ADSL vision and history in .NET

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102 ADSL vision and history
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With traf c patterns that mirror those of high-speed Internet access, ADSL and its implementation began with a completely different market than originally envisioned Video over ADSL was the rst application considered a bene cial match for ADSL The pattern of requesting a video recording from a central site and its transfer to the subscriber is an asymmetric data transfer The system requires a small bandwidth level for image requests and a larger bandwidth actual image transfer This transmission pattern was documented and patented by Bell Atlantic Network Services in 1998 [1] While such a bene cial means of combining services onto the proverbial single cable was devised, the result was that the marketplace was not yet interested in video delivery via phone line Rather, in the mid 1990s, organizations like the ATM Forum were interested in the last mile delivery of high-speed Internet data traf c The World Wide Web was becoming an active force on the Internet, and the traf c patterns of web clicks and graphical downloads matched those of video-on-demand Therefore, ADSL was developed with a vision of delivering highspeed Internet access, much faster than dial-up modems and not requiring an additional wire at the customer s premises The Telecommunications Deregulation Act of 1996 aided in the deployment of ADSL in the public switched telephone network (PSTN), because it allowed
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A Comparison of the Current State of DSL Technologies
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service providers that were not Regional Bell Operating Companies (RBOC) to have access to central of ces and lines on the poles to deliver ADSL services Today, ADSL maintains a reasonable stronghold in North America and maintains a commanding market share in Europe and Asia With regulatory agencies, such as the Federal Communications Commission (FCC), and RBOCs providing more guidelines and details on the service delivery of ADSL, the options available to the end user subscriber become more diverse and robust in terms of bandwidth speeds and value-added service offerings
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103 ADSL technology
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Originally envisioned as a transport mechanism for video-on-demand applications, and currently positioned as the favored mass-market DSL solution for Internet access, ADSL is capable of downstream rates (towards the customer) from 384 kbps to as high as 8 Mbps in some cases Downstream reach can extend up to 18 000 feet at approximately 1 Mbps, depending on line quality In order to achieve these downstream rates, ADSL allocates less bandwidth to the upstream Typical upstream speeds (away from the customer) are in the range of 64 kbps to 768 kbps, depending on the reach ADSL technology, as de ned in the ANSI T1413 1998, ITU-T G9921 (Gdmt) and ITU-T G9922 (Glite) standards, uses discrete multitone (DMT) as the line coding protocol This method, pioneered at Bell Laboratories, divides the frequency spectrum of the line into equally spaced subchannels, often considered to be subcarriers, in that different carrier waves are used to base the coding for each subarea of the frequency spectrum [2 5] The useful spectrum extends from 0 Hz to approximately 11 MHz, being divided evenly into 43125 kHz bands, allowing 256 subchannels to deal with interference and rate adaptation Should there be a frequency interferer in a particular band, the two network ends can negotiate, recognize the interference and agree to disable communication within a speci c bin While maximum theoretical bandwidth is diminished in these situations, even in very noisy environments, ADSL will still function at a reasonably expected level Telephone wires are bundled together in multipair binders containing 25 or more twisted wire pairs As a result, electrical signals from one pair can electromagnetically couple onto adjacent pairs in the binder This problem is known as crosstalk and can impede ADSL data rate performance As a result, changes in the crosstalk levels in the binder can cause an ADSL system to drop the connection Other issues that can cause dropped or impeded connections include AM radio signal disturbers, temperature changes and water in the binder [6] Additionally, the maximum bandwidth levels that can be achieved in the eld are dependent on the cable distance between devices The longer the cable, the lower the bandwidth levels, due to the increase in signal-to-noise ratio (SNR) resulting from the physical resistance of the medium to the current ow [7] The key elements of an ADSL network include the central of ce component, Digital subscriber Line Access Multiplexer, or DSLAM, the cable channel and the customer premises equipment (CPE), or modem DSLAMs typically multiplex several ADSL modem lines into one channel on the digital side, while splitter technology, either internal or external to the DSLAM, separates the voice channel to the plain old telephone system (POTS), see Figure 101 Digital sides typically are asynchronous transfer mode (ATM) devices or Fast Ethernet devices Modern DSLAM implementations are turning towards the installation and support of Internet
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