This Global System for Mobile communications Technical Specification ( GTS) defines three interface protocols for control of SMS functions wi thin a GSM mobile telephone from a remote terminal via an asynchronous interface. Clause 2 defines a binary protocol (Block Mode). The protocol includ es error protection and is suitable for use where the link may not be completely reliable. It will be of particular use where control of rem ote devices is required. Efficient transfer of binary encoded user dat a is possible. Clause 3 defines a character-based interfaced based on AT commands ( Text Mode). This mode is suitable for unintelligent terminals or ter minal emulators, and for application software built on command structu res like those defined in V.25ter. Some of the commands defined in cla use 3 will also be useful for implementations of clause 2 and/or claus e 4, for example enabling an indication of incoming SMS messages. Clause 4 defines a character-based interface with hex-encoded binary t ransfer of message blocks (PDU Mode). This mode is suitable for soft ware drivers based on AT command structures which do not understand th e 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 SM S/CBS transactions. This specification considers the mobile termination to be a single ent ity. Other GSM Technical Specifications describe the split of function ality between the mobile equipment and SIM. The three modes referred to above, are represented in figure 0.1/GSM 07.05. The Block mode is a self contained mode in its own right, and when e ntered, control will remain within that mode until the procedures to e oit the mode are executed, after which control is returned to the V.25 ter command state or on-line command state. The Text and PDU modes are not in themselves V.25ter states but ar e simply sets of commands which will operate in either the V.25ter co mmand state or on-line command state. The Text and PDU modes ar e transitory states and after each operation, control is automatically returned to the V.25ter command state or on-line command state. W hilst in the V.25ter command state, the MS is available to handle inco ming and outgoing calls such as Data or Facsimile.