Terminals in the field and the M2M applications communicate by means of small messages. Each message has a minimal overhead to allow identification of the message and the structure of the data it contains.

The Service Identification Number (SIN) (1 byte) is used to identify the service or application for which this message is used. SINs 0-15 are reserved for system use and SIN 16-255 can be defined by terminal manufacturer or M2M application developer.

The Message Identification Number (MIN) may be used to define a particular message type within the structure. It allows the application to interpret the data payload.

• SIN=0 is used for core modem operations. Within that service a number of different messages types are defined.
• MIN=0 identifies the modem registration message that is sent by the modem when it registers with the network and contains a variety of terminal and network details.

The remainder of the message contains the Payload Data: up to 6398 bytes from-mobile or 9998 bytes to-mobile. The structure of the payload is defined by terminal manufacturer, application developer or attached device and normally consists of several data elements such as latitude, longitude, speed, etc.

M2M applications can exchange message with the terminal by means of the IDP Messaging API. This API provides functions to submit messages destined for a particular terminal, retrieve messages sent by a terminal and query the status of messages sent.