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 with 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 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 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), or bytes per second (B/s). 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 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 could 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. These values are maximum values, and actual values may be slower under certain conditions (for example, noisy phone lines).[7] 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[8][9] 2400 Modem (600 baud) (V.22bis) QAM 2.4 1984 [8] 2400 Modem (1200 baud) (V.26bis) PSK 2.4 4800 Modem (1600 baud) (V.27ter) PSK 4.8 [10] 9600 Modem (2400 baud) (V.32) QAM 9.6 1984 [8] 14.4k Modem (2400 baud) (V.32bis) trellis 14.4 1991 [8] 28.8k Modem (3200 baud) (V.34) trellis 28.8 1994 [8] 33.6k Modem (3429 baud) (V.34) trellis 33.6 1996 [11] 56k Modem (8000/3429 baud) (V.90) digital 56.0/33.6 1998 [8] 56k Modem (8000/8000 baud) (V.92) digital 56.0/48.0 2000 [8] Bonding modem (two 56k modems) (V.92)[12] 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
Posted on: Wed, 26 Jun 2013 05:40:48 +0000
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