Figure: The SMS Gateway Architecture

The exchange of SMS messages is controlled by an SMS centre. Sometimes called a "Message Centre", this is a store-and-forward machine which keeps message until they can be delivered.

The GSM principle is that each network provides one or more Service Centres and each of these can send to any GSM mobile on any network. In practice, however, there are no agreements to allow SMS to travel between networks, although in many cases this is working at present. The ultimate goal is to set your service centre number to the one provided by your network, and forget it. You can enter any number and send a message, just like making a voice call. Until that ultimate goal is achieved, it is still possible to contact users on other networks.