Short Messages are two-way alphanumeric messages and binary messages that can be sent and received by GSM modems / modules or server equipment with Short Message Service (SMS) capabilities. The SMS service is provided by the Global System for Mobile Communications (GSM) . Short Messages sent from GSM units are called mobile originated, while messages received by GSM units are called mobile terminated. Short messages are stored and forwarded by the Short Message Service Centres (SMSC).
An SMSC is identified by a Global Switched Telephone Network (GSTN) number, which has to be known to the sender of a mobile originated message. The availability of the SMS service over different mobile networks depends on roaming agreements of the networks, as well as on a mechanism to deliver the messages. It is the network operators responsibility to inform the user about the success or failure of the message delivery.