This European Telecommunication Standard (ETS) defines three interface protocols for control of SMS functions within a GSM mobile telephone from a remote terminal via an asynchronous interface. Section 2 defines a binary protocol ('Block Mode'). The protocol includes error protection and is suitable for use where the link may not be completely reliable. It will be of particular use where control of remote devices is required. Efficient transfer of binary encoded user data is possible. Section 3 defines a character-based interfaced based on 'AT' commands ('Text Mode'). This mode is suitable for unintelligent terminals or terminal emulators, and for application software built on command structures like those defined in V.25ter. Some of the commands defined in Section 3 will also be useful for implementations of Section 2 and/or Section 4, for example enabling an indication of incoming SMS messages. Section 4 defines a character-based interface with hex-encoded binary transfer of message blocks ('PDU Mode'). This mode is suitable for software drivers based on AT command structures which do not understand the content of the message blocks and can only pass them between the MT and 'upper level' software resident in the TE. In all three modes, the terminal is considered to be in control for SMS/CBS transactions. This specification considers the mobile termination to be a single entity. Other GSM Technical Specifications describe the split of functionality between the mobile equipment and SIM.