Modem Configuration and Use

You can use modems for I/O communication between VTScada and a remote telemetry unit. They are also used for voice and pager calls by the Alarm Notification System.

Verify That Your Modem has Voice Support

If using a modem for the Alarm Notification System, you must choose one that has voice support. Manufacturer’s claims aside, you can verify this as follows:

1. Open the Windows® operating system control panel.
2. Open Phone and Modem options.
3. Select your modem.
4. Click on Properties
5. Select the Diagnostics tab
6. Click on Query Modem

The response will include a line that begins with:  AT+F class = ?

• If this line includes a  ,8    at the end, then you have voice modem support.

(e.g. AT+F class = ? 0,1,8)  

• If not, then your modem does not have the required voice support.

All modems are under the control of the Modem Manager, a VTScada service that enables management of modems distributed across a number of different workstations as a common pool. The Modem Manager Server and clients cooperate to provide modem pool management and to set up data paths between modem users and the allocated modems. The modem pool is managed using a time priority queue.

Outgoing calls are queued and are then executed at the requested time. You can receive calls on any modems, even on those on backup servers or clients. Call queue information is distributed to all copies of Modem Manager on a networked system so that queue integrity is maintained, even after switching to a backup server. If a call has not yet been established and server fail-over has occurred, the backup server that handles the Modem Manager's preserved queue will complete the call.

You must configure one Modem tag for each modem in the system. For use with the Alarm Notification System, leave the tag's area field blank. If using a data modem for I/O, you can force data calls to use that modem by configuring the area property to match that of its serial port and adding the application property, UseSerialAreaInModemCall, set to TRUE (1).

All modem access uses TAPI - the Microsoft telephony management layer that enables programs to share and pass calls. TAPI enables the sharing of telephony resources between multiple applications. It interfaces to a modem driver so that you do not need to configure VTScada for the specifics of the modem.

Both a modem audio driver and a TAPI Service Provider (TSP) are provided by Trihedral. These have been designed to avoid several of the problems associated with the older Unimodem V driver, including its limitation in detecting and switching between voice vs. data calls.

The Unimodem V Driver is still available, but will not display in the list of drivers unless you set the application property, UseUnimodem to 1.

if your system includes more than one modem, and those modems are attached to servers located in regions with different area codes, then use the canonical address format American Gas Association for each phone number and configure the server's location within the Windows™ Phone and Modem Settings dialog.

Some internal modems appear to enforce a "serial port" baud rate of 115200, even though they don't use a real serial port,  they just appear to be a COM port from a Windows software point of view. Because raw 16-bit linear audio at 8000Hz requires 160000 baud,  the decoder on the modem is starved for data,  and data accumulates in the ModemTSP queue. For this reason, you should avoid using the 16-bit linear format. Both μ-law and A-law are available on most modems and give excellent audio quality. With a data rate of ~80000 baud,  they are well under 115200 baud.