are used by a mobile application to control a wireless modem. The AT command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the parameters of the connection. The command set for GSM modems is specified in 3GPP specifications, TS 27.007 and TS 27.005 (for SMS-related commands). The standardized commands include some commands that are optional. Therefore, most wireless modem makers support most, but not all standardized commands. In addition, most modem makers include additional vendor-specific AT commands.