DSL
DSL (Digital subscriber line)
technology is developed by telephone companies to achieve high speed internet
access. This technology makes use of existing local loops. DSL technologies
like ADSL (Asymmetric digital subscriber line), ADSL Lite, VDSL (Very
high-bit-rate digital subscriber line), HDSL (High-bit-rate digital subscriber
line), SDSL (Symmetric digital subscriber line) are further described below.
ADSL
Asymmetric digital subscriber line uses
existing local loops for connection to the internet. It is suitable to connect
to internet at homes. It is an adaptive technology, i.e. it uses data rate
based on the condition of previously used channel and bandwidth. It uses DMT
(Discrete multi-tone technique) for modulation. Downstream rate for ADSL is
1.5-6.1 Mbps, Upstream rate is 16-640 kbps at distances of up to 12000ft. It
uses single twisted pair cable for communication.
ADSL Lite
ADSL Lite is less expensive than ADSL.
It uses DMT (Discrete multi-tone technique) for modulation. Downstream rate for
ADSL Lite is 1.5 Mbps, Upstream rate is 500 kbps at distances of up to 18000ft.
It uses single twisted pair cable for communication.
HDSL
HDSL uses 2B1Q encoding for modulation.
Downstream rate for HDSL is 1.5-2.0 Mbps, Upstream rate is 1.5-2.0 Mbps at
distances of up to 12000ft. It uses two twisted pair cables, one for upstream
communication and other for downstream communication.
SDSL
SDSL uses one twisted pair for
communication and so both upstream and downstream data rate uses 768 kbps speed
at distances of up to 12000ft which is half the speed of HDSL. SDSL uses 2B1Q
encoding for modulation.
VDSL
VDSL provides the highest value of
upstream and downstream data rate compared to above technologies. VDSL uses DMT
for modulation. It provides downstream rate of 25-55 Mbps, and upstream rate of
up to 3.2 Mbps at distances from 3000ft to 10000ft via single twisted pair
cable.
