At your typical Internet Provider, the connection from your computer to their computer goes through several cycles in order to accommodate current phone line technology. Illustration 1a shows what it takes with a normal Internet provider to be able to connect to the Internet.

1a

Computers are digital, in other words they understand 1's and 0's. Your modem turns these 1's and 0's into different tones which the remote modem listens to and converts them back to 1's and 0's. This is how modems transmit the data you receive every day on the 'net.

Problems with the above model

The phone company has used a digital network for their backbone for some time now. This is what ensures such excellent long-distance quality. However, the equipment from your telephone (or modem) is still analog until it reaches the CO. (Company Office) This varies from a few hundred feet to as much as several miles. The quality in these analog segments varies from carrier to carrier. As the old saying goes, a chain is only as strong as it's weakest link, in this case these weak links are the times when your data is being carried along these analog segments. Signals can (and do) degrade quickly over these analog segments. (sometimes phone equipment on these segments is as much as 40 years old)

(these are the problem areas we are talking about)