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band width

See bandwidth.



bandwidth

(1) Computer people may use the term for capability and time. For example, "not enough bandwidth to get the job done" means not enough staff or time to do it. Its true meaning follows.

(2) The transmission capacity of an electronic pathway such as a communications line, computer bus or computer channel. Digital bandwidth is the number of pulses per second measured in bits per second (bps). For example, Ethernet transmits at different speeds, including 10 Mbps, 100 Mbps and 1000 Mbps (see Mbps and baseband).

When transmitting alternating frequencies, as with all wired analog, many wired digital and most wireless communications, the bandwidth is the difference between the highest and lowest frequencies, measured in cycles per second, or Hertz (Hz). For example, 802.11n Wi-Fi transmits in 20 MHz and 40 MHz channels within the 2.4 GHz and 5 GHz bands. The 20 and 40 MHz channel frequencies are the bandwidths, and each channel is divided into subchannels.

From Hertz to Bits - A Complicated Process
When using alternating frequencies for digital transmission, the frequencies are modified (modulated) by the digital input. Using the 802.11n Wi-Fi example, the resulting bit rate can range from 6.5 Mbps to 600 Mbps. This extremely wide range is determined by the signal strength and interference in the environment at any given moment. Any one of more than 30 combinations of channel bandwidth (20, 40 MHz), modulation scheme, error correction rate, channel spacing and number of antennas may be selected, on a packet-by-packet basis. See modulation, video bandwidth, space/time and bandwidth junkie.




Bandwidth in Hertz to Bandwidth in Bits
This quadrature PSK (QPSK) example is one of the simplest modulation schemes. Each set of two input bits modifies the carrier into four phase angles. The amplitude remains constant, unlike QAM modulation, in which the amplitude is varied (see QAM).














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