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Improving Your DSL Connection
By: Gabor Bernat
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    Table of Contents:
  • Improving Your DSL Connection
  • The Theory
  • More Factors to Consider
  • Solutions
  • cFosSpeed
  • Conclusions

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    Improving Your DSL Connection - The Theory

    (Page 2 of 6 )

    DSL (Digital Subscriber Line) is a technology that provides digital data transmission over the wires of the local telephone network most of the time. This is achieved by dividing the transmitted frequencies into two major categories. The low frequencies assure the telephone's data stream while the higher ones are used for the DSL.

    In this way DSL can offer a bandwidth in the 512kbit/s to 24,000kbit/s range depending upon the type of technology. That's why DSL can offer some very high speeds, satisfying the ambitions of most users.

    DSL can be divided into two types. There is ADSL, which stands for Asymmetric Digital Subscriber Line, and SDSL, where the first S letter indicates, as you may think, Symmetric. With the first type, the upload/download bandwidth ratio is asymmetric (resulting in a higher download speed and a pretty low upload speed), while the second type offers the same speed for both of the data transfer actions.

    ADSL is more popular, for both marketing and technical reasons. The technical reason is that the asymmetric arrangement offers more than double the bandwidth of SDSL. The marketing reason is that, with ADSL, higher speeds can be advertised, and also most users download more often than they upload.

    ADSL and VDSL (Very high speed DSL) divide the "band" into two sections. The low "band" frequency (between 300 and 3400HZ) carries the voice and is named POTS (Plain Old Telephone Signal). The high "band" portion is reserved for DSL (between 10 and 100 KHz).

    On its own, DSL segments its band further into 4312.5 Hz channels. The channel allocation can continue up to 1.1 MHz for ADSL, until the the channels on demand are unusable. The speed of your Internet directly depends on the number of channels. The image below shows how it's divided for ADSL. PSTN stands for public switched telephone network, and is reserved for the use of the telephone.

    (Courtesy of Wikipedia.org)

    A filter is applied to the frequencies before they reach the telephone in a home with DSL, eliminating the DSL stream. This filter does not cause any interference or reduction in the quality of the phone signal received. Guided by this the idea you can easily see why you can speak on the phone and browse the Internet at the same time. The two have their own transfer "bands," and before they're used, filters eliminate the unnecessary data, processing only the streams  addressed to them. It's filter power all the way.

    The upper limit of the stream depends on the length and quality of the loop (the frequencies). Thus, you can see why the distance between where the data is streamed and the modem is so important. The higher frequencies travel a shorter distance. With the distance from the source (usually a DSLAM), some of the signal is wasted, and signal and speed are lost.

    DSL uses copper wire for optimal transfer quality. If the wire has faults the frequencies "travel harder" and again, signal will be lost. In an optimal situation, older ADSL provides an 8Mbit/s transfer rate for a 2km (1.25 miles) distance for unshielded twisted pair copper wires. The new ADSL2+ can do, under the same conditions, 24Mbit/s speed. Above this distance, bandwidth drops quickly and dramatically. This problem can be solved by installing ADSL loop extenders.

    (A DSLAM - Courtesy of Wikipedia.org)

    ADSL supports two methods of data transfer. The first is fast channel, which is best for streaming multimedia, where occasional bit drops can be allowed but lag is forbidden. The other method is the interlaced channel, designed for file transfer. No corrupted bit is allowed, and to prevent corruption the same bits can be resent, even with the cost of higher latencies.

    As for the potential hidden in DSL, you should know that in 2007 Dr. John Papandriopoulos (from the University of Melbourne) patented an algorithm that can boost DSL speed up to 205MBit/s. This said, we can conclude that we are talking about one of the most promising services.

    In most countries, copper wire connections have already been developed for the wire phone, and even with the cost of the additional equipment, it's still cheaper than creating a new optical fiber network from scratch. In 2004 the US started to see Dry-loop DSL after Qwest started offering it. This is also known as "naked DSL," consisting only of the DSL band; the POTS line is eliminated. This type of service has increased in popularity with the spread of cell phones, since many customers decided that they no longer needed landlines. 

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