Digital Subscriber Line(DSL) and Various Flavors:

Is it Worth It?

Digital Subscriber Line is a technology that utilizes the pre-existing copper telephone wires in the average home or building. According to ZDNET, there are two major types of DSL, or digital subscriber line: "ADSL (asynchronous, where download speed is faster than upload) and SDSL (synchronous, where upload and download speeds match)" [1]. There are, in fact 9 types of DSL total. The key to DSL technology is that advanced types of modems exist today which are capable of handling the amazing potential bandwidth(more on this later). DSL is basically "a high speed data 'pipe' that can be used to transmit any high speed data application" [2]. For example, heavy duty graphics, ultra-fast Internet access, interactive multimedia, on-line home banking now can be accomplished more efficiently from home or from a small business. DSL speeds can reach the 1.544 Mbps threshold, similar to the levels many large companies have with their T-1 lines. The advantages are obvious then when considering DSL uses "POTS"(Plain Old Telephone Service") lines! This is advantageous because installation of high-bandwidth lines such as T-1 is very expensive.

 

This is not to say installation of DSL lines/modems is cheap. DSL-compatible modems can be quite expensive. On top of this, users need to pay for installation, as most companies, like Pacific Bell will not provide the service unless they do the installation themselves. The installation must involve access to the copper wires. "DSL Modem transceivers must be installed at both ends of the copper line -- one transceiver is placed at the customer premises, and another at the telephone company Central Office (CO) building" [3]. The secret to DSL technology is that the modems contain "advanced digital signal processing (DSP) algorithms that produce mathematical models of the distortions caused by the line and produce automatic corrections" [4] . This way, data is passed (in both directions) on the line. There is one rule of thumb when determining data rate: the faster the data rate on the line, the shorter the transmission distance. Other factors: "the gauge (thickness) of the cable, and the amount of interfering noise present"[5].

 

Another major factor with DSL is that it uses digital signals, not analog signals, such as the telephones and modems common today use(i.e. 56 Kbps). As a result, lots of congestion problems are, or can be, avoided via the digital signaling. Many variations of DSL have been developed, or are being developed for future use, especially due to the lack of rapid implementation of the alternative: Cable Modem. Some of the common alternatives include ADSL and SDSL. With ADSL, or Asymmetric Digital Subscriber Line, downstream data rates (i.e. downloading web pages) of up to 8 Mbps can be reached; on the other hand, upstream rates(i.e. uploading, or FTP-ing web pages, to a web server from home for one's web site) of up to 768 Kbps can be achieved. Related to ADSL somewhat is SDSL, or Symmetric Digital Subscriber Line, which "uses only one of the two cable pairs{in a POTS world}. It transmits only on a single-pair, usually at the 784 Kbps (half-T1) data rate" [6]. In other words, data rates for Uploading and Downloading are equivalent: ~784 Kbps. Because most home users utilize the downloading aspect of the Internet more so than uploading, ADSL is probably the better choice of the two. However, for a uploading-heavy business or individual, SDSL is worth investigating.

 

One will notice that a search on the web in the late 90's, and into the next century, reveals many sites advocating the benefits of DSL. Of course, many of these sites are put up, or are financially backed, by DSL providers. One example: "DslLife.com"(or, http://www.dsllife.com/ [7] ). I noticed this site after a basic search on the web. In its its "About DslLife" link, one reads: "DSL ... More than a phone line - It's a lifestyle......DSL Life is sponsored by the ADSL Forum [which] represents nearly 300 of the world's leading companies from all sectors of the global communication and computer industry. This non-profit membership organization was founded in 1994 and is dedicated to helping service providers and their suppliers realize the enormous market potential of ADSL and to speed its deployment" [8]. At least they are honest and upfront about their role, and "non-profit" status.

 

On the other hand, for a hard, non-biased view of DSL, potential DSL customers should read "DSL: Don't Be Duped" , at TechWeb. This article pulls no punches, and unlike the fantasy-like opinions of DSL providers, TechWeb discusses the pitfalls of DSL extensively. The article is rather technical and is aimed at companies, as opposed to home users. It is nonetheless advantageous to peruse before shelling out the money for home use as well. It begins with: "Service providers are pitching DSL as the way to blow through last-mile bottlenecks at a bargain price. Corporate networkers who aren't very careful could see the technology blow up in their faces" [9]. This sets the tone for what turns out to be a very educational article from people who have nothing to gain from either cable modem or DSL services(or standard analog for that matter). Some potential problems with DSL, per the article : it is "distance-dependent ....Don't expect service providers to volunteer this sort of information—even if they know. It's time to get out the maps and do the math" [10]. As a general rule of thumb, service should be within 3 miles of the telco's "Central Office". Other matters to watch for: DSL providers who are oversubscribing the backbones, which could negate the benefits of high data rates; interference between DSL and other services running on the same lines; the quality of the copper lines; finally, watch for DSL providers who overstate the potential and maximum bandwidths: "With DSL, there's a definite tradeoff between speed and distance: The farther data has to go, the slower it travels" [11].

 

*******************************************************

This Table Compares The Different Varieties of DSL [Link/Reference: 12 below in "References"]

References:

1. http://www.kcie.com/index.htm

2. http://search.zdnet.com/cgi-bin/texis/zdhelp/zdhelp/single.html?Ueid=914908&b=tipzone

3. http://www.orckit.com/fr_newsa.html?/knowledge.html

4. http://www.orckit.com/fr_newsa.html?/knowledge.html

5. http://www.orckit.com/fr_newsa.html?/knowledge.html

6. http://www.orckit.com/fr_newsa.html?/knowledge.html

7. http://www.data.com/roundups/duped.html

8. http://www.dsllife.com/

9. http://www.data.com/roundups/duped.html

10. http://www.data.com/roundups/duped.html

11. http://www.data.com/roundups/duped.html

12. http://www.orckit.com/fr_newsa.html?/dsl_faq2.html

Done for a Telecommunications Class

[CIS 130: The Teacher's Page]

@ City College of San Francisco