What is HLR in GSM?
When a new sim card is purchased from the GSM network operator, the wireless carrier creates a new sim entry in a central database. The full form of HLR is Home Location Register. It is the database for sim provisioning along with software applications.
In addition to storing a subscriber profile, HLR has a full stack of GSM protocols for enabling voice, SMS, data, and other services. This tutorial will describe the role and detailed technical specifications for the GSM element.
Home Location Register and SS7 network.
Role of HLR and VLR in the telecom network.
Roaming is the main feature of mobile communication, and HLR plays a central role in roaming services. VLR is a network node in the visiting network for roaming. Once a mobile phone is switched on, it attaches to the nearest VLR.
During the attachment, the visiting network sends the location/roaming information to the HLR of the SIM card provider. Roaming information is the dynamic information stored in the HLR database that includes VLR, MSC, SGSN numbers, and other information.
A sim card configuration on HLR has IMSI and MSISDN as the primary keys. HLR assigns a mobile number, or MSISDN Number, to the SIM card on a gsm map location update procedure.
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 phone registration, 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 types 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 address. It is a gateway between a mobile device to the Internet or PDN.
HLR stores the GPRS subscription profile for a subscriber. This includes a list of APNs and Quality of service. An APN identifies a GGSN, and the Quality of Service consists of 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.