Mobile Network Architecture- High level elements

Mobile Network Architecture is a complex thing and its hard to figure out what fits in where. I at times try to explain it to different people and I tried to plot the below diagram as a basis for explanation. Its not perfect and each network has its own deviations, but I hope it would explain the main elements of the mobile network ignoring the GPRS/Data part to a beginner.

High Level Mobile Network Architecture

BTS – Base Transceiver Station
BSC – Base Station Controller
MSC- Mobile Switching Center
HLR – Home Location Register
VLR – Visitor Location Register
AUC – Authentication Center
LBS – Location Based Server
SMSC – Short Messaging Service Center
USSDC – Unstructured Supplementary Service Data Center
MMSC – Multimedia Messaging Service Center

As show in the diagram, a mobile subscriber at a given time will be associated with a single BTS. In turn many such BTSs shall be associated with a BSC. Many BSC will be associated with a MSC. The MSC in some networks are associated with a VLR. The VLR is a temporary database which will have the cell IDs of the subscribers associated with the associated MSC.

A network might have multiple MSCs which eventually get attached to the GMSC - Which acts as the gateway to the rest. The GMSC will be in turn connected via STPs to network capabilities. One such element is the HLR, which is the central database with a record for each IMSI number ( unique identifier for the SIM number ). HLR will have information like MSISDN, enabled services, the associated MSC of the subscriber, etc. HLR can be considered as the primary database in the network. So when a message is to be delivered to a given person, as the first step the HLR will be queried to find the MSC and then using that information the relevant VLR will be queried to find the relevant cell to eventually route the message to the right BTS for delivery.

The rest of the elements in the box are network services. They enable billing, authentication, MMS, MMS, USSD, Location Based Services, etc.