DSL - Digital Subscriber Line
(Located at http://library.advanced.org/27887/gather/advanced_technology/dsl.shtml)
DSL is a technology capable of providing high
bandwidth-information to individuals and business users by means of ordinary copper
telephone lines. DSL installations began in 1997 and have since become increasingly
popular with customers desiring a fairly inexpensive and fast connection. Depending often
on ones distance from the telephone company, DSL may be able to achieve up to 6 or 7
megabits on the downstream, allowing users to experience continuous transmission of motion
video, audio, and 3-D effects. Different types of DSL offer different types of rates. xDSL
refers to the family or type of DSL, such as ADSL, HDSL, and RADSL. With DSL, you need
only a single line for all your telecommunication needs.
Technology
Traditionally, phone lines have been used to transmit data such as a fax and phone using
an analog signal. Since the Telephone Company already has existing analog systems set up,
it’s easier to use simply use that system to transfer data from your computer modem
to the Telephone Company. When data is transferred from your home or business modem, data
is sent to and from the telephone company in an analog signal, it’s the
responsibility of the modem to convert analog into digital data, or vice versa. The thing
about DSL is that it assumes that data does not require this constant change from digital
to analog and back. Therefore, digital data is transmitted to your computer directly as
digital data, allowing the Telephone Company to use a wider bandwidth for transmitting.
Essentially, DSL takes existing voice cables that connect customer premises to the phone
company's central office and turns them into a high-speed digital link.
A DSL modem has a chip called a POTS splitter, which divides the existing phone line into
two bands: one for voice and one for data. Voice travels on the first 4kHz of frequency.
The higher frequencies—up to 2MHz depending on line conditions and wire
thickness—are used for data. Another chip in the modem, called a channel separator,
divides the data channel into two parts: a larger one for downstream Internet data and a
smaller one for upstream Internet data.
At the other end of the phone line, 18,000 feet away at most, is another DSL modem,
located at the phone company's central office. This modem also has a POTS splitter, which
separates the voice calls from the data. Voice calls are routed to the phone company's
public switched telephone network (PSTN) and proceed on their way as usual.
Data coming from your PC passes from the ADSL (most common DSL) modem to the digital
subscriber line access multiplexer (DSLAM). The DSLAM links many ADSL lines to a single
high-speed asynchronous transfer mode (ATM) line, which in turn connects to the Internet
at speeds up to 1Gbps.
ADSL
ADSL, Asymmetrical Digital Subscriber Line, is the most familiar type of DSL. It’s
called "Asymmetrical" because most of the duplex bandwidth is dedicated to the
downstream direction. Both ANSI (American National Standards Institute) and ETSI (European
Telecommunications Standards Institute) have already standardized ADSL. Only a small
portion of bandwidth is available in the upstream direction. For example, some telephone
companies may offer 640kps for the upstream, but give as much as 6.1megabits on the
downstream. This high downstream means the telephone line can provide continuos motion and
audio play. ADSL won’t compete with your neighbors for rates, like cable connections
do. Some areas may need to upgrade resources to allow for this, but many companies already
have existing services for DSL.
ADSL uses special modems called endpoints. Along with an endpoint, your PC needs a network
interface card that treats the modem like a local device on the network. Many ADSL modems
combine both functions into a single internal card.
ADSL comes in two competing varieties: Carrierless Amplitude/Phase modulation (CAP) and
Discrete Multi-Tone (DMT). Each of these is a different modulation system, which is a
method for putting data into a carrier signal on the ADSL wire and for reading the data at
the receiving end. CAP uses a single carrier; DMT uses multiple carriers. Currently 90
percent of ADSL services use CAP. But that doesn't mean it will win out, as DMT is the
basis of current ANSI and European standards for ADSL.
HDSL
HDSL (High bit-rate DSL) is another form of DSL equal to about a T1 in North America and
an E1 in Europe. It is the earliest variation of DSL to be used in wide-band digital
transmission within a corporate site and between the Telephone Company and a customer.
Unlike ADSL, HDSL is a symmetrical form, which means the upload and download rates are
equal. It uses less bandwidth and requires no repeaters. Using more advanced modulation
techniques, HDSL transmits 1.544 Mbps or 2.048 Mbps in bandwidth depending upon the
specific technique. HDSL provides such rates over lines up to 12,000 feet in length (24
gauge), the so-called Carrier Serving Area (CSA), but does so by using two lines for T1
and three lines for E1, each operating at half or third speed. Although HDSL has been the
maturest form, it is more likely that it will give way to ADSL and SDSL.
What You Will Need
Most DSL require that a signal splitter be installed at either your home or business, with
the Telephone Company giving you a nice bill for the service. Many companies will test
your lines for free; however, you may need to be within two miles of the Telephone Company
to ensure a good connection. Usually with the closer you get to the telephone company, the
greater transfer of data can take place. Another factor may include the gauge of the
copper wire. The heavier 24-gauge wire is capable of carrying longer distances than a
26-gauge wire. In general, the longest distance DSL can go without using repeaters is 5.5
kilometers. In addition, you may be required pay for a special modem and other equipment.
And depending on who your ISP is, you will have to pay a monthly fee for the service, in
addition to any cost from the telephone company.