The word "modem" is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line.
Modem modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. This enables communication between two digital networks like computers through a analogue medium like a telephone cable.
The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back.
Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines.
When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070-hertz tone for a 0 and a 1,270-hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025-hertz tone for a 0 and a 2,225-hertz tone for a 1. Because the originate and answer modems transmit different tones, they can use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare.
In order to create faster modems, modem designers had to use techniques far more sophisticated than frequency-shift keying. First they moved to phase-shift keying (PSK), and then quadrature amplitude modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K modems, which actually connect at something like 48 Kbps on anything but absolutely perfect lines.
The next step in the evolution of the modem was asymmetric digital subscriber line (ADSL) modems. The word asymmetric is used because these modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000-hertz signal needed for your phone's voice channel. If both the phone company's central office and your house are equipped with an ADSL modem on your line, then the section of copper wire between your house and the phone company can act as a purely digital high-speed transmission channel. The capacity is something like 1 million bits per second (Mbps) between the home and the phone company (upstream) and 8 Mbps between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data.
The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000-hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe.
Today we use our modems to connect to an Internet service provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see How Web Servers and the Internet Work for details). Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP.
The standard technique for routing these packets through your modem is called the Point-to-Point Protocol (PPP). The basic idea is simple -- your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmissio
Answered by
Kannan
, an ibibo Specialist,
at
11:13 AM on February 19, 2008