What [it] is [it] All About

A Home Location Register (HLR) is a database that contains semi-permanent mobile subscriber information for a wireless carriers’ entire subscriber base. HLR subscriber information includes the International Mobile Subscriber Identity (IMSI), service subscription information, location information (the identity of the currently serving Visitor Location Register (VLR) to enable the routing of mobile-terminated calls), service restrictions and supplementary services information.

The HLR handles SS7 transactions with both Mobile Switching Centers (MSCs) and VLR nodes, which either request information from the HLR or update the information contained within the HLR. The HLR also initiates transactions with VLRs to complete incoming calls and to update subscriber data.

Traditional wireless network design is based on the utilization of a single Home Location Register (HLR) for each wireless network, but growth considerations are prompting carriers to consider multiple HLR topologies.

Visitor Location Register (VLR)

A Visitor Location Register (VLR) is a database which contains temporary information concerning the mobile subscribers that are currently located in a given MSC serving area, but whose Home Location Register (HLR) is elsewhere.

When a mobile subscriber roams away from his home location and into a remote location, SS7 messages are used to obtain information about the subscriber from the HLR, and to create a temporary record for the subscriber in the VLR. There is usually one VLR per MSC.

International Mobile Subscriber Identity (IMSI) Number

The IMSI is a unique non-dialable number allocated to each mobile subscriber in the GSM system that identifies the subscriber and his or her subscription within the GSM network. The IMSI resides in the Subscriber Identity Module (SIM), which is transportable across Mobile Station Equipment (MSE). The IMSI is made up of three parts

(1) the mobile country code (MCC) consisting of three digits,

(2) the Mobile Network Code (MNC) consisting of two digits,

(3) the Mobile Subscriber Identity Number (MSIN) with up to 10 digits.

Mobile Subscriber ISDN (MSISDN) Number

The MSISDN is the dialable number that callers use to reach a mobile subscriber. Some phones can support multiple MSISDNs - for example, a U.S.-based MSISDN and a Canadian-based MSISDN. Callers dialing either number will reach the subscriber.