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Keywords: - : What is DSL, ADSL- Asymmetric Digital Subscriber Line, HDSL- High-bit-rate Digital Subscriber Line, IDSL
Author: - Surender Singh

There are several different types of DSL (or xDSL) connections.  DSL was designed to be a low-cost alternative to the current communications technologies (leased lines and
dial-up).  The greatest factor in the low price of DSL is that it runs over existing copper lines.  No new cabling has to be run to support DSL.  DSL connections can reach speeds
well over 2 Mbps--far better that a 56 kbps modem.  The differences between the types of DSL are the maximum bandwidth, number of conductors required, maximum distance,
and line coding (how data is formatted and sent).  Following is a breakdown of the main types of DSL.
Asymmetric Digital Subscriber Line (ADSL)

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 a 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.

High-bit-rate Digital Subscriber Line (HDSL)

Unlike ADSL, HDSL is a symmetric DSL technology.  This means that upstream and downstream data travels at the same rate.  HDSL runs over 2 pairs of wires and can achieve data rates
of up to 1.544 Mbps.  HDSL is the most mature DSL technology.  It does not support POTS on the same lines as data.

ISDN Digital Subscriber Line (IDSL)

IDSL is a cross between ISDN and DSL technologies.  The maximum transfer rate is the same as ISDN (minus the control channel) or 128 kbps.  It runs over a single pair of wires. 
The primary benefit of IDSL over ISDN is that it is always on (like any xDSL), and so there is no call setup time.

HDSL2/G.SHDSL

The newest version of HDSL is a relatively recent DSL technology.  HDSL2 maintains the 1.544 Mbps data rate of HDSL, but only requires 2 wires instead of 4.  The encoding
scheme for HDSL2 is more efficient than that of HDSL, giving HDSL2 a much longer reach than its predecessor.  HDSL2 has a maximum distance of 132,000 ft at 1.544 Mbps, and 225,000 ft
at 384 kbps.  HDSL2 supports several voice channels in the data stream.  G.SHDSL is a generalized standard from the ITU. Based on HDSL2, G.SHDSL can reach speeds of 2.3 Mbps. 
G.SHDSL can determine line conditions, and optimize performance based on those conditions.

Very high-bit-rate Digital Subscriber Line (VDSL)

Although not yet a standard, VDSL is the future of DSL technology.  VDSL will support downstream date rates of up to 52 Mbps at a range of 1000 ft, with upstream rates of
up to 2.3 Mbps.

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