I get poor and erratic results using a cellular phone with my PCMCIA modem on my Mac Powerbook.

Cellular modem connections can suffer from a variety of issues such as:

Signal Strength
Even though the phone itself indicates a strong signal, and voice calls are fine, data connections may be unreliable. If possible, try moving to a different area, preferably away from buildings and towards a known cell site.

Connection Speed
Connecting at 4800 bps or 7200 bps is often more successful than either 2400 or 9600 bps. If you add the AT command %B4800 or %B7200 to the modem initialization string of your communications program, your modem will only try to connect at that speed. It won’t negotiate a speed with the other modem. This can significantly improve connections.

Sensitivity To Temporary Carrier Loss
Cellular connections undergo temporary interruptions much more often than traditional land-based phone lines. For example, when your connection is handed off from one cell to another (which can occur even when you are not moving), it’s not uncommon for the carrier signal to disappear for an instant and then be reconnected. This happens automatically, usually without your noticing.

Modems use the S10 register to determine how long to wait before permanently hanging up the connection when a loss of carrier is detected. The duration of carrier loss which a modem will withstand before giving up the host varies from modem to modem, but many of the modems you’re likely to connect to (such as those at online services) are likely to have a very short duration specified in the S10 register.

Even if your cellular modem’s S10 register has been set (via a connection script or options file) to be tolerant of long periods of carrier loss, if the remote modem to which you’re connecting to has a small value in the S10 register, your connection may fail often. If possible, ask the person in control of the other modem to set a larger value in the S10 register. Most modems allow a value from 1 to 255 (measured in tenths of a second). The default is often 14 tenths. Increasing this value to, say, 100 or more tenths.

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