The distance from a switching station can also affect clarity. The modem works within these limits in creating sound waves to carry data across phone lines. But modem designers have become increasingly clever at squeezing more bits into that tonal range. Before transmission, these patterns are replaced with symbols or codes. The receiving-end modem takes the codes and reconstructs the original redundant bits, sending them to the receiving computer without any loss of information.
Modems use different tones to represent different bits of data.
Each set of tones is called a symbol. Baud measures the number of symbols, or discrete units of information expressed by phase and amplitude modulation, per second. The measurement is named for Jean-Emile Baudot, a Frenchman who developed a binary code in the 19th century. Modems were first widely used in the late 1960's. But the breakthrough that led to today's fastest modems was the idea of sending several tones at the same time, creating a mini-symphony of sounds in each symbol.