DSL, ADSL modem

Sunday, 21 January 2007

Communication systems are known to enable a plurality of communication devices to communicate among themselves and with communication devices in other communication systems. Such communication devices may be computers, modems, facsimile machines, printers, cell phones, and personal digital assistants. As the Internet continues to become more popular, there is an increasing need for reliably accessing and surfing the Internet at high speed and at low cost. The ever-increasing demand for high-speed data communications services and greater bandwidth is largely due to the popularity of the Internet and other data-intensive, high bandwidth applications. Both businesses and consumers are demanding higher bandwidth connections and faster Internet access. In particular, multimedia applications (which include textual, graphical, image, video, voice and audio information) have become increasingly popular and find usage in science, business, and entertainment. Local area networks (LANs) are essential to the productivity of the modern workplace; Ethernet-type networks have dominated the LAN market and have been continually enhanced (e.g., switched Ethernet, Fast Ethernet, and/or Gigabit Ethernet) to keep pace with the bandwidth intensive multimedia applications. A modem is a device that converts a desktop computer's digital data, for example, into audio-frequency analog signals to be transmitted over a telephone channel, and then back to digital form at the opposite receiving end. A modem may be connected to a telephone line, with potentially several telephone lines together providing connectivity to devices such as customer premises equipment (CPE), telephone sets and facsimile machines. Modems also include functionality to dial the line, answer a call, and control transmission speed. Modems provide part of the interface between subscriber lines and a digital network such as the public switched telephone network (PSTN). Standard analog modems (modulator / demodulator devices) used to be commonplace in the home and office environments, where they are used to enable electronic devices, such as personal computers, to transmit data over existing telephone lines to other electronic devices. Data is stored digitally in such devices, but is converted by an analog modem to be transmitted over the lines in analog form. Usually, a signal received on a local loop causes a corresponding signal to be transmitted on a telephone line and the receiving devices thus receive the signal on the telephone lines. Because the lines used by analog modems to transmit data only use a frequency range of about 0 3400 Hz, a data transmittance limit of about 56,000 bits-per-second (bps) exists for communication through the lines. As data and information transfer is becoming more and more voluminous and complex, using traditional data links such as analog voice-band modems is too slow in speed. The ability to provide such desired services as video on demand, television (HDTV), video catalogs, remote CD-ROMs, high-speed LAN access, electronic library viewing, etc., are similarly limited by the lack of high speed connections. The rapid increase in use and popularity of the Internet has motivated research and development of systems directed to advanced communication of information between remotely located computers, particularly in effecting higher bit-rates using existing infrastructure. One type of technology arising from this development is referred to in the art as digital subscriber line (DSL).

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.

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