Digital Subscriber Line (DSL) is a generic term categorizing services provided over copper wire. DSL subscribers may receive high speed Internet service and other services bundled with the DSL package. When a service provider or ISP offers Asymmetric Digital Subscriber Line (ADSL), the customer will receive service that is not guaranteed to be the same speed in both directions.
For example, many ISPs advertise their Internet service with a download and an upload speed, 1Mbps download and 256Kbps upload (or something similar). These advertised speeds are rarely the same in both directions. This would be the way that ADSL works, the upload and download speeds offered by the ISP to subscribers is different and stated that way.
DSL is more generic, implying any type of Digital Subscriber Line service, from ADSL where the upload and download speeds are different, to symmetric digital subscriber line (SDSL) in which the upload and download speeds are the same. Service providers are introducing new methods of ADSL style technologies, including tiered pricing which allows the ISP to charge higher fees for more bandwidth for either downloading, uploading, or both. ADSL is a specific type of service sold to allow subscribers to connect to high speed data networks. The majority of the DSL service sold for residential access is ADSL.
DSL, no matter the flavor, has some vast benefits over the dial-up technology used over the phone lines before it. The biggest, or most seen benefit of DSL, is the ability to use the phone to make or receive calls while connected to the Internet. With traditional dial-up service, this was not possible without the use of a second phone line. DSL accomplishes this by incorporating a filter on the phone jacks in a location that will have telephones connected to them. The signals on the wire under 4Khz are considered voice signals and anything above 4Khz is considered a data signal, the filter helps to ensure that these signals never cross.
One downside to DSL is the proximity issue. The closer a subscriber lives to the phone company’s central office (CO), the faster the DSL (ADSL or other) connection will be. Subscribers who live further away from the CO, but still within the reach of DSL service, (as determined by the phone company) will have a slower connection to the Internet. People living outside the predetermined boundary for DSL service will not be eligible for DSL and other means of high speed access will be needed.
Source: WiseGeek