Format to use for V.25bis protocol messages. You can modify
    this characteristic only when the entity is disabled. This
    characteristic is supported only when the characteristic
    connection type is non-switched. This characteristic applies only
    when the characteristic modem protocol type is v25bis or dmcl.

    asynchronous     Use the asynchronous format.

    hdlc             Use the HDLC format.

    synchronous      Use the synchronous format.

    The default value depends on the value of the characteristic
    communications mode. If communications mode is asynchronous, the
    default value of this characteristic is also asynchronous. If the
    value of communications mode is synchronous, the default value of
    this characteristic is hdlc.