Protocol Description Unit (PDU) - There are two ways of sending and receiving SMS messages: by text mode and by PDU mode. The text mode (unavailable on some phones) is just an encoding of the bit stream represented by the PDU mode. Alphabets may differ and there are several encoding alternatives when displaying an SMS message. If you read the message on your phone, the phone will choose a proper encoding. An application capable of reading incoming SMS messages, can thus use text mode or PDU mode. If text mode is used, the application is limited by the set of preset encoding options. However, if PDU mode is used, any encoding can be implemented.