SMS stands for Short Message Service, is the transmission of short text messages to and from a mobile phone, fax machine and/or IP address.
Message must be no longer than some fixed number of alpha-numeric characters and contain no images or graphics.
Once a message is sent, it is received by a SMSC (Short Message Service Center), which must then get it to the appropriate mobile device.
To do this, the SMSC sends a SMS Request to the HLR (Home Location Register) to find the roaming customer.
Once the HLR receives the request, it will respond to the SMSC with the status of the subscriber: (1) inactive or active (2) where subscriber is roaming.