The components of the GSM architecture comprise of databases and messaging systems functions:
Home Location Register (HLR)
Visitor Location Register (VLR)
Equipment Identity Register (EIR)
Authentication Center (AuC)
SMS Serving Center (SMS SC)
Gateway MSC (GMSC)
Chargeback Center (CBC)
Transcoder and Adaptation Unit (TRAU)
The following diagram shows the GSM network along with the added elements:
https://www.tutorialspoint.com/gsm/images/gsm-elements.gif


