A home location register (HLR) is a database that mobile network providers use to keep track of their clients. 2G and 3G services have their own home location registers specific to each carrier. 

4G and 5G networks popular today use a separate HLR for the same purpose. In 4G, the home subscriber server (HSS) performs this function. In 5G, the unified database management (UDM) takes this role. 

What information is accessible in a home location register? 

The HLR database stores the identities of a mobile network’s users, including their: 

• International mobile subscriber identities (IMSI) for identifying the subscriber identity module (SIM) on the device 
• Mobile subscriber integrated services digital network (MSISDN) to identify the numbers under each subscription 
• Phone numbers and number porting histories 
• Call divert settings 
• General packet radio services (GPRS) settings for wireless packet-switching protocols 
• Available services according to the contract 
• Billing information 
• Last known locations 

Information not available on the HLR includes data on the network operator’s roaming partners. In this case, the operator must check with the visitor location register (VLR) for information from a foreign HLR. The HLR also doesn’t have a real-time feed of the device’s location; it instead receives regular updates and remembers the last known location. 

Because these databases contain sensitive personal data of the consumers, network providers do not publicly release the information and protect it with the same cybersecurity tools they use for their other internal data. These databases must also be compliant with data privacy regulations like the European Union’s General Data Protection Regulation (EU GDPR). 

In some countries, law enforcement agencies may request information from the HLR as part of a crime investigation. 

How do home location registers operate? 

A mobile network provider must authenticate its subscribers before providing services. Whenever a mobile device connects to the network, the message switching center (MSC) looks up the international mobile subscriber identity (IMSI) code and cross-checks it with the HLR database. It then decides whether the subscriber is under contract to receive the services requested. 

The HLR Lookup service contacts the network operator’s HLR database for this purpose. It authorizes users to begin using the global system for mobile communications (GSM) network. For instance, whenever the device wants to send a short message service (​SMS​) message, the company scans the HLR database to find the appropriate MSC. 

Because cellular devices are mobile, an HLR must update itself as the location of the device changes. If the device does not obtain a connection, then the HLR waits until the signal reestablishes itself. For example, if a user flies from North America to Europe and loses connection during the flight, the device will contact the MSC upon landing and notify the HLR of the new location. The device will then receive any messages missed during the flight. 

Subscribers enter the HLR of the mobile network upon signing a contract with the provider. Their entries disappear once the contract expires or cancels. However, emergency calling functionality is still available in many jurisdictions regardless of contract status. 

What other uses does the HLR offer? 

In addition to providing cellular service, carriers often lookup information on the home location register to provide customer service and improve contact center efficiency. 

Carriers with their own SMS marketing campaigns targeted at their clients might use an HLR as a directory of its current active user base. Ensuring the HLR is clear of non-active users is paramount in this regard. 

What other components of cellular networking interact with the HLR? 

Cellular services use the home location register constantly for mobile network activities. Some related terms include the following. 

• Gateway mobile switching center (GMSC). This variant of an MSC routes calls going out of the mobile network. 
• Visitor location register (VLR). This database contains all roaming visitors within an MSC’s region and optimizes communication with the HLR regarding their activities. 
• Authentication center (AuC). This key component of the home location register authenticates users and ensures that unauthorized third parties do not gain access to the network. 

What are the benefits of HLRs? 

• HLRs (Home Location Registers) are databases that store information about authorized subscribers using a global system for mobile communication (GSM) core network. 
• HLRs help in ensuring that only authenticated and permitted users and devices can use a company's services for IoT purposes. 
• HLRs also facilitate short message services (SMS) by scanning through the HLR to identify the mobile switching center (MSC) currently used by the receiving party. 

How does Sinch work with HLRs? 

Sinch works with HLRs by providing HLR lookup services that allow customers to check the status and validity of mobile numbers.