What is HLR? Or Home Location Register?
When a person buys a new sim card, the mobile operator configures a new sim in a database. The database is the HLR. Along with storing a subscriber profile, HLR has a full stack of GSM protocols for supporting voice, SMS, data, and other services. HLR is located in the home network of the mobile subscriber. It is an SS7 node and has an SCCP global title (gt) for addressing. The gt address is called the HLR number.
As mobile moves from one place to another. So a mobile device keeps changing the location. The visiting network updates the HLR, for a subscriber. During the update, the visiting network sends the location/roaming information. Roaming information is the dynamic information stored in HLR. Dynamic data is VLR, MSC, and SGSN numbers along with other information.
HLR for voice and SMS:
Voice and SMS service for a sim card is not configured by default. HLR stores a teleservice list for Voice and SMS. Upon location update the list of services is transferred to VLR/MSC, then the roaming network uses the information to allow or disallow the call. The following are the key parameters stored in HLR.
VLR Number :
This is the dynamic information stored in the database. Upon registration of the phone, HLR gets a location update from the network. The location update has the VLR number. The VLR number is an MSISDN. If location updates get successful, the VLR number is stored in the database. Requests initiated from HLR will have a VLR number, as the destination address to reach the roaming network of a mobile device.
This is similar to the VLR number. Most of the time, MSC and VLR are co-located. This is the dynamic information stored in the database.
This is the mobile number of the subscriber. Upon location update, HLR assigns the mobile number to the sim card. It is static information.
It is a service. A mobile operator provides to its subscribers. Allows a subscriber to receive calls on another number. Mobile users can activate call forwarding from the phone. On HLR, call forwarding data have, Call forwarding type and the mobile number to forward the call. Call forwarding type can be, Call Forwarding Unconditionally, Call forwarding when busy and call forwarding when not reachable.
Camel Subscription Info:
Static data. MSC or GMSC uses this data for real-time prepaid charging. It is a list of camel subscriptions or CSIs. A CSI can be for mobile originated calls or MO-CSI, T-CSI, etc.
GPRS data subscription in HLR:
A mobile operator provides various types of data services to their subscribers. The service may be for 2G, 3G, and 3G+. Each type has a Quality of Service associated with it. For accessing data service, the roaming network should know the GGSN for a gateway from a mobile device to the internet or PDN.
HLR stores the GPRS subscription profile for a subscriber. This includes APNs, an APN identifies a GGSN to get data. Quality of Service includes the maximum and minimum bandwidth. On location update for GPRS, SGSN downloads the GPRS subscription data. Based on data, SGSN creates a bearer towards the mobile device. The APN received from HLR, identifies the GGSN. GGSN allocates the IP address to the device. The HLR may have a static IP configuration also for an APN.
Prepaid Online Charging:
When roaming, a mobile subscriber is billed in real time for a call. This is possible because of the intelligent network. The mobile operator deploys the charging platform, which is Camel Protocol-based. The charging node is the in-home network. On location update, HLR sends the charging gateway address for online prepaid charging to the VLR/MSC. The gateway address is the global title of the charging node.