DSL stands for the following 'Digital Subscriber Line' and is a wired Internet access technology that has been the most common access technology used in the UK for the past 15 years. DSL replaced dialup as the most common access technology - dialup was the only access technology available to most UK home users prior to 1998. DSL and dialup are similar in one respect, they both use the copper line telephone network to provide Internet access, the difference between the two technologies is that DSL is a broadband technology and dialup is a narrowband technology. DSL is categorised as a broadband technology because it is a digital data transmission that can support multiple signals / channels: enabling the telephone network to be used simultaneously with the Internet access (something dialup does not provide). DSL is also defined as broadband because the connection it always 'on' and it is at least 10 times faster than dialup (dialup is maxed at 60 kbit/s, dsl at a minimum should provide 250 kbit/s). Due to DSL's implementation across the copper line telephone network, end-users do not need to purchase alot of equipment to get a DSL connection up and running: a DSL modem, DSL filter, ethernet cable to connect the modem to the computer/tablet, and line cord to connect the modem to the telephone socket.
It is not surprising that the digital subscriber line technology has proved to be so popular: the copper line telephone network has long been established, and the cost of laying new cabling - like fiber optic - usually requires some subsidy from national governments to make it a commercial reality. The drawback with DSL is that many households have a connection speed below 8 mbit/s (megabits per second), congestion occurs at peak times, crosstalk occurs, rural locations are especially 'hit' by poor download speeds due to their distance from the telephone exchange, and due to the age of the UK copper line network, many households have unreliable connections due to noise on the line. Even though DSL is reliable and cost effective, it's drawbacks are such, that it is not fit for purpose for modern Internet services: high definition video streaming etc. Fiber optic cabling is slowly being implemented in the UK - and in most developed and developing countries - and should replace DSL as the most common wired access technology in 3-7 years.
DSL is not a single technology, there is two broad categories of DSL: asymmetric digital subscriber line (ADSL) and symmetric digital subscriber line (SDSL). ADSL refers to a DSL technology that has a higher bit rate is one direction (higher download bit rate), and SDSL refers to a DSL technology that has an equal bit rate in both directions. ADSL is usually marketed to home users - who download data more than they upload data - and SDSL tends to be marketed to business users - who upload more data to provide business services. John Cioffi, an American engineer, is credited as being the 'father' of DSL, or essential in it's development, due to his research of discrete multitone modulation (DMT) in the mid 1980's. DSL technologies have been developed by a range of research organisations and telecommunication companies, such as: AT&T Paradyne, Texas Instruments, and the American National Standards Institute (ANSI).
ADSL is by far the most popular DSL technology in the UK, because it supports the highest download bit rate (up to 24 mbit/s), and the phone can be used at the same time as the Internet. The drawback with ADSL is that it's speed drops considerable the further away a user is to the telephone exchange; making it a bad technology for people who are over 3 miles from the telephone exchange: as they will probably have a download speed of under 2 mbit/s. Openreach (BT) has deployed the DSL technology Broadband Enabling Technology (BET) - which is a version of Symmetrical high speed digital subscriber line (SHDSL) - to improve download speeds for rural areas who are over 8 miles from the telephone exchange. Standard ADSL only supports a download speed of 8 mbit/s (1 mbit/s upload), but has been updated to Asymmetric digital subscriber line 2 (ADSL2) download 12 mbit/s and upload 3.5 mbit/s, and Asymmetric digital subscriber line 2 plus (ADSL2+) download 24 mbit/s and upload 3.5 mbit/s.
SDSL is nowhere near as popular as ADSL, due to it being more suitable for medium sized businesses: who upload far more data than home users or small business users. Some versions of SDSL can support an upload bit rate of up to 22 mbit/s when paired over multiple copper lines. Some popular versions of SDSL are: High bit rate digital subscriber line (HDSL), High bit rate digital subscriber 2 (HDSL2), High bit rate digital subscriber line 4 (HDSL4), and Single pair high speed digital subscriber line (G.SHDSL). HDSL2 is provided over two copper wires and HDSL4 is provided over four copper wires; HDSL4 is more expensive to implement, but is more reliable for essential business services.
Some niche DSL technologies are used to improve performance over distance, tolerance of copper line noise distortion, or to increase bandwidth up to 400 mbit/s; some examples are: Rate adaptive digital subscriber line (RADSL), DSL Rings (DSLR), Uni-DSL (UDSL), Multi rate symmetric DSL (MSDSL / MDSL), Etherloop, Very high bitrate digital subscriber line (VDSL), Very high bitrate digital subscriber line 2 (VDSL2) and DSL bonding.