DSL Connections - Digital Subscriber Line
By Erik Rodriguez
This article describes types, uses, and configurations of DSL connections.
DSL stands for Digital Subscriber Line. There are several different types of DSL, and they may not be available in all areas. Basically, your phone company determines what types are available.
History of DSL
The history of DSL is rather interesting. Initially, DSL was developed in the early 1990's by U.S. Incumbent Local Exchange Carriers. The original purpose for DSL
was to deliver video over existing copper lines. NOTE: The copper lines that run from the phone company to residential or commercial areas are sometimes called the
"local loops." In the early years of DSL, the economic benefit was not present. However, two key events raised the bar and, in turn, created a viable market for DSL. The first incident was the
"dot com" boom of the mid 1990's. The second event was the enactment of the Telecommunications Act of 1996.
The main step for the construction and infrastructure to support DSL technology was support by competitive local exchange carriers (CLECs). The competitive market
of broadband was expanding and cable companies were gathering a large share of the market. Cable has always had a larger market share and is not losing ground to
DSL because of the increasing number of cellular telephones. Consumers are canceling their land lines and using cell phones for long distance.
Future of DSL
By 2005, Yankee Group predicts that DSL will have 10.5 million U.S. subscribers.
While cable will have15.7 million U.S. subscribers. However, statistics provided by DSL
Forum state that by 2005, there will be approximately 55 million DSL subscribers worldwide.
On a more personal note, I spoke with a Telco engineer who had 33 years of experience in the telecommunications field. He claimed that with
the innovation of wireless networks, DSL will be an obsolete technology in the years to come. From my experience, cable connections seem to be more consistent than
DSL connections. The physical infrastructure of DSL has more factors to affect its speed and overall QoS (Quality of Service). Shown below is a table of various
"flavors" of DSL. Remember that these figures are VERY rough estimates of real-time performance.
||64 Kbps-1.54 Kbps Upload
256 Kbps-9 Mbps Download
||512 Kbps Upload
1.5 Mbps Download
|1.544 Mbps or 2.048 Mbps
(Upload and Download)
(Upload and Download)
(Very High Bit-Rate)
|13-52 Mbps Download
2.3 Mbps Upload
These figures indicate maximum speeds. In most cases, unless you are neighbors with the phone company, you won't get anything close to these speeds.
ADSL is the most common among home users today. The other types are geared towards businesses or other large campuses. HDSL is most like a T1 line. It can be
integrated with phone systems and offers a higher level of bandwidth. SDSL is aimed at users that need symmetric transmissions such as video conferencing. VDSL
can be very expensive.
Also, as I mentioned above, the physical infrastructure of DSL uses existing copper wire or "local loop." Often, the limitations of DSL for your area depend on how
far away you are from a DSLAM. DSL range cannot be extended using repeaters like T1 lines or packet-based networks because even though
DSL travels over the same physical wire as regular telephone conversations, data is sent over ultra-high frequencies that span above the audible
spectrum of a human ear. It would be very expensive to design a repeater that would boost only those frequencies (not to mention that each ISP may be using a different range of frequencies).
Running a Server
Running a server over DSL can go either way. If your DSL connection is static, it makes your life a whole lot easier. Bandwidth may vary, but I have
known of people having good results with their web/FTP servers. If your provider uses PPPoE, setting up out-bound services can be difficult. It most cases, your
ISP will take certain measures to stop people from running servers. These measures can include port filtering (blocking out-bound port 80 to prevent web servers),
stateful packet inspection, or bandwidth monitoring. Every ISP has a monthly bandwidth quota, and most of them also invoke a daily quota. Stateful
packet inspection is used so that a port is not completely closed. Traffic is "inspected" coming in and out of your connection to determine if it is communicating with a server.
DSL vs. Cable
From my experience, I would choose cable. Although DSL providers claim you get a "dedicated" bandwidth, I have seen better speeds from cable. Business class DSL
performs significantly better than standard ADSL. NOTE: The telephone company will try to BS you and claim that business class DSL requires them to install special
equipment. That is a lie; the only difference between standard and business service is a setting in the computer at the central
office. I guess that is how they justify charging almost double the price. The main advantage of cable is coverage. Nearly everyone can get cable unless you are in a very remote or rural area. In such a case, DSL would also be unavailable, and you would be stuck using satellite. I place an emphasis on the word "stuck."