|DSL, ADSL modem|
|Sunday, 21 January 2007|
Digital subscriber line (DSL) is a modem technology that allows existing copper telephone lines to carry high bandwidth information. DSL technology provides one approach to addressing the demand for high-speed telecommunications service. DSL technology utilizes a modem both at a customer's location and a remote location, such as a service provider's central office. These modems allow communication between the customer and the service provider according to a DSL protocol. DSL modem elements permit high data rate transmission of data over the public switched telephone network (PSTN) at multiple megabit speeds using sophisticated signal processing techniques that permit voice and data to travel simultaneously over the same analog copper twisted pair wire. The frequencies at which data are transmitted and received according to DSL technology are higher than the frequency spectrum used for voice traffic on a standard telephone line. DSL modems typically perform three functions including voice and data separation, channel separation, and encoding and decoding. DSL uses one of a number of technologies to encode and decode data on a single carrier frequency. These technologies include quadrature amplitude modulation (QAM), carrierless amplitude and phase (CAP) modulation, and discrete multi-tone (DMT) modulation. Digital data is transported across the PSTN by converting the data into an analog signal that is transmitted by varying, or modulating, the frequency, phase, amplitude or other characteristic of a carrier signal. The modulation is performed by a DSL modem attached to a standard telephone line referred to as a "local loop". When analog signals are received from other DSL modems in the PSTN, the receiving DSL modem performs an opposite function by demodulating the received analog signal to convert it back into digital data. A DSL modem conventionally has a component referred to as a plain old telephone service (POTS) splitter, which divides the existing phone line to two bands, one for voice and one for data. Voice travels between zero and four kilohertz. The higher frequencies are used for data. Another component in a DSL modem, referred to as a channel separator, divides the data into two parts. Typically a DSL network comprises a plurality of customer premise equipment (CPE) devices connected to a DSLAM (digital subscriber line access multiplexer) via a bundle of twisted-pair wires. The DSLAM is also connected to a network for sending and receiving data to and from the respective CPE. The DSLAM may further be connected to other devices, such as routers, for directing and switching data through the DSL network. A DSLAM comprises a plurality of DSL modems which may be implemented in software residing on one or more digital signals processors (DSP). Certain DSL systems provide a downstream data transfer rate from the DSL point-of-presence (POP) to the subscriber location. Since the digital subscriber line provides a plurality of separate channels used for transmission of audio telephone signals, such as audio sound, fax, etc., the digital subscriber line serves high speed data communications to be transmitted and received or both the audio telephone signals and the high speed data communications to be simultaneously transmitted and received through the conventional telephone line. Digital subscriber lines (DSL) technology and improvements thereon including: ADSL (asymmetric digital subscriber line), VDSL (very high bit-rate digital subscriber line), SDSL (symmetric digital subscriber line), MDSL (multi-rate digital subscriber line), RADSL (rate adaptive digital subscriber line), HDSL (high bit-rate digital subscriber line), etc. all of which are broadly identified as X-DSL or xDSL have been developed to increase the effective bandwidth of existing subscriber line connections.
One type of DSL technology is referred to as asymmetric digital subscriber line (ADSL) and corresponds to the ANSI standard T1.43. ADSL is a high-speed communication technology that allows one client modem, such as that at a remote terminal (RT), to be connected to only one central office (CO) modem through one twisted pair telephone line forming a loop. Asymmetric digital subscriber line (ADSL) modems transmit high rate digital data over existing twisted pair telephone lines to connect residential customers to digital networks. A frequency duplexed ADSL modem transmits in one frequency band and receives in a second, disjoint frequency band. ADSL modems use two competing modulation schemes: discrete multi-tone (DMT) and carrierless amplitude/phase modulation (CAP). Discrete multitone (DMT) is a modulation technique commonly employed in various digital subscriber line (xDSL) communication systems. Discrete multi-tone modulation is an orthogonal frequency-division multiplexing (OFDM) technique based on modulating bits on a sequence of N baseband tones (sub-channels or bins), generally evenly spaced. A DMT line code comprises multiple bins or subchannels implemented through a discrete fourier transform (DFT). Each bin is independently modulated to some carrier frequency. The number of bins available to carry information is generally equal to or slightly less than half of the DFT size. ADSL technology is a popular choice for high-speed data transmission over existing telephone infrastructures. A digital modem operable under the ADSL protocol, used for data transmission, is about 100 times faster than a conventional analog modem having data transfer rate of 56 Kbps. Another advantage of ADSL is that a telephone line is still available for use as a voice connection or for an analog modem connection when ADSL traffic is passing through the line. This is made possible by splitters at both ends of the line that separate the ADSL signals and conventional telephone signals at either end. A host-signal processing (HSP) based ADSL modem implements most of its ADSL modulation functions via the computer processor. Therefore, it reduces the need for a separate high-speed digital signal processor (DSP). Through software implementation, it also has advantages for programming flexibility. It allows an easy software update for improved protocols and algorithms for better performance and new standards requirement.