Cellular Networking Funadmentals

An analog signal is a continuously variable waveform (such as a sound wave) that can represent an infinite number of values. Wireline telephone systems use analog signals to provide voice communication. Analog transmission over phone lines, however, has some inherent problems when used for sending data. First, the digital signals of the sending computer must be converted to analog signals in order to be transmitted. The signals must then be converted back to digital signals by the receiving system. Second, analog signals lose their strength over long distances and often need to be amplified. Any distortion that exists in the signal becomes amplified as well, raising the possibility of errors.

In contrast to the waveform of analog signals, digital signals are transmitted over wireline connections by varying the voltage across the line between a high and a low state. Typically, a high voltage level represents a binary digit 1 and a low voltage level represents a binary digit 0. Because they are binary, digital signals are inherently less complex than analog signals; over long distances, digital signals are more reliable. If a digital signal needs to be boosted, the signal is simply regenerated rather than amplified.

Digital wireless connections vary slightly from the approach used with wireline connections. In digital wireless connections, modulation of a radio carrier signal, rather than voltage levels, is used to indicate a 1 or 0.

Like wireline phone systems, cellular networks initially offered only analog transmission of voice messages. Recently, however, digital transmission across cellular connections has become available and, because of its advantages, is quickly becoming the preferred method for data transmission.
Digital transmission across cellular connections is the basis of both CDPD as well as digital cellular and PCS systems.