Baseband refers to the original frequency range of a transmission signal before it is converted, or modulated, to a different frequency range. For example, an audio signal may have a baseband range from 20 to 20,000 hertz. When it is transmitted on a radio frequency (RF), it is modulated to a much higher, inaudible, frequency range.
Signal modulation is used for radio broadcasts, as well as several types of telecommunications, including cell phone conversations and satellite transmissions. Therefore, most telecommunication protocols require original baseband signals to be modulated to a higher frequency before they are transmitted. These signals are then demodulated at the destination, so the recipient receives the original baseband signal. Dial-up modems are a good example of this process, since they modulate and demodulate signals when they are transmitted and received. In fact, the word "modem" is short for modulator/demodulator.
While most protocols require the modulation of baseband signals, some can transmit in baseband without any signal conversion. A common example is the Ethernet protocol, which transfers data using the original baseband signal. In fact, the word "BASE" in "10BASE-T," "100BASE-T," and "1000BASE-T" Ethernet refers to baseband transmission. These Ethernet protocols do not require signal modulation. However, unlike broadband networks, baseband Ethernet networks are limited to single transmission channel.