DSL MODEMS

DSL modems are very suitable gateway devices for boosting the Internet bandwidth available to appliances running on an in-home network. DSL modems support data transmission over standard telephone lines as much as 50 times faster than the analog modems used by most homes and small businesses.

A Digital Subscriber Line (DSL) circuit consists of two DSL modems connected by a copper twisted-pair telephone line. To maintain backward compatibility with the standard telephone system and to avoid disruption of service due to equipment failure, the voice part of the frequency spectrum is separated from the digital modem circuitry by means of a passive filter called a POTS splitter. This means that if the DSL modem fails, the POTS service is still available. Under this configuration, users are able to simultaneously make voice calls and transmit Internet data over the same broadband DSL pipe. When a DSL transmission is received at the central office, a more advanced POTS splitter is used to send the voice traffic to the public telephone network and data to the Internet.

DSL modems can be based on one of two different modulation schemes: DMT (Discrete Modulation Tone) and CAP (Carrierless Amplitude Phase).

DMT

The official standard for DMT is International Telecommunications Union G.992.1. DMT divides the spectrum from 4 kHz to 1 MHz into 256 components called subchannels. Each subchannel's noise level is analyzed, and information is sent along a subchannel according to its noise level. So if a given subchannel has a lot of noise, few or no bits are transmitted on it. DMT uses a discrete Fourier transform to split the frequency band into subchannels.

CAP

CAP is not a standard that is defined by any of the major international bodies; however, it was the first to market. CAP doesn't divide the frequency band into subchannels. Instead, it uses the whole band at once, relying on the amplitude modulation and phase shifts of signals to transmit at the high bit rates of xDSL. CAP is based on the quadrature amplitude/phase modulation (QAM) technique used by high-speed digital set-top boxes and cable modems.

CAP-based xDSL is easier to implement than solutions built on DMT, and so CAP equipment was used in early field trials of xDSL. But CAP has disadvantages—for a start, it's more susceptible to noise than DMT. Also, CAP is a proprietary technology, whereas DMT is defined in a standard. Consequently modems that use the DMT modulation scheme have been chosen as the standard line modulation for DSL in the future. Many manufacturers still build CAP-based modems; however, most vendors are building their current products based on the DMT standard. So if you are selecting a DSL modem, you need to make sure that it supports DMT; otherwise you can experience problems connecting to your local service provider. From a consumer's perspective, using a DSL modem as a gateway device to the outside world has become significantly easier over the past couple of years. In 1998, large PC manufacturers such as Dell and Compaq began shipping computers with high-speed DSL modems.