An alternating electromagnetic signal with a steady frequency upon which information is superimposed by some form of modulation. The specific frequency at which the carrier signal runs is called the carrier frequency and is measured in hertz (Hz).
The modulation of the carrier signal enables information such as voice or data traffic to be integrated into the carrier signal. The carrier signal thus “carries” the voice or data information using modulation technologies.
The type of modulation used in digital communication systems depends upon whether the underlying carrier signal is analog or digital. For example, in digital radio or microwave communication, some form of digital-analog modulation, such as frequency-shift keying (FSK), is used to impose the digital (binary) information on the analog carrier wave. On the other hand, in Ethernet networking, a digital-digital encoding scheme called Manchester coding is used to enable the digital signal to carry binary 1s and 0s.
In Ethernet networks, the carrier signal plays an important role in the media access control method that Ethernet uses, namely the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method.