Asymmetric Digital Subscriber Line
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ADSL (Asymmetric Digital Subscriber Line) is one of the leading technologies for providing high-speed Internet service to subscribers. ADSL is a variant of DSL, and there exists a common misconception that the A stands for Asynchronous. This is incorrect, as ADSL is a syncronous servies. It is, however, asymmetric because the available upstream bandwidth is much less than the downstream bandwidth. Though there are some limitations imposed on the technology such as maximum bandwidth, and maximum distance from the Central Office, ADSL continues to be a competitive force in the Internet service market.
ADSL provides much greater bandwidth than traditional voiceband modems, but does so while utilizing the same copper phone lines.
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Introduction to ADSL
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History
Historically Internet was served to people's homes through the use of voiceband data modems connect to the existing copper telephone line network. This network consists of twisted pair copper wires which run from a subscriber's home back to the telephone company's central office. The voiceband data modem made use of the same frequencies as human speech, up to about 3.4kHz, and as such was only able to achieve speeds of up to 56Kbps. This also had the disadvantage of tying up the phone line whenever the modem was in use. ADSL makes use of other frequencies which are not needed by traditional telephony services, allowing an ADSL signal to exist on a copper line in harmony with a voice signal.
Traditionally, when ADSL was run to a home, a whole home splitter was installed at the demarcation point, the point where the phone line enters the home from the street. The splitter would filter all existing phone lines in the home, and provide an unfiltered line for use by the ADSL modem. This approached proved to be rather expensive for the ISP as they would have to cover the cost of the splitter, as well as the technician who would go out and install the device. Most ISPs have now adopted a much more passive stance wherein they ship the subscriber a number of microfilters which users will have to place on each of their telephones. While cheaper for this ISP, this can lead to degradation of service for the consumer if they have more than 5 voiceband devices in their home. The reason for this is that the branching of the various phone lines causes reflections which interfere with the ADSL signal. A whole home splitter solves this problem in that there are no reflections; the ADSL modem is connected directly to the splitter, and is alone on an unfiltered line.
The Technology
ADSL differs from standard DSL in that the data flow is asymmetric; data flow is greater in one direction. This is due to the observation that most people use the Internet in a fairly passive fashion. Requests are placed to Internet servers for websites or files, which are then downloaded to the subscriber's PC. Requests are much smaller than the files and websites they lead to, so there is little need for large upstream transmission capability. As the Internet grows, so too does the size of the files people are downloading, and the size of the websites they are visiting. It makes sense that users should have a greater ability to download and view these files. In fact, even though the upload pipeline is very small, only recently has its bandwidth been fully used. The advent of technologies such as Bit Torrent, which focuses on equal uploading and downloading, has finally begun to use the full upload bandwidth.
Functionality
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The Physical Setup
The physical setup of the ADSL system is fairly symetrical. The core of the setup is the DSLAM (Digital Subscriber Link Access Multiplexer), a device which is responsible for intermixing the voice and data signals on the downstream side of the connection, and for seperating out the voice from the data signal on the upstream side. On the upstream side, once the voice signal has been removed from the line, it is passed on to the phone company's telephony network. The data signal is passed onto the ISP's Internet backbone. On the downstream side, the DSLAM combines the voice and data signals coming from the phone network and the Internet, and transmits them down the twister pair copper line to the subscriber's home. There is a physical distance restriction impossed here, of about 5km. This is the maximum distance that a signal from a DSLAM can travel without degredation. Beyond this point, internet service may be intermittent, or slow, or just not possible. For this reason, ISPs will not provide ADSL to people living outside of the 5km service area of a DSLAM.
Transmission
Traditional voice telephony only utilizes a very small portion of the possible bandwidth in a copper wire. In fact, all voiceband communications fall within the range between 0 and 4kHz. ADSL makes use of the higher frequencies supported by the copper wire. There is an unused buffer area between 4kHz and 28.875kHz. The range from 28.875kHz to 138kHz is used for upstream transmission, while the region from 138kHz to 1104kHz is used for downstream. ADSL is a full duplex technology, which means that signals can be traveling in opposite directions at the same time, on a single wire.
Each of the two frequency ranges is further subdivided into smaller units, called channels, which span approximately 4kHz. When an ADSL link is first established the modem on the user end and the DSLAM on the provider end test the various channels on the line to establish which channels have the best signal-to-noise ration, a measure of how much interference exists in that channel versus how strong the ADSL signals are. Interference in a channel can come from poor wire quality, noise on the line, or even AM radio signals (a copper phone line can be thought of as a large antennae). Once the best channels are determined, these channels are used for communication. If the signal ever becomes degraded to the point that communication is no longer possible, the channels are re-evaluated. It should be noted, however, that the amount of available bandwidth decreases with each unusable channel.
The Future of ADSL
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In recent years ADSL has enjoyed a huge surge in popularity. As the Internet grows and the demand for streaming video and other such high bandwidth applications increases, voiceband technology has been all but abandoned in favour of Highspeed Internet. Due to the ease of installation, and the fact that it utilizes technology already present in every home with a telephone, ADSL has become extremely popular.
In the coming years, ADSL service providers will continue to expand their coverage. As things stand at the moment, ADSL is only available in densely populated areas, and even then there are gaps in the service. This stems from the fact that the ADSL signal degrades rapidly and becomes unstable at distances great than 5km. Service providers are solving this problem by running fibber optic lines from the central office to remote DSLAMs in locations where service was previously unavailable. The practice can also increase the speeds available to people who already fall within an ADSL service area by connecting them to a DSLAM which is closer to their home (for instance if they were previously being served from a DSLAM 4.5km away, and a new one is installed less than 1km distant).
Despite its recently surge, however, when one looks to the future of ADSL over the next ten years, the outlook does not look good. New technologies are emerging that could eclipse and even replace ADSL altogether. Some ISPs have already begun to roll out the successor to ADSL, ADSL2, which features greater bandwidth and speeds through the use of greater frequency ranges. However, even the new ADSL2 is eclipsed by the emerging VDSL (also known as VHDSL). VDSL, Very High Speed Digital Subscriber Line, provides speeds up to 100Mbits/s in both directions through the utilization of frequencies up to 30MHz. Though it is still an emerging technology in the Western world, VDSL has seen widespread adoption in Japan and South Korea where it is the predominant Highspeed Internet technology.
See Also
References
- http://http://en.wikipedia.org/wiki/Adsl
- http://electronics.howstuffworks.com/vdsl1.htm
- http://electronics.howstuffworks.com/vdsl3.htm
- http://www2.rad.com/networks/2005/adsl/main.htm
- http://www.esatclear.ie/~aodhoh/adsl/report.html
- http://en.wikipedia.org/wiki/VDSL
External Links
--Durandal 10:47, 9 April 2008 (EDT)
