alog and Digital Transmission a voice-grade analog circuit, bandwidth-limiting filters are put on that circuit to fil- ter out all frequencies above 4,000Hz. That's why analog circuits can conduct fairly low-speed data communications. The maximum data rate over an analog facility is 33.6Kbps when there are analog loops at either end. How 56Kbps Modems Break the 33.6Kbps Barrier With 56Kbps modems, only one end of the loop can be analog. The other end of the connection has to be digital. So, in other words, if you're using a 56Kbps modem to access your ISP, you have an analog connection from your home to the local exchange. But the ISP has a DSL or a digital termination facility from its location to its exchange. Analog facilities have limited bandwidth, which means they cannot support high-speed data. Another characteristic of analog is that noise is accumulated as the signal traverses the network. As the signal moves across the distance, it loses power and becomes impaired by factors such as moisture in the cable, dirt on a contact, and critters chewing on the cable somewhere in the network. By the time the signal arrives at the amplifier, it is not only attenuated, it is also impaired and noisy. One of the problems with a basic amplifier is that it is a dumb device. It knows only how to add power, SO it takes a weak and impaired signal, adds power to it, and brings it back up to its original power level. But along with an increased signal, the amplifier passes along an increased noise level and adds its own internal noise. So in an analog network, each time a signal goes through an amplifier, it accumulates noise. After you mix together coffee and cream, you can no longer separate them. The same concept applies in analog networks: After you mix the signal and the noise, you can no longer separate the two, and, as a result, you end up with a lot of noise and very high error rates. Digital Transmission Digital transmission is quite different from analog transmission. For one thing, the signal is much simpler. Rat