What Is an Asymmetric Digital Subscriber Line (ADSL)?

Asymmetric Digital Subscriber Line (ADSL) is a new technology that provides high transmission speeds for video and voice to homes over an ordinary copper telephone wire. It will be most cost-effective in areas with a low market penetration of cable TV.

Understanding an Asymmetric Digital Subscriber Line (ADSL)

Asymmetric Digital Subscriber Line (ADSL), sometimes just called DSL, is considered to be the major competition to cable modems. DSL and cable systems are compared by bandwidth, a measure of how much data a network can transfer. Internet providers typically denote bandwidth speeds in millions of bits per second, or megabits (Mbps), and billions of bits per second, or gigabits (Gbps). Generally speaking, the higher the bandwidth, the faster the speed with which a computer downloads information from the internet whether users view emails or watch streamed movies.

The U.S. Federal Communications Commission (FCC) defines broadband internet speeds as connections with a bandwidth of 25 Mbps for downloads and 3 Mbps for uploads.

Providers state the bandwidth measurement to customers, but that may not be the actual bandwidth speed a customer receives. The connection could have a bottleneck where one network is limited by the lowest speed going to several computers at once. More computers connected to the same bandwidth speed can slow down the bandwidth for everyone who shares the same connection.

Cable vs. Internet vs. Fios

In terms of theoretical peak performance, a cable modem generally has greater bandwidth than DSL. Cable technology, which transmits data over coaxial copper cables buried underground originally intended for television, currently supports approximately 300 Mbps of bandwidth in many areas, while DSL speeds typically peak at 100 Mbps. Actual speeds can vary in practice depending on the quality of the copper phone line installation. In addition, the length of the phone line needed to reach the service provider's central office also can limit the maximum speed a DSL installation can support.

In 2017, Verizon Communications Inc. (VZ) introduced a new service, Fios Instant Internet, that increased the internet speeds it offers to both residential and business customers, The broadband service provides customers with equal upload and download speeds of 750 Mbps. Verizon claimed that with symmetrical speeds, the new service could handle multiple devices connected to the internet without compromising the performance of any of them.

Most types of DSL service are asymmetric, or ADSL. Typically, ADSL offers higher download speeds than upload speeds, which is usually not a disadvantage because most households download more data from the internet than they upload. Symmetric DSL maintains equal data rates for both uploads and downloads.

The main selling point of DSL is widespread availability; telephone infrastructure is already deployed basically everywhere, so it doesn’t take much setup to connect most customers to the internet via DSL, especially in rural areas where cable is less likely to be an option.