ShortMessageService (SMS) is the transmission of short text messages to and from a mobile phone, fax machine and/or IP address. Messages must be no longer than 160 alpha-numeric characters and contain no images or graphics.

Once a message is sent, it is received by a ShortMessageServiceCenter? (SMSC?), which must then get it to the appropriate mobile device.

To do this, the SMSC? sends a SMS Request to the home location register (HLR) to find the roaming customer. Once the HLR receives the request, it will respond to the SMSC? with the subscriber's status: 1) inactive or active 2) where subscriber is roaming.

If the response is "inactive", then the SMSC? will hold onto the message for a period of time. When the subscriber accesses his device, the HLR sends a SMS Notification to the SMSC?, and the SMSC? will attempt delivery.