To connect two computers via an analogue telephone line, a modem (MOdulator-DEModulator) is required at each end of the telephone line. The modem provides the interface between the digital computer system and the analogue telephone line. The modem accepts a bit stream from the computer and uses it to modulate a sine wave carrier on the telephone line. For incoming signals, the process is reversed, and the carrier wave is demodulated to reproduce the original stream of bits. The modem may be an add-on card mounted directly into the motherboard of the PC (using an ISA or PCI slot) or implemented as an external unit in its own case (see below). It is connected to a standard telephone wall socket via a cable. External modems connect to the PC via a DB9 or DB25 serial communication port, or a USB port. PCMCIA (Personal Computer Memory Card International Association) modems are designed for notebook computers.
External fax modem
Most PCI modems are software modems (or winmodems), which means they use the services of the system processor. ISA modems are called hardware modems, because they include the necessary electronic components to function independently of the PC's processor. A modem that can send and receive faxes is called a fax modem. Modern modems conform to the V.90 standard, and have a theoretical maximum download rate of 56 kbps (only possible due to data compression), with a theoretical maximum upload rate of 33.6 kbps. Due to the relatively low bandwidth of an analogue phone line (around 3 kHz), the standard signalling rate available to modems is 2,400 baud. The data rate achieved depends on the number of bits that can be represented per baud, but this in turn is dependent on line conditions, which are always less than perfect. Data rates of 48 kbps can generally be considered good. Modems negotiate a mutually acceptable connection speed when the connection is initially established, and can subsequently renegotiate connection speeds up or down, depending on the line conditions at any given time.
Internal fax modem