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Commercial modems largely did not become available until the late 1950s, when the rapid development of computer technology created demand for a method of connecting computers together over long distances, resulting in the Bell Company and then other businesses producing an increasing number of computer modems for use over both switched and ...
The modem uses the Data Carrier Detect (DCD) pin to signal whether it's connected to a host. The computer can generally just read the DCD pin at any time and always know if the modem is in command or data mode. DCD high means data mode, and low means command mode. DCD is a signal sent from the modem to the computer.
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The modem connects to a single computer or router, through an Ethernet port, USB port, or is installed in a computer PCI slot. The more common DSL router is a standalone device that combines the function of a DSL modem and a router, and can connect multiple computers through multiple Ethernet ports or an integral wireless access point.
A PCI softmodem (left) next to a conventional ISA hardware modem (right). A software modem, commonly referred to as a softmodem, is a modem with minimal hardware that uses software running on the host computer, and the computer's resources (especially the central processing unit, random access memory, and sometimes audio processing), in place of the hardware in a conventional modem.
An example handshake of a dial-up modem. Modern dial-up modems typically have a maximum theoretical transfer speed of 56 kbit/s (using the V.90 or V.92 protocol), although in most cases, 40–50 kbit/s is the norm. Factors such as phone line noise as well as the quality of the modem itself play a large part in determining connection speeds.
Cable modems can have a problem known in industry jargon as "flap" or "flapping". [23] A modem flap is when the connection by the modem to the head-end has been dropped (gone offline) and then comes back online. The time offline or rate of flap is not typically recorded, only the incidence.