22 What Kind of Modem Should I Buy?

Refer to the previous two sections.

We have always recommended external modems. In the past, the main reasons for this were that:

• They can be connected to any kind of computer that has a serial port.
• You can monitor the lights and speaker sounds for troubleshooting.
• They don't cause interrupt conflicts or address confusion, as internal modems almost always do.

In recent times, the reasons to stick with external modems are all the more compelling:

• Almost any recent-model modem is bound to have bugs and defects. Therefore it is better to keep it outside your PC, where it can't affect the internal bus, configuration, or interrupt structure of your computer.
• An external modems can be turned off and on to return it to its power-up configuration, as is often necessary when the modem becomes hopelessly confused. Internal modems can be power cycled only by turning your whole computer off and on.
• External modems are almost never "controllerless". To the best of our knowledge, all RPI modems, Winmodems, and other "software assisted" modems are internal PC modems.
• External modems are never "Plug and Play". Plug and Play modems need special OS-specific loaders to be initialized correctly. They can't be used with DOS applications, even in a Windows 95 DOS window. To the best of our knowledge they work only in Windows 95/98, and maybe to some degree also in Linux through the isapnptools software.

We do not recommend or endorse any particular brand of modem. However, we do recommend the following attributes:

• It should be external rather than internal. The extra price is worth it.
• It should follow established ITU-T (formerly CCITT) standards like V.32bis, V.34, V.90, V.42, and V.42bis. If a modem claims to "exceed" standards or "set" standards, beware; it is unlikely to interoperate correctly, or at all, with modems from other manufacturers.
• It must not depend on operating-system-specific drivers or loaders for any of its signal processing, modulation, error-correction, or compression functions. The operating system should be able to make full use of it through its serial-port driver, with the application providing the interface to the modem's command language. Thus you should be able to change or upgrade operating systems without losing the ability to use your modem.

Read the box carefully before buying.

56K modems are designed for only one purpose: to dial up an Internet Service Provider (ISP) that offers 56K service and has a digital connection to the telephone network. If they don't work for other purposes, this is not surprising, since they were not designed for any other purpose.

V.90 is the recently approved ITU-T standard 56K method, whereas X2 and K56flex are competing proprietary methods that preceded the standard. While V.90 is based on the other two, it does not include either one of them, and is only just now appearing on the market.

Basic connection problems can occur if:

• There is more than one analog segment (and therefore more than one analog/digital conversion point) in the telephone circuit between two 56K modems. Of course, you have no control over this. But it is likely if you are calling any host or service that does not have a direct digital link to the phone network. It might also be the case if you are calling out from a PBX, which can involve multiple A/D D/A conversions.
• There is only one analog segment, but it is too long or too noisy.
• Your modem tickles bugs in the other modem, or vice versa.
• Your modem and the other modem (which might or might not be a 56K modem) can not negotiate a common modulation or protocol. It takes more memory than most modems have to accommodate one or two 56K "standards" plus all the others (v.34 and below) and so essential fallback procedures might be lacking.

In theory, the modems should be able to recover from such situations automatically, and agree upon a lower modulation and connection speed. In practice, sometimes the modems become "frozen" or disconnect entirely. The most common complaint is that the modem makes the connection, but there is only a "blue screen" on the other end. That is, the modems are connected, the local modem reports carrier, but no characters are transmitted.

Performance is a totally separate question, and generally hinges upon the specific pair of modems and the connection between them.