Dsl modem modulation types

Written by jc torpey
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Dsl modem modulation types
Different types of modems and routers (cable modem image by IKO from Fotolia.com)

DSL is a Digital Subscriber Line, which is the technology used to connect the Internet to homes and businesses through regular telephone lines. A DSL line transfers both digital and voice information, meaning a subscriber can use the Internet and use the phone at the same time. A modem is the box the DSL line connects to that directs the signals to either the phone or the computer. The DSL signal directed to the phone passes through a filter that modulates, or changes, the DSL signal into the voice that we hear when we talk on the phone.

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DMT

DMT, or Discrete Multitone, is the modulation type most preferred. It uses the FFT, Fast Fourier Transform, algorithm to modulate and demodulate the signals. It then divides the DSL signal's spectrum of frequencies into 256 channels. This makes it easier for the DMT to work better in the presence of an AM radio signal. Each divided channel then uses the Quadrature Amplitude Modulation type, or QAM, which is the standard of digital cable in the United States.

CAP

Carrierless Amplitude/Phase Modulation is the first ASDL, or Asymmetrical Digital Subscriber Line. It is a technique that divides the DSL frequency range into voice signals, upstream data signals and downstream data signals. This method is no longer preferred since the advent of DMT.

G.DMT

This is the official ITU-T standard known as G-992.1.1, which uses a splitter instead of a filter to send the DSL signals to their destination while using the DMT method. This method allows for a higher rate of bandwidth. It can utilise speeds of up to 12 Mbits per second for a downstream signal and 1.3 Mbits per second as an upstream signal.

G.Lite

This is a version of DSL also knows as "splitterless" DSL, ASDL and Universal DSL, according to TechTarget.com. This is a DSL signal that runs slower through the lines, so it does not require a splitter or a filter of any kind on the user's side. However, in the phone company, where the signal originates, the signal is split remotely, saving the phone company time for not needing to go on a call to the customer's house or place of business.

HDSL

High Bit-Rate DSL is used for larger business sites. It is used to transfer wideband digital signal transmissions from the phone company to the customer. This is a symmetrical line, as the transmissions for both upstream and downstream signals are equal. However, the maximum total data transfer rate is much less than that of the ASDL lines.

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