Cellular phone technology is an analog radio-based technology originally developed for use with mobile voice phones for handheld and in-car use. For the phone to work, there must be a network of cellular towers to service a call area. Each tower creates a "cell" in which the phones operate. The cell is capable of handling a certain number of active calls per cell, each assigned to it’s own analog frequency.

Transmission level between the phone and the tower varies based on the environmental conditions, the phone in use, the equipment in the tower, the distance between the two, and any objects in between the two devices. In addition, other devices in the area create interference in the signal. This means that the cellular modem environment is an order of magnitude more complicated for modems to deal with than regular voice telephone lines.

If the modem is to be used in a truly mobile fashion, there is one more complication. Since the cells are arranged in an overlapped fashion, the can support a user is moving between cells and they can be "handed off" from one cell to another in the middle of a call. This hand-off is not usually noticeable by human callers, but it is enough to cause problems for cellular data use.

Cellular Modems require support for at least one of the protocols designed for use with a cellular modem. The two most common ones are ETC and MNP-10.

ETC (Enhanced Throughput Cellular) is the newer of the two protocols.

There are some additional details to remember when you are using a Cellular modem. The most important thing is that every call you make is under unavoidably different conditions; noise levels will vary, and if your location is not fixed, each call may be placed from a different cell. Your transmission levels will vary as they are received by the cellular tower, and your incoming signal strength will vary. Most external cellular modems should have a signal strength indicator, or else they will be attached to a cellular phone with a signal strength indicator. Making a voice call is a good idea if you can’t get a cellular connection working, because it will give you an audible idea of how noisy your environment is.

Cellular capable modems have a number of unique features, which you will need to experiment with to find the correct settings for your environment:

• Most cellular modems have automatic transmission level adjustment features, to compensate for the apparent rising and falling of signal strength.
• Special error correction and compression protocols are used, such as ETC (Enhanced Throughput Cellular) and MNP-10 from Microcom.
• Most cellular modems are limited to 1200 or 9600 bps data rates, depending on signal strength and noise conditions.
• Every modem has a carrier-loss hang up delay time, which is set to the amount of time the modem should wait after losing carrier before it hangs up. This should be set to a reasonably long delay for cellular use, since the carrier will be interrupted periodically by noise.

The programs you use over cellular connections must also be very tolerant. Even with error correction and compression, some noise will get through. This means that protocols have to be resistant to noise, use proper checksums, and have to be able to survive "blank-outs" where the modems cease data transfer for a short period of time, usually up to 30 seconds, because of extreme noise levels.

Even with all of this, cellular modems are a questionable proposition, and may be too unreliable for some applications. In the near future there may be better mobile connectivity alternatives, such as CDPD, and Digital PCS Modems.