Modems are typically most affected by the phone line conditions and the type of modem used.
So besides the distance to CO (phone company Central Office) you need to know the condition of lines in your locations and the type of modem you are using.
If a modem has a controller it's usually called hardware modem.
If it doesn't have a controller but has a DSP it's called a 'winmodem' by USRobotics.
If it doesn't have a controller or a DSP its called a softmodem.
A controller based modem ('hardware' controlled modem) is usually more resilient and maintains higher transfer speeds than a controllerless modem ('software', CPU controlled modem). Most newer modems are controllerless and may or may not have a DSP.
Avoid controllerless modems without DSP's -- don't get such a modem or buy a new one if you have such a modem. If you plan to travel alot, especially around the world, I recommend the Multitech modems. Xircom was bought out by Intel, but they don't display specs as much as they should so it's hard to find info about the Xircom Realport modems. Personally, I think the realport connection beats Xjack and all other dongles by a significant margin. Do your own research on the stuff.
Here are some other models with controller based modems.
Internal PCMCIA
USRobotics.com, model 3CXM756-US
USRobotics.com, model 3CCM756-US
Zoom.com, model 3075
External USB
Multitech.com, model MT5634MU
[this small USB modem has been around for a few years, so it's only V.90 capable but has some other limitations in protocols]
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Here are two PCMCIA modems without controllers but having a DSP and V.90, but no V.92/V.44 capability.
Multitech.com, model MT5634ZLXI
http://www.multitech.com/DOCUMENTS/DataSheets/415.asp
Actiontec.com, three models: MDV9012; FM560LK; 56K DataLink
Both of the above DSP modems (controllerless) reviewed below.
http://www.lucent.com/press/0398/980318.mea.html
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