Load reduction of a visitor location register

Details Load reduction of a visitor location register Load reduction of a visitor location register

1998-06-26

2000-11-14

Trost, William G.

Telecommunications

Radiotelephone system

Zoned or cellular telephone system

455435, 455458, 455422, 455446, 455449, H04Q 720

Patent

active

061482005

DESCRIPTION:

BRIEF SUMMARY
The invention relates to mobile communication systems and particularly to reducing the load of their visitor location registers.
FIG. 1 illustrates the roaming of a mobile station MS in a mobile communication network or in a network subsystem NSS. In this example, the NSS comprises six base transceiver stations BTS1 to BTS6, which can be controlled by one or more base station controllers BSC. Each base station BTS serves one cell. When an MS terminated call is switched through the network NSS, the NSS must know the area in which the MS is to be paged. The area in which the MS is paged is called a location area LA.
The size of the location area LA is a compromise. If the location area is very large, the MS must be paged in the area of several base stations BTS. This means that mobile stations are also paged in base station areas where they are not located. This consumes the capacity of radio channels. If the location area is small, location updatings must be performed frequently. FIG. 1 shows, by way of illustration, three location areas LA1-LA3, which cover 1, 2 and 3 base stations, respectively. The base stations BTS continuously broadcast information on themselves and their environment, such as a base station identifier BSI and a location area identifier LAI. On the basis of the LAI, an MS registered in a BTS knows in which LA it is located at each moment. If the MS, when changing the BTS, observes that the LAI has changed, it sends a location update request or message to the network NSS. The LA of the MS is updated in the visitor location register VLR in whose area the MS is located at that moment. The VLR typically stores e.g. the subscriber's IMSI (International Mobile Subscriber Identity) number, MSISDN number, subscriber service data and LAI.
Information on the VLR in whose area the MS is located is sent to the home location register of the MS. The HLR stores permanent subscriber data, which are not dependent on the subscriber's location at a given moment: e.g. the subscriber's MSISDN and IMSI, subscriber service data, and routing data to the VLR in whose location area the MS is located.
FIG. 2 illustrates the setting up of an MS terminated call in a GSM-type mobile communication system. If the MS of FIG. 1 is not located within the area of its own home location register, the parts of FIG. 2 correspond to those of FIG. 1; however, the HLR and VLR are associated with different mobile services switching centres MSC. In step 1, a call arrives at a gateway mobile switching centre GMSC. In step 2, the GMSC defines the subscriber's home location register HLR on the basis of the called subscriber's directory number MSISDN, and sends it a request for routing data. Information on the VLR in whose area the subscriber is located is updated in the subscriber's HLR in connection with location updating. On the basis of this information, the HLR sends the VLR a `Provide Roaming Number` request in step 3. The IMSI of the MS subscriber is also sent to the VLR in this request. In step 4, the VLR stores the data it has received and assigns a mobile station roaming number MSRN. In step 5, the VLR sends the MSRN it has assigned to the HLR, which in step 6 sends it to the switching centre GMSC that requested the routing data. The roaming number domain is defined in such a way that a call is always directed to the MSC whose VLR has assigned the roaming number in question. On the basis of the MSRN, the GMSC is thus able to route the call forward by sending an initial address message in step 7 to the MSC indicated by the MSRN.
In this example, the MSC finds out on the basis of the MSRN that the call will terminate in its own area. In step 8, the MSC requests the data of the called subscriber from its own VLR call set-up. In a normal case, the VLR returns the requested data in step 9 for setting up the call. This is indicated by arrows 10 and 11.
A problem arises when the data of the VLR are lost for some reason. This may happen, for instance, when the VLR is reinitialized, possibly during software updating or equipment main

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