A significant part in delivering SMS messages is receiving delivery information. Apart from the basic delivery report, there are protocols that help us to identify the status of a phone number more precisely.
So what is HLR and VLR?
The Home Location Register, aka HLR, is a mobile operator database that includes details of subscribers, such as the phone number, billing details, and phone status parameters that are valuable if an SMS didn't get delivered. This data is accessible by all Mobile Switching Centres (MSC) and Visiting Location Registers (VLR).
Essential parameters in the HLR are:
The Visiting Location Register supports roaming functions for users outside the coverage area of their own HLR. It contains mostly the same data from the HLR, yet this information is stored temporarily, containing mostly duplicated information relevant to the subscriber network location. As a phone number may operate outside their home network at times, the VLR can locate in which network the number is currently roaming.
- Phone number
- Current network location
- Supplementary Services
- Number status (registered or deregistered)
- Authentication key and AUC Functionality
- Mobile subscriber roaming number
VLR helps to:
- Monitor subscribers’ locations within the VLR’s jurisdiction
- Determine whether a subscriber may access a particular service
- Locate roaming numbers during incoming SMS
- Delete the records of inactive subscribers
- Accept information passed to it by the HLR
When sending SMS messages, the operator initially checks whether the number is sending and receiving signals from the its home network. If it's not in the home network, the VLR sends a request for information about its current status and location by network via HLR. The VLR sends routing information back to the Mobile Switching Center, which allows it to locate the network it’s currently roaming to deliver the message. This communication is based on Signaling System (SS7) protocols and signaling.
The main difference between HLR and VLR is the data that is stored in them. HLR has more permanent data while VLR’s data changes all the time and is temporary.
Essential parameters in the VLR are:
- Location Area Identity (LAI)
- Temporary Mobile Subscriber Identity (TMSI)
- Mobile Station Roaming Number (MSRN)
- Mobile status