HLR , VLR

A Home Location Register (HLR) is a database of user (subscriber) information, i.e., customer profiles, used in mobile (cellular) networks. It is a key component of mobile networks such as GSM, TDMA, and CDMA networks. A HLR contains user information such as account information, account status, user preferences, features subscribed to by the user, user’s current location, etc. The data stored in HLRs for the different types of networks is similar but does differ in some details.

 

HLRs are used by the Mobile Switching Centers (MSCs) to originate and deliver arriving mobile calls.

 

A Visiting Location Register (VLR) is a database, similar to a HLR, which is used by the mobile network to temporarily held profiles of roaming users. This VLR data is based on the user information retrieved from a HLR. MSCs use a VLR to handle roaming users.

 

How HLR & VLR are used

Each mobile network has its own HLRs and VLRs. When a MSC detects a mobile user’s presence in the area covered by its network, it first checks a database to determine if the user is in his/her home area or is roaming, i.e., the user is a visitor.

·         User in Home Area: HLR has the necessary information for initiating, terminating, or receiving a call.

·         User is Roaming: VLR contacts the user’s HLR to get the necessary information to set up a temporary user profile.

 

The user’s location is recorded in the HLR, and in case the user is roaming, it is also recorded in the VLR.

 

In case the user wants to make a call:

·         User in Home Area: MSC contacts the HLR prior to setting up the call.

·         User is Roaming: MSC contacts the VLR prior to setting up the call.

 

In case there is a call for the user (call goes to the home MSC):

·         User in Home Area: Home MSC delivers the call immediately.

·         User is Roaming: Home MSC contacts the VLR to determine the appropriate switch in the roaming area to handle the arriving call and then transfers the call to the roaming area MSC.

 

Issues with HLRs:

·      Slow Performance, due to lookups.

·     Questionable adaptability in handling different types of networks, including 3G networks.

·         Limited capability/data to support user authentication.

·         Limited Support for data backups, fault tolerance, and reliability.

·         Scalability.