CHAPTER 9 COMMUNICATIONS AND NETWORKS

WHAT IS A MODEM???

The word modem is short for modulator-demodulator. A modem 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 decode to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.

Modems are generally classified by the amount of data they can send in a given time unit, normally measured in bits per second (bit/s, or bps). They can also be classified by the symbol rate measured in baud, the number of times the modem changes its signal state per second.

The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrics signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.

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.

T-Mobile universal mobile
Telecommunications system PC card modem

ADSL modems, a more recent development, are not limited to the telephone’s voice band audio frequencies. Some ADSL modems use coded orthogonal frequency division modulation (DMT, for Discrete MultiTone; also called COFDM, for digital TV in much of the world.

DSL modem

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.

New types of broadband modems are beginning to appear, such as doubleway satellite and power line modems. Broadband modems still be classed as modems, since they use complex wave forms 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. Most broadband modems include the functions of a router (with Ethernet and WiFi ports) and other features such as DHCP, NAT and firewall features.
                                                                            
                          
sources from http://en.wikipedia.org/wiki/Modem