What Kind of Modulation Is Used in High-Speed Internet Connections?
The speed obtained when accessing the Internet depends in part on the modulation protocols used. The media-intensive content of many websites requires high-speed connections for an optimal browsing experience. Asymmetric digital subscriber line, or ASDL, modems and cable modems use different methods of modulation to achieve high Internet speeds. The different protocols define how the information is encoded and the rate or speed of data transfer.
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Modulation Protocols
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The word modem comes from the words “modulator” and “demodulator.” A modem converts digital signals from a computer into transmittable signals destined for another computer. The method used to convert the signal is referred to as the modulation protocol.
ADSL
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ADSL technology is the most common type of modem technology in North America used over DSL, an alway-on connection using a telephone line that does not tie up your phone, as a dial-up connection would. The modulation technology used by ADSL modems includes discrete multitone technology (DMT), carrierless amplitude modulation (CAT) and multiple virtual line (MVL). ADSL connections use most of the available bandwidth for the downstream direction to send information to the user. The upstream bandwidth of an ADSL connection, on the other hand is limited, resulting in longer upload times.
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CAP and DMT
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CAP is one type of modulation used by ADSL connections. CAP uses signals with frequency ranges divided into three bands: downstream, upstream and voice data. MVL uses fast courier transform algorithms for both modulation and demodulation. However, neither method is used by the majority of ADSL service providers. DMT is the preferred method for ADSL signal modulation because it offers 256 frequency bands or channels separated in usable frequency ranges.
Cable Modems
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Modulation on cable modems converts digital computer information into radio frequency signals used for transmission. Cable modems use burst modulators because of the irregular nature of transmissions and Internet traffic. Quadrature amplitude modulation is the name for this type of modulation. QAM combines two amplitude-modulated signals into one channel. The signals separate during transmission and combine once reaching their final destination. Because cable modem signals travel over coaxial cables rather than phone lines, the speed is much faster than that achieved by ADSL modems.
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