DSL

Internet access is one of the demands for the new era and incumbent local exchange carriers want to have better transmission speed and larger bandwidth using copper wire installed in the local loop. With new technology arises, Digital Subscriber line was born and saved the ILEC on providing high-speed internet for users using its traditional copper wires. A figure below shows how DSL is configured and connected using local loop networks.

Here are the families of DSL and their technical descriptions:

Asymmetrical DSL

There are a family of DSL with different speeds and capability. One of the most famous xDSL is the ADSL or the Asymmetrical DSL. According to Regis Bates, ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities. In reality, ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current technology.

The "asymmetric" in ADSL refers to the fact that the downstream data rate, or the data coming to your computer from the Internet, is traveling faster than upstream data, or the data traveling from your computer to the Internet. Upstream data rates are slower because Web page requests are fairly miniscule data strings that do not require much bandwidth to handle efficiently.

ISDN DSL

ISDN Digital Subscriber Line (IDSL) uses ISDN-based technology to provide a data communication channel across existing copper telephone lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection at 128kbit/s. The digital transmission bypasses the telephone company's central office equipment that handles analogue signals. IDSL uses the ISDN grade loop without Basic Rate Interface in ISDN transmission mode. The benefits of IDSL over ISDN are that IDSL provides always-on connections and transmits data via a data network rather than the carrier's voice network. IDSL also avoids per-call fees by being generally billed at a flat-rate.

HDSL

HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. HDSL can carry as much on a single wire of twisted-pair cable as can be carried on a T1 line (up to 1.544 Mbps) in North America or an E1 line (up to 2.048 Mbps) in Europe over a somewhat longer range and is considered an alternative to a T1 or E1 connection. It was developed to overcome the problem of sending data on copper wires using T1 lines. HDSL does not also require repeaters over longer wires and also bridge taps is not a primary problem.

SDSL

Symmetric Digital Subscriber Line is a technology that allows more data to be sent over existing copper telephone lines (POTS). SDSL works by sending digital pulses in the high-frequency area of telephone wires and cannot operate simultaneously with voice connections over the same wires. SDSL requires a special SDSL modem. SDSL is called sysmmetric because it supports the same data rates for upstream and downstream traffic.

RADSL

Rate Adaptive Digital Subscriber Loop was developed by Westell and has a potential of 2.2 Mbps downstream and 1.0 Mbps upstream. What makes RADSL more flexible is that the sync rate can be dynamically adjusted up or down as line conditions change. This makes it more of a viable alternative where line conditions are marginal due to distance or other factors. In many respects, RADSL is an enhanced ADSL to meet a more diverse set of line conditions. Like ADSL, RADSL can piggyback on the POTS line. RADSL does not require any splitters or filters.

VDSL

Very high rate Digital Subscriber Loop is a DSL still in development with a current downstream capacity of 52.8 Mbps, and upstream of 2.3 Mbps. At this time, VDSL is limited to very short loop lengths, and is not yet a viable alternative. It may find application where there is fiber to the neighborhood, and thus the copper loop segment is relatively short.

G.SHDSL

G.SHDSL is an international standard for symmetric DSL developed by the ITU. G.SHDSL provides for sending and receiving high-speed symmetrical data streams over a single pair of copper wires at rates between 192 kbps and 2.31 Mbps. 

A table below is shown to summarize the capabilities of each DSL network and its transmission rate.

Sources:

http://www.informit.com/articles/article.aspx?p=31699&seqNum=3

http://www.webopedia.com/TERM/G/G_SHDSL.html

http://searchnetworking.techtarget.com/definition/HDSL

http://en.wikipedia.org/wiki/ISDN_digital_subscriber_line

http://www.wisegeek.com/what-is-adsl.htm