Recently, a Windows 2000 Magazine reader sent me the following question: I'm having a curious problem with RAS. I have a Windows NT Server 4.0 Service Pack 6a (SP6a) system that acts as a RAS server. The server has four USR 56Kbps V.90 modems. When I connect to the Internet from this server, I usually get a connection rate of about 49Kbps. However, when remote users access the RAS server, they get a connection rate that varies between 24.6Kbps and 28Kbps—occasionally connecting at speeds as fast as 33Kbps. How can I configure the server so that remote users can achieve higher speeds? This question reminded me that many misconceptions and myths surround 56Kbps-modem technology. I want to take this opportunity to set the record straight.
I'll start by explaining 56Kbps-modem technology's fundamental concepts. Although both of the modems involved in a modem-to-modem connection might be 56Kbps capable (e.g., V.90, K56Flex, x2-compliant), standard analog phone lines don't support these faster connection rates for modem-to-modem connections. To achieve 56Kbps speeds (or any speed greater than 33.6Kbps), your ISP must connect to the local phone company with digital lines that accommodate 56Kbps connections (i.e., the lines support either ISDN or V.90/56Kbps analog modem traffic). These special digital lines and the hybrid analog and digital connection that they create between you and your ISP have let modem manufacturers develop standards that support download speeds as fast as 56Kbps (e.g., K56Flex, x2, V.90, which is the vendor-independent international standard for 56Kbps). However, the upload speed is still subject to the aforementioned 33.6Kbps limit as a result of older V.34-style encoding, which is a predecessor to the current genre of 56Kbps-encoding schemes. Thus, when you connect your server's modem directly to an ISP that provides a 56Kbps-compatible pathway, you achieve a faster connection speed. However, modem-to-modem connections through RAS are capable of only a 33.6Kbps connection because the connection from the phone company's central office switch to the customer's site is usually analog rather than digital.
If you're wondering why 33.6Kbps keeps popping up, it's a result of Shannon's Limit. This communication theory determines the optimum achievable transfer rate of a given medium—in this case, a standard analog phone line—with a given signal to noise ratio. The limit for analog phone lines works out to be 33.6Kbps.
Another requirement to achieve a 56Kbps connection is that you can't have more than one Analog to Digital (A-D) conversion between you and your ISP's network access server. If you happen to be one of the unfortunate souls who connects to the phone company through a special type of analog circuit called a pair gain or line concentrator connection, or if you're connected through an internal PBX system, you might be out of luck. These types of connections introduce additional A-D conversions that can prevent you from reaching a 56Kbps connection.
Even if you're on a telco connection that doesn't suffer from these maladies, you're not guaranteed a 56Kbps connection. Factors such as noisy lines and buggy modem firmware can significantly reduce your maximum connection speed. You probably can't do much about noisy phone lines, but you can reduce the possibility of firmware-related problems by downloading and installing the latest firmware update for your modem. Depending on how long ago you purchased your 56Kbps modem—especially if you purchased it before the ratification of the V.90 standard—a newer firmware version might exist for your modem. The newer version might support additional standards and the ability to connect more reliably at higher speeds. Fortunately, you can download firmware updates for most modems from the vendor's Web or FTP site. Finally, you should be aware that current Federal Communications Commission (FCC) telecommunications regulations don't permit connections faster than 53Kbps, even if a full 56Kbps connection is possible.
If you're curious about whether your telco line has 56Kbps potential, 3Com U.S. Robotics provides a test line that lets you use your existing V.34 modem to dial in and discover whether your phone line can support 56Kbps. For information about what you need to use this line test, go to http://www.3com.com/56K/need4_56K/linetest.html.