Cellular modems provide wireless data communication over worldwide cellular networks.
All modems modulate an original analog signal (such as a human voice) in order to transmit it to a specific destination. When the signal reaches the destination it is then demodulated back to the original signal. (The term "modem" is actually a portmanteau of these two concepts: "modulation-demodulation.") In cellular modems, the destination for the modulated signal is a cellular tower; the tower then retransmits the signal to cellular phones, tablet computers, and other wireless devices. The source for a cellular modem's signal is usually a computer or similar digital device.
Cellular modems may use one of several different network types, including Global System for Mobile (GSM), 3G, 4G, and Code Division Multiple Access (CDMA). In terms of form factor they may exist as external (requiring a physical computer port to operate) or as an embedded card within a computer's hardware.
A cellular modem within an industrial cellular network. Image credit: SixNet