MODEM

T-DSL Modem

                  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 can be used over any means of transmitting analog signals, from light emitting diodes to radio. The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in thevoice 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 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. For example, the ITU V.21 standard used audio frequency-shift keying, that is to say, tones of different frequencies, with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which was able to transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.

SOFT MODEM
A Winmodem or softmodem is a stripped-down modem that replaces tasks traditionally handled inhardware 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. One downside is that the software generating and interpreting the modem tones is not simple (as most of the protocols are complex), and the performance of the computer as a whole often suffers when it is being used. For online gaming this can be a real concern. Another problem is lack of portability such that non-Windows operating systems (such as Linux) often do not have an equivalent driver to operate the modem.

A PCI Winmodem/softmodem (on the left) 

next to a traditional ISA modem (on the right)

List of dialup speeds

Note that the values given are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines). For a complete list see the companion article list of device bandwidths. A baud is one symbol per second; each symbol may encode one or more data bits.

Connection	Modulation	Bitrate [kbit/s]	Year Released
110 baud Bell 101 modem	FSK	0.1	1958
300 baud (Bell 103 or V.21)	FSK	0.3	1962
1200 modem (1200 baud) (Bell 202)	FSK	1.2
1200 Modem (600 baud) (Bell 212A or V.22)	QPSK	1.2	1980
2400 Modem (600 baud) (V.22bis)	QAM	2.4	1984
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	1984
14.4k Modem (2400 baud) (V.32bis)	QAM	14.4	1991
28.8k Modem (3200 baud) (V.34)	QAM	28.8	1994
33.6k Modem (3429 baud) (V.34)	QAM	33.6
56k Modem (8000/3429 baud) (V.90)		56.0/33.6	1998
56k Modem (8000/8000 baud) (V.92)		56.0/48.0	2000
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