DSL (Digital Subscriber Line)
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.
Difference between Dial up and DSL Connection:
1. Occupy a phone line
Phone line is used for either voice service or internet connection
2. Limited speeds of up to 56.6 Kbps made available through a telephone
network and modem.
3. Have to dial an access number to connect to the internet.
4. Limited speed restricts download time. Exposed cables results in the
possibility of damage.
5. Unique IP address for each connection
6. Chances of connectivity during peak hours is remote, Drawbacks include
disconnections, repeated dialing and blocked telephone lines.
7. Connected using a computer modem
8. Easy setup, no setup fee
9. Low monthly fee
1. DSL Phone line is not affected by internet connection
Simultaneous transmission of data (DSL) and voice or fax services.
2. Hi-Speed Internet: Speed of up to 2 Mbps.
3. Always on
4. Download is up to 10 times faster. Underground cables eliminate damage.
5. Static IP address (higher security risk)
6. Available when you need it. Easy to use, no dialing, no disconnections.
Single-click connectivity, always online.
7. Connected through a DSL modem
8. Self installation is usually free. A setup fee is needed if installed by a
9. Higher monthly fee
After traditional modems reached their peak data rate, telephone companies
developed another technology, DSL, to provide higher-speed access to the
Internet. Digital subscriber line (DSL) technology is one of the most promising for
supporting high-speed digital communication over the existing local loops. DSL
technology is a set of technologies, each differing in the first letter (ADSL, VDSL,
HDSL, and SDSL). The set is often referred to as xDSL, where x can be replaced
by A, V, H, or S.
A technology that transforms a normal copper telephone line into a high-
speed digital line. This technology gives you always-on access to telephone
services and the Internet at the same time. An ADSL line has a higher
downstream speed (into the end user) than upstream speed (away from the
SDSL stands for Symmetric Digital Subscriber Line. A version of DSL where
the upload speeds and download speeds are the same.
High-bit rate Digital Subscriber Line. The earliest variation of DSL. It is used
for wideband digital transmission within a business and between the
telephone company and a customer. HDSL is symmetrical meaning an equal
amount of bandwidth is available in both directions.
VDSL (Very High Speed DSL) is a type of digital subscriber line, a new
technology allowing more data to be sent over existing copper lines. It's not
yet widely deployed as it is currently going through a standards issue.
Define Upstreaming and Downstreaming?
The term Upstreaming means that to transfer digital data or computer
software from a computer or other digital device to a server or another
Downstreaming is the method by which users access and save or "pull
down" software or other files to their own computers from a remote
computer, usually via a modem.
Working of ADSL:
In the implementation and deployment of a high-speed wide area public digital
network, the most challenging part is the link between subscriber and network:
the digital subscriber line. With billions of potential endpoints worldwide, the
prospect of installing new cable for each new customer is daunting. Instead,
network designers have sought ways of exploiting the installed base of twisted-
pair wire that links virtually all residential and business customers to telephone
networks. These links were installed to carry voice-grade signals in a bandwidth
from zero to 4 KHz. However, the wires are capable of transmitting signals over
a far broader spectrum – 1 MHz or more.
ADSL is the most widely publicized of a family of new modem technologies
designed to provide high-speed digital data transmission over ordinary telephone
wire. ADSL is now being offered by a number of carriers and is defined in an
The term asymmetric refers to the fact that ADSL provides more capacity
downstream (from the carrier’s central office to the customer’s site) then
upstream (from customer to carrier). ADSL was originally targeted at the
expected need for the video on demand and related services. This
application has not materialized. However, since the introduction of ADSL
technology, the demand for high speed access to the Internet has grown.
Typically, the user requires far higher capacity for downstream than for
upstream transmission. Most user transmissions are in the form of keyboard
strokes or transmission of short e-mail messages, whereas incoming traffic,
especially Web traffic, can involve large amounts of data and include images
or even video. Thus ADSL provides a perfect fit for the Internet requirement.
ADSL uses frequency division multiplexing (FDM) in a novel way to exploit
the 1-MHz capacity of twisted pair. There are three elements of the ADSL
1) Reserve lowest 25 KHz for voice, known as POTS (plain old telephone
service). The voice is carried only in the 0 to 4 KHz band; the additional band
width is to prevent crosstalk between the voice and data channels.
2) User either echo cancellation or FDM to allocate two bands, a smaller
upstream band and a larger downstream band.
3) Use FDM within the upstream and downstream bands. In this case, a
single bit stream is split into multiple parallel bit streams and each portion is
carried in a separate frequency band.
Echo cancellation is a signal processing technique that allows transmission
of digital signals in both directions on a single transmission line
simultaneously. In essence, a transmitter must subtract the echo of its own
transmission from the incoming signal to recover the signal sent by the other
When echo cancellation is used the entire frequency band for the upstream
channel overlaps the lower portion of the downstream channel. This has two
advantages compared to the use of distinct frequency bands for upstream
The higher the frequency, the greater the attenuation. With the use of echo
cancellation, more of the downstream bandwidth is in the “good” part of
The echo cancellation design is more flexible for changing upstream
capacity. The upstream channel can be extended upward without running
into the downstream; instead, the area of overlap is extended.
The disadvantage of the use of echo cancellation is the need for echo
cancellation logic on both ends of the line.
The modulation technique that has become standard for ADSL is called the
discrete multitone technique (DMT) which combines QAM and FDM. There is
no set way that the bandwidth of a system is divided. Each system can
decide on its bandwidth division. Typically, an available bandwidth of 1.104
MHz is divided into 256 channels.
Each channel uses a bandwidth of 4.312 kHz, as shown in Figure.
Figure shows how the bandwidth can be divided into the following:
Voice: Channel 0 is reserved for voice communication.
Idle: Channels 1 to 5 are not used and provide a gap between voice and
Upstream data and control: Channels 6 to 30 (25 channels) are used
for upstream data transfer and control. One channel is for control, and 24
channels are for data transfer. If there are 24 channels, each using 4 kHz
(out of 4.312 kHz available) with QAM modulation, we have 24 x 4000 x 15,
or a 1.44-Mbps bandwidth, in the upstream direction. However, the data rate
is normally below 500 kbps because some of the carriers are deleted at
frequencies where the noise level is large. In other words, some of channels
may be unused.
Downstream data and control: Channels 31 to 255 (225 channels)
are used for downstream data transfer and control. One channel is for
control, and 224 channels are for data. If there are 224 channels, we can
achieve up to 224 x 4000 x 15, or 13.4 Mbps. However, the data rate is
normally below 8 Mbps because some of the carriers are deleted at
frequencies where the noise level is large. In other words, some of channels
may be unused.
Physical connection must come first. On the customer side, a DSL modem is
hooked up to a phone line. The telephone company (Telco) connects the
other end of the line to a DSLAM, which concentrates a large number of
individual DSL connections into a single box. The location of the DSLAM
depends on the Telco, but it cannot be located too far from the user because
of attenuation, the loss of data due to the large amount of electrical
resistance encountered as the data moves between the DSLAM and the
user's DSL modem. It is common for a few residential blocks to be connected
to one DSLAM.
When the DSL modem powers up it goes through a sync procedure. The
actual process varies from modem to modem but generally involves the
1. The DSL modem performs a self-test.
2. The DSL modem checks the connection between the DSL transceiver and
the computer. For residential variations of DSL, this is usually
the Ethernet (RJ-45) port or a USB port; in rare models, a FireWire port is
used. Older DSL modems sported a native ATM interface (usually, a 25
Mbit serial interface). Also, some variations of DSL (such as SDSL) use
synchronous serial connections.
3. The DSL modem then attempts to synchronize with the DSLAM. Data can
only come into the computer when the DSLAM and the modem are
synchronized. The synchronization process is relatively quick (in the
range of seconds) but is very complex, involving extensive tests that allow
both sides of the connection to optimize the performance according to the
characteristics of the line in use. External or stand-alone modem units
have an indicator labeled "CD", "DSL", or "LINK", which can be used to
tell if the modem is synchronized. During synchronization the light flashes;
when synchronized, the light stays lit, usually with a green color.
The accompanying figure is a schematic of a simple DSL connection. The left
side the shows a DSLAM residing in the Telco’s central office. The right side
shows the customer premises equipment.
Comparison of xDSL Alternatives:
Table shows a summary of DSL technologies. Note that the data rate and
distances are approximations and can vary from one implementation to another.
Brief Explanation of xDSL Types:
The asymmetric part of the name refers to the fact that downstream transfer
rates are higher than the upstream transfer rates (In the case of home
service, you can download data faster than upload). The data rate of ADSL
is dependent on the distance from the CO. ADSL can operate at 8 Mbps
downstream at 9000 ft, and 1.544 Mbps at 18000 ft.
Downstream rates peak at 640 kbps. ADSL operates at frequencies of 30
kHz to 1.1 MHz, leaving the lower frequencies available for Plain Old
Telephone service (POTS).
ADSL runs over a single pair of wires, and so can be run almost anywhere
there is phone. A variant of ADSL called G.Lite has a lower peak data rate
(1.544 mbps), but is more tolerant to line noise and is easier to install.
The high-bit-rate digital subscriber line (HDSL) was designed as an
alternative to the T-1line (1.544 Mbps). The T-1line uses alternate mark
inversion (AMI) encoding, which is very susceptible to attenuation at high
frequencies. This limits the length of a T-l line to 3200 ft (1 km). For longer
distances, a repeater is necessary, which means increased costs.
HDSL uses 2B1Q encoding, which is less susceptible to attenuation. A data
rate of 1.544 Mbps (sometimes up to 2 Mbps) can be achieved without
repeaters up to a distance of 12,000 ft (3.86 km). HDSL uses two twisted
pairs (one pair for each direction) to achieve full-duplex transmission.
The symmetric digital subscriber line (SDSL) is a one twisted-pair version of
HDSL. It provides full-duplex symmetric communication supporting up to 768
kbps in each direction. SDSL, which provides symmetric communication, can
be considered an alternative to ADSL. ADSL provides asymmetric
communication, with a downstream bit rate that is much higher than the
upstream bit rate. Although this feature meets the needs of most residential
subscribers, it is not suitable for businesses that send and receive data in
large volumes in both directions.
The very high-bit-rate digital subscriber line (VDSL), an alternative approach
that is similar to ADSL, uses coaxial, fiber-optic, or twisted-pair cable for
short distances. The modulating technique is DMT. It provides a range of bit
rates (25 to 55 Mbps) for upstream communication at distances of 3000 to
10,000 ft. The downstream rate is normally 3.2 Mbps.