A "null" string constant, or a string constant of zero length, is represented by two adjacent delimiters
("").
Compound Values
Actions may have more than one sub parameter associated with them, and parameters may
have more than one value. These are known as "compound values", and their treatment is the
same in both actions and parameters.
A compound value consists of any combination of numeric and string values (as defined in
the description of the action or parameter). The comma character must be included as a
separator, before the second and all subsequent values in the compound value. If a value is not
specified (i.e., defaults assumed), the required comma separator must be specified; however,
trailing comma characters may be omitted if all associated values are also omitted.
2.4.3 Action Commands
Action Execution Command Syntax
There are two general types of action commands: those that have associated sub parameter
values that affect only that invocation of the command, and those that have no sub parameters.
If sub parameters are associated with a command, the definition of the action command
indicates, for each sub parameter, whether the specification of a value for that sub parameter is
mandatory or optional. For optional sub parameters, the definition indicates the assumed
(default) value for the sub parameter if no value is specified for that sub parameter; the
assumed value may be either a previous value (i.e., the value of an omitted sub parameter
remains the same as the previous invocation of the same command, or is determined by a
separate parameter or other mechanism), or a fixed value (e.g., the value of an omitted
sub parameter is assumed to be zero). Generally, the default value for numeric sub parameters
is 0, and the default value for string sub parameters is
""
(empty string).
The following syntax is used for actions that have no sub parameters:
+<name>
The following syntax is used for actions that have one subparameter:
+<name>[=<value>]
The following syntax is used for actions that have two or more subparameters:
+<name>[=<compound_value>]
For actions that accept sub parameters, if all sub parameters are defined as being optional, and
the default values for all sub parameters are satisfactory, the data terminal equipment (DTE)
may use the first syntax above (i.e., omit the "=" from the action execution command as well
as all of the sub parameter value string).
If all other relevant criteria are met (e.g., the modem is in the proper state), the command is
executed with any indicated sub parameters. If <name> is not recognized, the modem issues
the ERROR result code and terminates processing of the command line. An ERROR is also
generated if a sub parameter is specified for an action that does not accept sub parameters, if
too many sub parameters are specified, if a mandatory sub parameter is not specified, if a value
is specified of the wrong type, or if a value is specified that is not within the supported range.
Action Test Command Syntax
The DTE may test if an action command is implemented in the modem by using the syntax: +<name>=?
If the modem does not recognize the indicated name, it returns an ERROR result code and
terminates processing of the command line. If the modem does recognize the action name, it
returns an OK result code. If the named action accepts one or more sub parameters, the modem
sends an information text response to the DTE, prior to the OK result code, specifying the
values supported by the modem for each such sub parameter, and possibly additional
information. The format of this information text is defined for each action command.
2.4.4 Parameter Commands
