., the MGT is of variable length. Within the MGT, the Country Code (CC) is derived from the Mobile Country Code (MCC). The National Destination Code (NDC) is derived from Mobile Network Code (MNC) or from the MNC and some initial digits of the Mobile Station Identifier Number (MSIN).

Derivation of MGT from roamer's IMSI.

Figure 6-5 Derivation of MGT from roamer’s IMSI.

MGT-based routing.

Figure 6-6 MGT-based routing.

Each PLMN consists of one logical HLR. In practical implementations, one physical HLR covering an entire network may not be feasible. In most of the implementations, more than one HLR may exist, grouped under one logical HLR. The SCCP gateway/GMSC at the edge of a network decides to route the message received by a VPLMN to the right HLR on the basis of the MGT in the SCCP called party address. As shown in Figure 6-6, VLR5 derives SCCP called party address (CdPA) from the roamer’s IMSI. The gateway MSC in the HPLMN makes the decision to route the message to HLR 3 by looking at NDC and MSIN digits.

Figure 6-7 shows partial protocol decodes of an update location request and an update location response message. In this example, a roamer from a Singapore network is trying to register in a network in Malaysia. The serving VLR uses MGT translation to route the UL message to the HLR in the roamer’s home PLMN in Singapore. The HPLMN GMSC routes the UL request to the HLR, which contains subscriber information. The HLR, in a response message, inserts its own address in the CgPA. The CgPA received in the request message is used as the CdPA in subsequent messages from the VLR.

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