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Original Publication Date: 2001-Apr-03
Included in the Prior Art Database: 2001-Apr-03
Document File: 2 page(s) / 8K

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Jheroen P. Dorenbosch: AUTHOR [+1]



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by Jheroen P. Dorenbosch and Grant M. Emberson

Many cellular systems use Visited Location Registers (VLRs) and Home Location Registers (HLRs) for mobility management. Such systems typically have large numbers of VLRs and HLRs.

The HLR of a Mobile Station (MS) stores the MS's current VLR. The system uses the stored value to forward provisioning changes and calls to the MS. The system uses a directory to resolve the HLR of an MS.

When an MS from one Service Provider (SP) wanders into the system of another SP, the MS will not get service unless it is in the system directory. Hence, a way to couple systems of different service providers is then to connect the VLRs and HLRs of both systems to a common network and to merge the system directories (Figure 2). It should be clear that this solution does not scale well and introduces coupling in the maintenance of the system directories of the coupled systems.


An alternative way to couple systems is to use a pair of HLR/VLR gateways (Figure 3). On the system side these gateways expose standard HLR and VLR interfaces. It looks like a normal HLR and a normal VLR have been added to the system.

Other than that, one needs to update the directories of both systems. One must map all MSs of a neighbor system into the HLR gateway. The details of the neighbor system remain hidden; the neighbor system's HLRs do not appear in the directory.

As shown in Figure 3, when a neighbor's MS wanders into a system and registers, the gateway HLR maintains the true current VLR. The MS's true HLR in the neighbor system maps the MS into the gateway VLR. This way, provisioning changes and calls can be forwarded to the MS with one level of indirection. The MS can also call MSs in the current system, including other wandering MSs.


The main benefit of HLR/VLR gateways is decoupling. One can even couple systems that use different, otherwise incompatible software versions. A well-designed gateway allows the software in one system to be upgraded without a need for software changes in the neighbor system. Similarly one can use HLR/VLR gateways to couple systems that use different protocols, like iDEN and TETRA systems.

When an MS (bbb) from the neighbor system wanders into the system, the Visited LR (VLR A4 will try to register it with the gateway HLR. The gateway HLR forwards the registration request to the gateway VLR of the neighbor system. That gateway VLR uses that system's directory to resolve the MS's true HLR (B4) and does a registration. The true HLR will store the gateway VLR as 'current VLR' of the MS. The gateway HLR will store the true VLR (A4) as the MS's current VLR. It can easily be verified that provisioning changes and calls can then be forwarded to the wandering MS without too much difficulty, and without changes to the existing network elements.