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| Reference Series |
Table of Contents For This Issue
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| How Computers Work, Part I | |
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August 2001• Vol.5 Issue 3 Page(s) 144-149 in print issue | |
Speed Up Your Internet Connection With DSL See What Digital Subscriber Line Technology Has To Offer |
According to a study by DSL provider SBC Communications, nearly all DSL users (96%) consider their
high-speed Internet access an important household technology, more significant than the microwave (88%), remote control (87%), VCR (81%), cable TV (70%),
and the garage door opener (59%).
For several years, DSL (Digital Subscriber Line) technology has been touted as a prime candidate to meet the increasing worldwide demand for high-speed Internet access. Its user base has been growing exponentially. And according to leading technology research firm IDC, the worldwide DSL subscriber base jumped 447% between 1999 and 2000 to 4.5 million users. That number is expected to skyrocket in the next few years, as well to 66.4 million by 2004. With this incredible growth, it is not surprising that most Internet users have heard of DSL. Telephone companies around the world are selling it aggressively, running advertising campaigns that offer free modems, reduced rates, and other incentives to new subscribers. These marketing efforts are designed to give DSL an edge over its chief competitor, cable Internet access, by convincing potential customers of the technology’s superiority. What these campaigns often fail to do, however, is explain to new users exactly how DSL works and what they can expect from it. In fact, much of this information is designed to do exactly the opposite—to convince users that an appealing, yet imperfect, technology is a panacea for all their Internet access woes. Is DSL an improvement over standard dial-up connections? Most definitely. Is it always the best solution for every user? Maybe not. In the next few pages, we’ll explore the inner workings of current DSL technology and provide you with information to help decide if it is the right solution for you. We will also take a peek at exciting new developments that are just over the horizon.  Broaden The Information Highway.
DSL is one of a family of communication technologies known as broadband. That term has become synonymous with high bandwidth, (
bandwidth being the amount of information that can move through a connection during a given amount of time). In its most precise definition, however, broadband
communication is that which can carry several channels or data streams at once.Standard telephone connections, for example, are not broadband because they can carry either voice or data, but not both at the same time. Cable connections are broadband, because they carry hundreds of television and music channels, and now Internet connections, at one time. DSL connections are also broadband because they carry both data and voice signals. And all broadband technologies are faster, with higher bandwidth than dial-up Internet access, which may explain why the broadband definition has shifted somewhat. For the average user, the important thing to remember is that broadband connections, including DSL, offer an efficient method for users to increase the speed of their Internet connections while piggybacking them with other data feeds. In the case of DSL, that speed can theoretically be 8.448Mbps (megabits per second), which is nearly 200 times faster than a 56Kbps (Kilobit per second) modem. We say theoretical, because current FCC (Federal Communications Commission) restrictions limit the maximum speed of consumer-level DSL to 1.544Mbps, up to 50 times faster than 56Kbps. Even though companies sell modems with theoretical speeds of 56Kbps, the FCC restricts transmission speeds to 33.6Kbps for upstream (requests and data going from the customer to a Web server), and 53Kbps for downstream (Web site information and files being sent from a Web server to a user). The 1.544Mbps figure given above for DSL is for downstream delivery only; consumer-level DSL is an asynchronous technology (its true acronym is ADSL), which means the send and receive speeds are different. This allows a larger portion of the bandwidth to be set aside for data delivery and downloads, which are the areas of greatest need. On the upstream side, most DSL providers set the connection speed at 128Kbps or less, which is around three times the speed of an upstream request on a 56Kbps modem. This is fine for most users, but those who frequently upload files to a Web server or transfer them to other users may find their upstream connection speeds a little disappointing. Even on the downstream side, the technology has limitations that prevent many users from attaining the highest possible connection speed. (We will discuss these later in the Ups & Downs section.) Depending on their location and line quality, some users may experience connection speeds as low as 150Kbps. This is still three times as fast as the speediest dial-up modem, but it is a far cry from 1.544Mbps, which is 10 times faster. A Union Of New & Old.
All broadband technologies, including DSL, utilize a fairly new development known as end-to-end digital transmission of data. (All the data in a computer is digital; a series of ones and zeros. With the exception of dedicated high-speed networks, the networks
on which the data has traveled have not been digital.) Ironically, in the case of DSL, however, this digital revolution is taking place across a technology that has been available for more than a century. That technology is the standard telephone line,
which has euphemistically been dubbed POTS (plain old telephone service) line by the telecommunications community. This marriage of new and old is giving DSL an edge that service providers are using to speed its deployment worldwide. Other broadband systems, such as cable, satellite, and wireless, can become operative only after substantial expenditures have been made on the proper infrastructure. For DSL, however, the capability to put the power of digital feed to work has been in place all along. The ability to route a broadband transmission over existing telephone lines is one of the most confusing aspects of DSL. If broadband can run over existing telephone lines with its combined voice and data and higher bandwidth, why have the majority of Internet users been slogging along for the past decade, toggling between data and voice connections and cruising the Internet at a snail’s pace? The answer lies in the digital transmissions mentioned earlier. When we speak, our voice creates a sound wave that converts to an electromagnetic wave for transmission across a telephone network. All electromagnetic waves, whether telephone, radio, or microwave, are analog. This means they travel in a continuous flow or a wave. These waves are described by the speed at which they cycle, which is determined by how they move from peak to valley within a given time. (This isn’t the same speed at which the wave travels, because all waves travel at the speed of light. This is a measurement of how closely the cycles are packed together.) This measurement, the number of cycles per second, is called a hertz. The specific hertz measurement of a particular wave is called its frequency, and the total range of all possible frequencies is called the spectrum. All electromagnetic waves travel within a different range of frequencies or a band, hence the term bandwidth. Some, such as microwaves and gamma waves, have very high frequencies and are measured in gigahertz, which are equal to 1 billion cycles per second. The range of frequencies for voice signals have to match that of human speech, which is very low and cycles very slowly; between 0Hz and 3,400Hz. High-frequency waves not only cycle faster, they can also deliver more data in a shorter period of time than low-frequency waves. Since voice signals are at the bottom of the spectrum in terms of cycle speed, they are unable to carry data very rapidly. The physical limit of voice signals is 33.6Kbps. (A 56Kbps modem achieves faster speeds using sophisticated compression technologies.) At the time telephone networks were initially created, none of the high-bandwidth technologies we know today had been conceived of yet, let alone invented. Consequently, telephone equipment was designed to handle only this portion of the spectrum. When digital data transmission became a practical necessity for computer users, data files were still very small, and it seemed logical and efficient to use existing telephone networks, since at the time they were perfectly adequate for data transmission. Modems were developed to do just that—convert, or modulate, digital signals into a format that can travel across an analog network and then convert the signals back to digital on the other end. (This explains why modem stands for modulation-demodulation.) When the Internet came along with streaming video and audio, real-time conferencing, and large file downloads, this antiquated snail’s pace transmission system was suddenly outdated. Enter the lowly copper telephone line. Copper is highly conductive and can efficiently transfer very high frequency waves, up to 2MHz (a megahertz equals 1 million hertz) in many cases. Digital transmissions are not continuous. They are broken into packets. That means they can be distributed across a wide range of frequencies to achieve the most efficient transfer possible. Since neither digital data nor copper lines are restricted to the miniscule frequency band used for voice signals, there was no reason current technology could not be put to better use. Developers recognized this potential and began working on modems and equipment that could process digital transmissions and distribute them across the copper lines directly, rather than within the framework of the voice network. An added benefit would be that since the transmissions would move across a much higher portion of the spectrum than voice transmissions, there would be no concern about crossover or interference. This meant they could travel concurrently with voice signals, and DSL broadband was born. Going The Distance.
Now that you know it’s possible for DSL to travel across existing networks, perhaps you’re curious to know how it gets there and what happens once it arrives.Like telephone signals, DSL transmissions are sent to your home from a central office that houses the main telephone equipment. These facilities are little more than rooms full of switches and cables that route calls and data from one place to the next. Telephone signals arrive at the central office via either standard copper lines, or more commonly now (depending on the sophistication of your telephone company’s equipment), via high-speed fiber-optic cable. In the case of DSL, however, the Internet connection is sent from the ISP’s (Internet service provider) servers to the central office along a high-speed access line that is tied into the system. Since traditional telephone switches are designed to handle the low-bandwidth analog feed we discussed earlier, the telephone company builds special switches that can handle this high-speed digital data stream. Other equipment combines the data stream with the voice signal, scrambling the data portion, so when it arrives at your home you can pick up the phone and send and receive calls normally. A special DSL modem (either internal or external) attached to your computer receives this dual feed, strips away the voice signals, unscrambles the data feed, and provides you with data access. It also splits the feed into two channels, a slower one for upstream and a faster one for downstream. The upstream requests are processed in the same fashion as downstream information, moving from your home to the closest switching office, to the server, and then to the Internet. On a related note, a newly developed technology called the splitterless DSL technology separates the voice and data feeds at the telephone office, eliminating the need for special installations. It has recently been made a standard, but it’s not yet widely in use for consumer installations. When using DSL, data delivery is not continuous. However, the connection is. In other words, no information will be exchanged unless a valid, recognized data request is initiated on either end of the connection, in much the same as with a network. Data requests can be automatic, however. Since the user does not have to log on, he can leave his browser and e-mail client active and set them to perform tasks, such as checking e-mail or downloading files automatically. This continuous connection feature of DSL is very convenient, but it can present a security risk. Malicious parties can potentially access your computer and its files at any time with a DSL connection, whether or not you are browsing the Internet. Your system will be particularly vulnerable if you have enabled file sharing but failed to create passwords. Ups & Downs.
DSL has a number of advantages, the greatest of which is probably its ability to run across existing networks. It also has drawbacks, of which potential subscribers should be aware. Unlike cable Internet access, where all users within a given local area actually share one high-bandwidth connection, DSL subscribers are not part of a giant network. With DSL, the feed is dedicated for each user and no bandwidth is shared. This is beneficial for two reasons. First, the data is secure. Since cable subscribers share one big connection, it is possible, though unlikely, for one user to hijack another user’s transmissions. Secondly, since the feed is dedicated, a DSL subscriber’s connection speed will be fairly consistent. With cable Internet access, if a user is online at a time when there are only a few people using the network, he might achieve connection speeds much faster than can be achieved with DSL. During times of high traffic, though, the reverse could be true. Now for the bad news. At the beginning of this article, we told you that a user’s connection speed would depend on location and line quality. Line quality is a factor, but it is not really a variable. Users with newer, heavier-gauge phone lines will have speedier connections, period. Location, however, is a serious consideration for the potential subscriber. The strength of any electromagnetic signal degrades over time and space, which is why you cannot pick up a New York radio station in California. Low-frequency transmissions, such as voice signals and AM radio broadcasts, travel better than high-frequency ones. Therefore, voice signals travel more efficiently than high-frequency digital data feeds. Furthermore, small amplifiers called loading coils help voice transmissions along their route. These amplifiers are incompatible with digital signals. Consequently, when DSL feeds begin to deteriorate and data is lost, the equipment that processes the packets must resend some of the lost information, consuming more bandwidth and slowing the connection. If you are in the immediate vicinity of a central office, you might be able to receive data at rates up to the 1.544Mbps as mentioned earlier. The farther away you travel, the slower your connection, until a limit of 18,000 feet (5 kilometers) is reached. After this point, the signal degrades to the point that it is undecipherable. Some ISPs use digital repeaters to pick up the data feed, amplify it, and transport it even further, but signal degradation can still be issue. Will the telephone company tell you whether you location is prime for DSL? Probably not, but you should request some guarantee of average connection speeds and be allowed to cancel your contract if the reality does not match your expectations. A Glimpse Of The Future.
If your location makes DSL impractical, or if what you have heard so far leaves you unimpressed, do not write DSL off entirely. A new generation of technology is on the horizon.Called SHDSL (Single-Pair High-Speed DSL), this new technology was recently standardized and approved by the ITU (International Telecommunications Union. The ITU publishes the commonly accepted standards for communications technologies). Unlike standard consumer DSL, SHDSL is a symmetric transmission technology, which means upstream and downstream data delivery rates are the same. In the case of SHDSL, these rates can vary from 192Kbps to as high as 2.320Mbps, depending on the user’s distance from the central office. With dual-feed lines, one for sending and one for receiving, transmission rates of up to 4.72Mbps are possible. This blasts through the old limit of 1.544Mbps, thanks to new developments in equipment that make these speeds possible. In addition, symmetric data delivery will make two-way technologies, such as high-speed video conferencing, much more viable. (With asymmetric data delivery, the sending party cannot transmit data as fast as the recipient can receive it.) SHDSL also offers better signal strength across greater distances than other DSL technologies, further enhancing its appeal. SHDSL is not the first new version of DSL. A number of versions have been available for several years, including the consumer version discussed here and several others used in business applications. SHDSL is more exciting than other versions because it is spectrally compatible with all prior DSL technologies, so it can be deployed across existing networks and used in tandem with older technologies. SHDSL is also a variable-rate technology, meaning providers will be able to offer a variety of services operating at different speeds without any equipment changes or line upgrades. The SHDSL technology is currently being developed only for business applications. Specifically, it is designed to replace leased 1.544Mbps T-1 lines, making ultra-high-speed connections more affordable for small businesses. As with the great majority of business applications, however, once SHDSL has become accepted in business, there is an excellent chance it will make its way down to the consumer. If this happens, Internet users will be able to enjoy connection speeds of which most current users only dream. Imagine watching a movie across the Internet, full screen and in real-time, with perfect sound and video quality. These innovations, and more, may become possible in the near future with SHDSL. Final Thoughts.
Even though cable currently enjoys a huge lead, DSL technology is considered by many to represent the future of high-speed Internet access. Cable connections are present in approximately 70% of U.S. homes, according to telecommunications research firm Insight
Research, but are very rare in business environments. Telephone service (and potentially DSL), on the other hand, is ubiquitous across the board. It is present in virtually all businesses, and by definition it is already present in 100% of homes that
currently access the Internet via a dial-up modem. Furthermore, according to Insight Research, telephone companies in the United States have already invested $170 billion in telephone cabling, but many cable companies still need to conduct digital upgrades to their equipment. Consequently, the “roll-out” of DSL systems may soon outpace that of cable. DSL has the distance limitations we mentioned earlier, but it enjoys the advantage of consistency. Once you have achieved connection at a certain speed, chances are good your service will continue to fall within that range. Unlike cable connections, you will not suffer if the company adds 10 subscribers or 10,000 subscribers. And unlike satellite and fixed wireless Internet access, you will not be affected by weather disruptions or line-of-site problems. Finally, more than one telephone company may offer DSL in your area, whereas you probably have only one cable provider. If the service is much better or prices are much lower with one or the other, you can change telephone providers (and telephone numbers) to avail yourself of that advantage. If DSL, or any broadband technology for that matter, appeals to you, be sure to visit DSL Reports (http://www.dslreports.com). It is an excellent resource Web site that offers reviews of service providers, news digests about broadband, and other interesting information. 
by Jennifer Farwell View the graphics that accompany this article. (NOTE: These pages are PDF (Portable Document Format) files. You will need Adobe Acrobat Reader to view these pages. Download Adobe Acrobat Reader) View the chart that accompanies this article. (NOTE: These pages are PDF (Portable Document Format) files. You will need Adobe Acrobat Reader to view these pages. Download Adobe Acrobat Reader) View additional graphics that accompany this article. (NOTE: These pages are PDF (Portable Document Format) files. You will need Adobe Acrobat Reader to view these pages. Download Adobe Acrobat Reader)
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