A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data.

Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps). Modems can alternatively be classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal.

Prior to the Hayes Smartmodem, dial-up modems almost universally required a two-step process to activate a connection: first, the user had to manually dial the remote number on a standard phone handset, and then secondly, plug the handset into an acoustic coupler. Hardware add-ons, known simply as dialers, were used in special circumstances, and generally operated by emulating someone dialing a handset.

With the Smartmodem, the computer could dial the phone directly by sending the modem a command, thus eliminating the need for an associated phone instrument for dialing and the need for an acoustic coupler. The Smartmodem instead plugged directly into the phone line. This greatly simplified setup and operation. Terminal programs that maintained lists of phone numbers and sent the dialing commands became common.

A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally handled in hardware with software. In this case the modem is a simple interface designed to create voltage variations on the telephone line and to sample the line voltage levels (digital to analog and analog to digital converters). Softmodems are cheaper than traditional modems, since they have fewer hardware components.

Direct broadcast satellite, WiFi, and mobile phones all use modems to communicate, as do most other wireless services today. Modern telecommunications and data networks also make extensive use of radio modems where long distance data links are required. Such systems are an important part of the PSTN, and are also in common use for high-speed computer network links to outlying areas where fibre is not economical.

Wireless modems come in a variety of types, bandwidths, and speeds. Wireless modems are often referred to as transparent or smart. They transmit information that is modulated onto a carrier frequency to allow many simultaneous wireless communication links to work simultaneously on different frequencies.

Transparent modems operate in a manner similar to their phone line modem cousins. Typically, they were half duplex, meaning that they could not send and receive data at the same time.

Smart modems come with a media access controller inside which prevents random data from colliding and resends data that is not correctly received. Smart modems typically require more bandwidth than transparent modems, and typically achieve higher data rates.

Wireless data modems are used in the WiFi and WiMax standards, operating at microwave frequencies.

Modems which use a mobile telephone system (GPRS, UMTS, HSPA, EVDO, WiMax, etc.), are known as wireless modems (sometimes also called cellular modems). Wireless modems can be embedded inside a laptop or appliance or external to it. External wireless modems are connect cards, usb modems for mobile broadband and cellular routers. A connect card is a PC card or ExpressCard which slides into a PCMCIA/PC card/ExpressCard slot on a computer. USB wireless modems use a USB port on the laptop instead of a PC card or ExpressCard slot. A cellular router may have an external datacard (AirCard) that slides into it. Most cellular routers do allow such datacards or USB modems. Cellular Routers may not be modems per se, but they contain modems or allow modems to be slid into them. The difference between a cellular router and a wireless modem is that a cellular router normally allows multiple people to connect to it (since it can route, or support multipoint to multipoint connections), while the modem is made for one connection.

ADSL modems, a more recent development, are not limited to the telephone’s voiceband audio frequencies.

Cable modems use a range of frequencies originally intended to carry RF television channels. Multiple cable modems attached to a single cable can use the same frequency band, using a low-level media access protocol to allow them to work together within the same channel.

Broadband modems should still be classed as modems, since they use complex waveforms to carry digital data. They are more advanced devices than traditional dial-up modems as they are capable of modulating/demodulating hundreds of channels simultaneously.

Voice modems are regular modems that are capable of recording or playing audio over the telephone line. They are used for telephony applications.

Connection Modulation Bitrate [kbit/s]
110 baud Bell 101 modem FSK 0.1
300 baud (Bell 103 or V.21) FSK 0.3
1200 modem (1200 baud) (Bell 202) FSK 1.2
1200 Modem (600 baud) (Bell 212A or V.22) QPSK 1.2
2400 Modem (600 baud) (V.22bis) QAM 2.4
2400 Modem (1200 baud) (V.26bis) PSK 2.4
4800 Modem (1600 baud) (V.27ter) PSK 4.8
9600 Modem (2400 baud) (V.32) QAM 9.6
14.4k Modem (2400 baud) (V.32bis) QAM 14.4
28.8k Modem (3200 baud) (V.34) QAM 28.8
33.6k Modem (3429 baud) (V.34) QAM 33.6
56k Modem (8000/3429 baud) (V.90) 56.0/33.6
56k Modem (8000/8000 baud) (V.92) 56.0/48.0
Bonding modem (two 56k modems)) (V.92) 112.0/96.0
Hardware compression (variable) (V.90/V.42bis) 56.0-220.0
Hardware compression (variable) (V.92/V.44) 56.0-320.0
Server-side web compression (variable) (Netscape ISP) 100.0-1,000.0

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