This attribute indicates whether conversion between synchronous
and asynchronous data format (specifically, the stripping or
insertion of start and stop bits) is being performed. The value is on
unless the V.42 layer is active or it is a fax call. This attribute has
one of the following values:
on: Conversion between synchronous and asynchronous data is
being performed.
off: Conversion between synchronous and asynchronous data is not
being performed.