xDSL

   The motivation of digital subscriber line technology was the Integrated Services Digital Network (ISDN) specification proposed in 1984 by the CCITT (now ITU-T) as part of Recommendation I.120, l. xDSL is similar to ISDN in as much as both operate over existing copper telephone lines and both require the short runs to a central telephone office usually less than 20,000 feet. However, xDSL offers much higher speeds - up to 32 Mbps for upstream traffic, and from 32 Kbps to over 1 Mbps for downstream traffic. DSL technologies use sophisticated modulation schemes to pack data onto copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations. A new form of communications was needed to work over the existing copper cable plant. One of the technologies selected was the use of xDSL. The DSL family includes several variations of what is known as digital subscriber line. The lower case x in front of the DSL stands for the many variations. These will include: 

      First we have, Asymmetrical digital subscriber line (ADSL), it is used in a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. Some of its advantages was no expensive modification is required to central office switch, simple splitter splits ADSL signal from the existing analog line, and it had a high bandwidth. Then we have, ISDN Digital Subscriber Line (IDSL),  also called the last mile, a system in which digital data is transmitted at 128 Kbps on a regular copper telephone line (twisted pair) from a user to a destination using digital or analog transmission, by passing the telephone company's central office equipment that handles analog signals. We have also  what we called High bit−rate Digital Subscriber Line (HDSL), it uses two-pair copper cable for full-duplex transmission, using echo cancellation with each pair carrying 784 kb/s. The lower bit rate allows for a lower frequency range of operation that reduces channel loss and next. Then we have, Consumer Digital Subscriber Line (CDSL), a version of Digital Subscriber Line (DSL) service that is somewhat slower than Asymmetric DSL up to 1 Mbps downstream, probably less upstream and has the advantage that a splitter does not need to be installed at the user's end. It allows more data to be sent over existing copper telephone lines and it supports data rates up to 3 Mbps.

Digital Subscriber Line Advantages

• Independent services: Loss of high speed data does not mean you lose your telephone service. Imagine your telephone, television, and Internet access going out when a cable company amplifier/repeater dies.
• Security: Unlike cable modems, each subscriber can be configured so that it will not be on the same network. In some cable modem networks, other computers on the cable modem network are left visibly vulnerable and are easily susceptible  to break ins as well as data destruction.
• Integration: DSL will easily interface with ATM, Nx64, and WAN technology. Telecommuting may get even easier.
• High bandwidth
• Cheap line charges from the phone company.
• Good for "bursty" traffic patterns

Digital Subscriber Line Disadvantages

• No current standardization
• Expensive
• Distance dependence: The farther you live from the DSL Access Multiplexer, the lower the data rate. The longest run lengths are 18,000 feet, or a little over 3 miles.
• Limited availability
• Low or no Committed Information Rate.  This means that as traffic across the telco switch increases your data could in effect, be locked out, until call volumes and other traffic subsides.
• Reliability and potential down time issues makes DSL a very risky choice for mission critical systems unless backup / fail over links are put in place.
• Sometimes the process of getting DSL may be an adventure.

We have also Single High Speed DSL (SHDSL), was developed to provide high−speed communications on that single cable pair but at distances no greater than 10K. Despite this distance limitation, SDSL was designed to deliver 1.544 Mbps on the single cable pair. Typically, however, the providers offer SDSL at 768 Kbps.Then we have Rate−adaptive digital subscriber line (RADSL), was designed to increase range and noise tolerance by sacrificing up stream speed. We have also Very high−bit rate digital subscriber line (VDSL),a digital subscriber line technology providing faster data transmission over a single flat untwisted or twisted pair of copper wires up to 52 Mbit/s downstream and 16 Mbit/s upstream. And lastly, we have what we called  Single or symmetric digital subscriber line (SDSL), it works by sending digital pulses in the high-frequency area of telephone wires and can not operate simultaneously with voice connections over the same wires.

Benefits & Applications

Benefits

• High-speed data service

• DSL typically >10x faster than 56-kbps analog modem

• Always on connection

• No need to “dial-up”

• Uses existing copper wires

• Co-exists w/ POTS service

• Reasonably priced today and getting cheaper

Applications

• High speed Internet access
• SOHO
• Multimedia, Long distance learning, gaming
• Video on Demand
• VPN
• VoDS

Reference:

•  http://www.webopedia.com/TERM/X/xDSL.html
•  Richard D. Gitlin, Sailesh K. Rao, Jean-Jacques Werner, Nicholas Zervos. "Method and apparatus for        wideband transmission of digital signals between, for example, a telephone central office and customer    premises". US Patent 4,924,492. Filed March 22, 1988. Issued May 8, 1990
• http://www.state.net/dsl/DSLadvantages.html
• IDSL Frequently Asked Questions" (Press release). Cisco. February 26, 2008.