BTS – Base Transceiver Station (Antenna)
BSC – Base Station Controller
MSC – Mobile Switching center
HLR- Home Location Register
VLR – Visitor Location Register
SMSC – Short Message Service Center
When a user sends an SMS, the request is placed via the MSC. The MSC forwards the SMS to the SMSC where it gets stored. The SMSC queries the HLR to find out where the destination mobile is and forwards the message to the destination MSC if the destination mobile is available. If the mobile is not available the message gets stored in the current SMSC itself. In most installations If a mobile is not available for SMS delivery the SMSC will not retry. Instead the destination MSC will inform the SMSC when the mobile comes back in range.SMS handling is a store and forward operation unlike USSD.
SMS has got a validity period, for which it will wait for the destination mobile to the available. After that time the SMSC will delete the message. The validity period can be set by the user. Normal validity is 1 day.