All commands except two, must begin with the characters AT. The two exceptions are the escape sequence (+++), and the repeat command (A/). The command line prefix (letters AT) and the command sequences which follow, can be typed in upper case, or lower case, but case must not be mixed. More than one command can be typed on one line, separated by spaces if you wish for easier reading. The spaces are ignored by the modem's command interpreter. The command line buffer accepts up to 39 characters including "A" and "T". Spaces, carriage return, and any line feed characters do not go into the buffer, and don't count against the 39 character limitation. If more than 39 characters are entered, or a syntax error is found anywhere in the command line, the modem returns an ERROR result code, and the command input is ignored.
Extended AT Command Set Main Page