Fundamentals of DSL Technology
The proliferation and ever-increasing demand for high-speed Internet access worldwide has necessitated the development of new technologies. DSL, which stands for digital subscriber line, emerged as one of the main ways to provide broadband Internet access to a huge segment of the population. In spite of advantages in terms of accessibility and lower costs, DSL faces stiff competition from the likes of cable providers and wireless technologies in the residential broadband market.
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How It Works
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DSL utilizes the existing phone line network, specifically the twisted copper wire connecting home and small office users to the telephone company central offices. DSL takes advantage of the copper wire using a technique called frequency division multiplexing (FDM). To understand this technique, think of a communication channel as a multi-lane expressway. What FDM does is place barriers between each lane so only a certain type of signal go exclusively on certain lanes, or in this case, sections of bandwidth. FDM achieves this by dividing each lane by the frequency of each type of signal (see Reference 4). While voice operates below the four-kilohertz (kHz) range, data through DSL can be sent at frequencies ranging from 26 kHz to one megahertz (MHz), according to consulting firm Core Competence (see Reference 2).
Advantages
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Using this technique, DSL essentially maximizes the capacity of copper cables to deliver broadband data and voice through the same channel. This makes DSL attractive to certain markets, because it enables delivery of high-bandwidth data rates to dispersed locations with relatively small changes to the existing telecommunications infrastructure, according to telecommunications giant Cisco (see Reference 1). Also, no additional wiring is necessary for DSL, only the equipment at the user's end and at the telephone company end of the cable must be upgraded to new equipment. Under DSL, each user has a separate network with the service provider, which minimizes potential intrusions and security breaks.
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Disadvantages
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All DSL connections experience a trade-off between the data rate and cable distance: the longer the cable, the steeper the data rate drop and the more unreliable the service. For instance, higher-speed DSL services sometimes require for clients to be within relative close proximity to the central office, and any DSL connection may be limited to a certain radius as well. DSL connections also are typically faster on the downstream, meaning the data that comes from the service provider to the end user than on the upstream. This may make the technology unattractive to people or corporations who need to upload large amounts of data such as video, according to Dsl-isp-info.com (see Reference 5).
Asymmetric DSL
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Different flavors of DSL exist. In ADSL, the "A" stands for asymmetric, meaning that the downstream data rate is much higher than the upstream data rate. Because average Internet users typically download much more information than they send, this makes ADSL ideal for Internet surfing and for services like video-on-demand. Downstream rates via ADSL range from 1.5 to 6.1 megabits per second (Mbps) while upstream rates top off at 640 kilobits per second (kbps). One mbps equals roughly 1,000 kbps. Although very similar to ADSL, another type called VDSL (very high-data rate DSL) operates a much higher data rates, topping off at speeds of 52 Mbps over distances of less than 1,000 feet, as well as higher upstream rates in the 1.5-Mbps to 2.3-Mbps range. Lastly, RADSL (the "RA" stands for rate-adaptive), although also similar to ADSL, has a feature that adjusts the transmission speed to the quality and length of the line, according to the McGraw-Hill Encyclopedia of Networking & Telecommunications (see Reference 3).
Symmetric DSL
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HDSL (high-speed DSL) operates differently than asymmetric DSL technologies in the sense that it has the same downstream and upstream speeds. HDSL speeds reach up to 1.5 Mbps both ways and can reach distances of up to 12,000 feet. HDSL-2, an updated version of the technology, has generally replaced the earlier version. Also, a new flavor of this technology, dubbed G.SHDSL, can reach higher speeds of up to 2.3 Mbps. Other technologies of this type include SDSL (Symmetric DSL) and SHDSL (Symmetric high-speed DSL).
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References
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