Different xDSL technologies
The DSL technology is based on discrete
multi-tone modulation technique and covers a number of similar yet competing
forms of DSL (collectively termed as xDSL) including IDSL, HDSL, SHDSL, ADSL,
RADSL, UDSL, Etherloop, VDSL and GDSL. xDSL is drawing significant attention
from implementers and service providers because it promises to deliver
high-bandwidth data rates to dispersed locations with relatively small changes
to the existing telco infrastructure.
These technologies are differentiated by:
1. Speed of transmission
2. Maximum distance of signal transmission
3. Variation in speed between upstream and downstream
4. Symmetric or asymmetric character of the connection
Currently, the primary focus in xDSL is on
the development and deployment of ADSL and VDSL technologies and
architectures.
ISDN digital subscriber line (IDSL)
This is an integrated services digital
network (ISDN)-based technology that provides data flow rates of 144 kbps,
which is slightly higher than the dual-channel ISDN data rate of 128 kbps. The
goodness of IDSL lies in its 'always-on' connectivity which transmits data via
a data network rather than the carrier's voice network. Thus it gives freedom from
overloading of voice switches by data users.
IDSL uses a 2B1Q (two-binary,
one-quaternary) line code on a single copper pair line to transmit information
through the ISDN 'U' interface. The major limitation of IDSL is that customers
cannot access ISDN signaling or voice services. But if the requirement is Internet
browsing at higher speed, IDSL is a better option than ISDN.
High-bit rate digital subscriber line
(HDSL)
High-bit
rate digital subscriber line (HDSL)
HDSL is a symmetric technology that provides
the same amount of bandwidth for upstream and downstream traffic. It offers
speed of 2.048 Mbps over two copper pairs with operating distances of 3.6 km to
4.6 km, and is ideal for connecting PBX systems, digital local loops, point of
presence, Internet servers and campus- based networks.
HDSL-II
HDSL-II, another technology proposed
within the American National Standard Institute (ANSI) and the European
Telecommunication Standard Institute (ETSI), offers the performance of HDSL
but over a single pair.
HDSL was originally developed in USA, as a
better technology for high-speed local exchange carrier systems which carry
high-speed data links and voice channels using T1 lines. T-carrier circuits
operate at 1.544Mbps and were carried using alternate mark inversion (AMI) line
code. Due to limited range of AMI, the line code B8ZS (bipolar 8-zero
substitution) has been used. Later, 2B1Q line code was used, which allowed a
784kbps data rate over a single twisted-pair cable and 1.544Mbps with two
twisted-pair cables. But, the problem still continued due to the differences
between the T1 (1.544Mbps) and E1 (2.048 Mbps) standards.
A new standard for HDSL has been developed
using the carrier less amplitude phase modulation (CAP) line code, which
reached the maximum bandwidth of 2.048 Mbps using two pairs of copper. HDSL can
be used either at the T1 rate or the E1 rate. Multiple of 64kbps channels
inside the T1/ E1 frame can be used to provide slower speeds to customers but
the line rate is still the full T1/El rate.
HDSL further gave birth to two new
technologies, called HDSL2 and SDSL. HDSL2 offers the same data rate over a
single pair of copper and can work up to longer distances over a low-quality or
lower-gauge copper. On the other hand, single-line rate digital subscriber line
(SDSL) is a multi-rate technology offering speeds ranging from 192 kbps to 2.3
Mbps using a single pair of copper.
Single-pair high-speed digital subscriber
line (SHDSL)
Single-pair high-speed DSL technology
supports symmetrical data rates. It is best suited for PBX, VPN, Web hosting
and other data services that do not need the service guarantees of frame relay
or the higher performance of a leased line. It cannot support voice service on
the same pair as it takes over the entire bandwidth.
The ITU-T recommendation G.991.2 defines
the standards for SHDSL. With one pair of copper line, the SHDSL having
multiple of 64kbps payload provides symmetrical download and upload data rates
ranging from 192 kbps to 2.304 Mbps. Moreover, the SHDSL provides symmetrical
data rates from 384 kbps to 4.608 Mbps in 128kbps increments for two pair
applications.
The distance covered is about 3 km and
depends on the loop rate and noise conditions. One option to increase the
coverage area is to decrease the data rates. Higher data rates can be achieved
using two or four copper pairs, and one such extension of SHDSL provides data
rates up to 5.696 Mbps.
The payload may be either unstructured,
T1, E1, multiple ISDN basic rate access (BRA), asynchronous transfer mode (ATM)
cells or Ethernet packet transfer mode (PTM). In order to share the SHDSL
bandwidth, a dual bearer mode can be used, which allows a combination of two
types of payloads.
Asymmetric digital subscriber line
(ADSL)
By studying different scenarios, it was
realized that it was possible to transmit data more quickly from an exchange to
a user. But when the user sent information to the exchange, it was more
sensitive to the noise caused by electromagnetic disturbances (the nearer the
subscriber to the exchange, the greater the concentration of cables, generating
more crosstalk). So the idea was to use an asymmetric system, imposing a lower
speed from the subscriber to the exchange. This idea gave birth to the
asymmetric digital subscriber line technology, which was originally developed
at Bellcore (now Telcordia Technologies) in 1988.
Asymmetric
digital subscriber line (ADSL)
ADSL caters specifically to connections
between ISPs and customers. The Internet is used largely for downloading files,
HTML and graphical content. Processes like uploading files or other content to
servers are limited to very few users. Hence the bandwidth required for
downstream data (from ISP to client) is more than that required for upstream
data (from client to ISP).
This DSL-based technology enables
transmission and reception of data at speeds higher than legacy copper media.
The modulation technique used allows several bits to be represented by one
transmission symbol.
In ADSL, bit rate allocation for a channel
within the available bandwidth is not the same as for the other channels, and
hence the term 'asymmetric.' In other words, the upstream bandwidth is smaller
than the downstream bandwidth. ADSL offers an upstream data rate of 500 kbps
and a downstream data rate of up to 8 Mbps.
ADSL Lite, another variant of the ADSL
standard, offers upstream speeds up to 500 kbps and downstream speeds up to
1.5 Mbps. Further, ADSL has many variants like ADSL2, splitterless ADSL2,
ADSL2+ and ADSL++.
ADSL2/G.DMT.bis is defined in ITU G.992.3
and is an improved version of ADSL with data rates of 12 Mbps in downstream
and 3.5 Mbps in upstream. Splitterless ADSL2/G.lite.bis is defined in ITU
G.992.4 and is capable of providing 1.536Mbps downstream and 512kbps upstream.
ADSL2+ defined in ITU G.992.5 can provide
up to 24Mbps theoretical downstream speed, which is double of the ADSL2 speed.
The upstream speed is up to 3.5 Mbps. Thus ADSL2+ doubles the frequency band of
typical ADSL from 1.1 MHz to 2.2 MHz. More importantly, ADSL2+ provides port
bonding known as G.998.x or G.Bond. This is a very attractive feature of ADSL2+
in which the download and upload speeds are the sum of individual speed of all
provisioned ports to the end user. It means if two lines with 24Mbps were
bonded, the net result would be a speed of 48 Mbps.
ADSL++, another variant of ADSL, developed
by Centillium Communications, is capable of providing download speeds up to
50Mbps, and uses the frequency band up to 3.75 MHz.
