of FTTC networks available today. xDSL Technology (how it works) XDSL signals are designed to maximize the rate of transmission of digital signals through non loaded twisted pairs, making use of bandwidths that can be greater than 1MHz, much greater than the 3000Hz or so allocated for voice transmission. There are several types of xDSL signal in commercial use today. Each signal type is implemented in circuitry with accompanying software, called a transceiver. The transceiver design includes the encoding or modulation scheme along with decoding or demodulation applied to convert serial binary data streams into a form suitable for transmission through twisted wire pairs. The transceivers may also employ various signal processing, equalization, amplification, and shaping techniques to adapt transmission for physical attenuation and phase distortions experienced by signals transmitted through twisted wire pairs. The transceiver software and circuitry may also use coding techniques to detect and correct noise that is present on a twisted wire pair. A variety of signal processing techniques have been developed over the past10 years to increase the bit rate of digital transmission through telephone loop twisted pairs. The following sections will describe these technologies. 2B1Q The DSL acronym was first used as shorthand to refer to the line code designed to support basic rate integrated services digital network (ISDN) transmission through twisted wire pair loops. The ISDN basic rate signal is required to carry an information payload of 144kbps, consisting of two "B" channels of 64kbps each and one packet data or "D" channel of 16kbps added for framing, error detection, and other overhead functions. The ISDN line of "U" interface operates at a raw data rate of 160kbps. In the mid 1980's the T1 committee in the United States created a standard U interface using a four-level line code referred to as 2B1Q for two binary bits per symbol carried ...
