Modem Arcana

What follows is a series of questions and answers about modems, which have been found to be of general interest. The answers to these questions have been drawn from various publicly available sources, as well as my own personal experience.

Q. What is a Winmodem¹?

A. Winmodem_ is a registered trademark of 3Com, but generally refers to several varieties of "controllerless" or "software" modems. Some functions of these modems are performed by the system's CPU (Central Processing Unit), rather than by hardware within the modem. For some modems (controllerless modems) the CPU only performs minimal control functions for the modem, resulting in a modest increase in load, while in others (software modems) the CPU must handle DSP (Digital Signal Processing) functions, significantly increasing the load on the CPU. These modems also require a software "driver" before they will work at all and are generally tied to a single operating system. It is generally recommended that these modems only be used in fast (133MHz or faster) Pentium systems. Operating system upgrades can be problematic with these modems, as can other software that modifies your system.

Q. What does V.90² mean, and how does it relate to X2 and K56flex?

A. V.90 is an international standard set by the International Telecommunication Union (ITU) describing the technical specifications for 56K modems. V.90 merged the two major competing proposals for 56K modems, USR's X2 and Rockwell/Lucent's K56flex, but is not identical to either. V.90 provides for receive speeds up to 56 Kbps. Your actual speed depends on line conditions and interoperability issues. V.90 is an asymmetric protocol, where the "downstream" speed (from your Internet Service Provider, ISP, to your modem) may be up to 56Kbps, while the "upstream" speed (from your modem to your ISP) is limited to V.34 speeds (up to 33.6Kbps). Some modems may support both V.90 and X2, while others may support V.90 and K56flex. It is unlikely that you'll find a modem that supports all three.

Q. Why don't I get a 56K connection with my modem?

A. There is no easy answer to this question, because there are so many factors involved. Some of these are:

• Power limitation to help prevent crosstalk, the FCC has limited the power a modem is allowed to output over the phone line. This power limitation has the effect of reducing your possible speed to 53Kbps.
• Noisy lines as the speeds go up, the modems are more sensitive to the condition of your phone lines. If you can pick up the phone and hear buzzing, crackling or other noise, chances are your connect speeds are going to be lower. If you have more than one phone or answering machine on the same line with the modem, you might try temporarily disconnecting them to see if your connect speeds improve.
• Multiple conversions V.90 only works when there is a single Analog to Digital conversion between you and the phone company's central office. If additional equipment such as DLCs (Digital Line Carrier) or SLCs (Subscriber Line Carrier) are present between your modem the central office, you may not be able to get V.90 connections at all.
• Compatibility issues even though any V.90 modem should talk to any other V.90 modem, the sad truth is that some parts of the V.90 standard have been interpreted and implemented differently by the manufacturers. If your modem has trouble communicating with a particular brand of modem, check the manufacturer's support site for any code upgrades that might address the problem.

Q. My modem says "connected at 115200": is it really talking that fast?

A. Your modem is not really communicating with the other end at 115200 bps. Your system is reporting the speed at which it is talking to your computer, not the distant modem. The two primary causes are:

• Your modem is set to report DTE speed rather than DCE speed. Many modems accept the ATW2 command in the initialization string to change this to DCE speed reporting. Check your manual.
• The modem reported a connection speed that isn't present in your system's INF file (for Windows 95/98), so it defaulted to the serial port speed. Download the latest INF file from your modem manufacturer's support site and install it.

Q. I acquired a cheap modem that doesn't have a manufacturer's name. How do I find out who made it?

A. If your modem has a FCC ID number (they all should), you may be able to look up the manufacturer on the FCC site:
http://www.fcc.gov/oet/fccid/

References:

The Navas 28800-56K Modem FAQ:
http://www.aimnet.com/~jnavas/modem/faq.html

V.90 Frequently Asked Questions:
http://www.V90.com/faq.htm

Curt's High Speed Modem Page:
http://www.teleport.com/~curt/modems.html

3Com V.90 Technology White Paper:
http://www.3com.com/technology/tech_net/white_papers/500659.html

¹ http://www.3com.com/legal/trademark.html

² http://www.itu.int/newsroom/press/releases/1998/98-04.html