HLR vs VLR
Home Location Register (HLR) and Visitors Location Register (VLR) are databases that contain the mobile subscriber information as per the GSM architecture. In general there is one central HLR per mobile network operator and one VLR per each Mobile Services Switching Centre (MSC) but this can vary according to the different vendor implementations. Capacity of HLR and VLR can directly affect the subscriber capacity of the mobile network operator.
HLR
HLR contains entries for each and every subscriber (MSISDN Number) within a mobile network. Mostly HLR contains static and permanent information about a subscriber. For an example subscriber status, service subscriptions (Voice, Data, SMS etc.), supplementary services, permissions etc. Other than this static information, it has temporary information such as current VLR number and MSC number. HLR works as the central location to route calls within the respective mobile operator’s network. Most of the administrative activities regarding the subscribers are controlled and centralised around the HLR. In most vendor implementations Authentication Centre (another element in GSM architecture) is integrated to HLR to provide more efficient and effective mobile network design. In this case HLR contains authentication information as well.
VLR
VLR is a database that contains part of the data available in the HLR and other dynamic information about the mobile stations’ currently roaming in the administrative areas of the associated VLR. Data in the VLR are more dynamic than the other because of the mobility nature of the mobile stations. When a mobile station moves from one Location Area to another their information is updated in the VLR, so as to locate the mobile stations. When a subscriber moves out to new VLR area then HLR inform the old VLR to remove the information related to the given subscriber. Interface between HLR and VLR is called as D-Interface as per the GSM standard which help to share information between nodes. Information about location such as LAI (Location Area Information), attached status and Temporary Mobile Subscriber Identity (TMSI) are stored in the VLR. Also some of the authentication information is passed from HLR to VLR for the authentication requirements.
What is the difference between HLR and VLR?
HLR and VLR have their own functionality within the GSM architecture. There is also a communication interface between HLR and VLR as per the GSM architecture. Number of communications take place within HLR and VLR nodes to share their information. For an example when one subscriber move from one VLR area to another area their locations are updated in the VLR and new VLR information is updated in the HLR. But if a subscriber moves within the same VLR area no such interaction with the HLR is needed.
Both HLR and VLR store the subscriber information as per the GSM architecture to provide mobile communication services to subscribers registered within the network. In general HLR contains information about all subscribers within a network while VLR contains more dynamic information relevant to subscribers roaming within the VLR area. This can vary depending on the network architecture design because in most cases HLRs’ act as centralized nodes while VLRs are mostly geographically diversified nodes. HLR acts as a fixed reference point to a given mobile station (subscriber) while his VLR can vary depending on the mobility and network design.
Even though both HLR and VLR act as databases within the same mobile network, in most designs VLRs are assigned limited geographical area to handle all dynamic data about the subscribers within that area while HLR act as more centralized node that provide more static information about the subscribers within whole network. HLR handles the subscriber administration activities within the network while VLR supports the mobility function and other dynamic information about the subscribers.
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