Communication via an Inmarsat C terminal will always involve a transfer of data (see picture below the text). This data transfer will always be "store and forward". This means there will never be a direct connection between the Inmarsat C terminal and the final destination, and for this reason it is not possible to make a telephone call via this Inmarsat C system. Normally the terminal will send his message to the choosen LES on via the NCS-directed channel. The complete message will be stored in a computer in the LES. The message will be given a reference number by the LES. After the transmission of this message, the LES will forward the stored message to the appointed address. When the message has been delivered, the LES will give the sender a Positive Delivery Notification (PDN), when the sender has asked for this service.
This PDN will be charged by the LES. When a message cannot be delivered to the appointed address, the sender will get a Non Delivery Notification (NDN). In this NDN will be a code (see fault codes in the appendix) that explains why the delivery has failed. Normally the NDN is free of charge, but this depends on the policy of the LES being used .
In the Inmarsat system you can use the Two Digit service codes to obtain some special services directly via the selected LES. E.g. when you wish to obtain radio medical advice you should enter service code 32, and then the selected LES will forward your message directly to a doctor or hospital. This list containing the Two Digit service codes can be found in the appendix.

Data transfer via the Inmarsat C system

The Inmarsat station on board a ship is called a Ship Earth Station (SES), an Inmarsat station on board a car or airplane is called a Mobile Earth Station (MES).

Types of Inmarsat C Messages

The Inmarsat C system is capable of transmitting different types of messages. It can be used for the transmission of:

• TELEX.
  
• Fax.
  
• Data,
  
• E-mail and also
  
• SMS messages (Short Message Service).

The type of messages and how (the format of) these messages should be transmitted depends on the protocol which has been used by the provider of the LES.