Using Generic USB Controllersdows editions and service packs have brought improvements in usbser.sys, so
using a recent and updated Windows edition is likely to give the best results.
An alternate USB virtual COM-port driver from Walter Oney Software
(www.oneysoft.com) provides enhancements such as the ability for an applica-
tion to retain an open handle when a device is removed and re-attached. Other
sources for USB virtual COM-port drivers include MCCI (www.mcci.com)
and Thesycon Systemsoftware & Consulting (www.thesycon.de).
For telephone functions, the Windows Telephony Application Programming
Interface (TAPI) defines a standard way for applications to control telephone
functions for data, fax, and voice calls. TAPI manages signaling, including dial-
ing, answering, and ending calls and supplemental services such as holding,
transferring, and conference calls./ ' # #
The POTS model encompasses several models that vary in the amount of data
processing the device is responsible for. Each model supports different requests
and notifications. Processing can include modulation and demodulation, error
correction, and data compression.
As Chapter 8 explained, modulation is the process of encoding information by
varying the amplitude, frequency, phase, or a combination of these on a signal
to be transmitted. Demodulation is the reverse process of extracting data from a
modulated signal. In a basic wired, digital interface, the data doesn’t use a car-
rier or modulation. 1!
#
The most basic POTS model is the direct line control model, which in turn
encompasses the Direct Line (DL), datapump, abstract control, and telephone
control models.
In the DL model, the device converts between the phone line’s analog signal
and digital data, and the host handles modulation and demodulation, error cor-
rection, and data compression. The data uses an audio interface. The datapump
model is similar except the device handles modulation and demodulation, and
the data uses a vendor-specific interface.
In the abstract control model, the device handles modulation and demodula-
tion and detects and responds to V.25ter commands. The host or device can
handle error correction and data compression. The model supports requests