What Are the Different Types of DSL Technology?

DSL technology is either asymmetric or symmetric

Close-up of functioning Internet modem

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DSL (digital subscriber line) high-speed internet service for homes and businesses competes with cable and other types of broadband internet services. DSL delivers a broadband network using a copper phone line. DSL internet service is either asymmetric or symmetric; most DSL service is asymmetric. The service that is best for you depends on whether you do a lot of streaming or require support for simultaneous voice and video communications.

Asymmetric DSL

Asymmetric types of DSL connections are faster and provide more network bandwidth for downloading from the internet service provider to the subscriber's computer than for uploading in the other direction. By reducing the amount of bandwidth available upstream, service providers can offer more bandwidth downstream, which reflects the typical subscriber's needs.

Asymmetric DSL technology is best suited for subscribers who require mostly downstream bandwidth.

Common forms of asymmetric DSL include:

  • ADSL (asymmetrical digital subscriber line) has a downstream rate of up to 8 Mbps and an upstream rate of 384 Kbps. It supports telephone service and data transmission at the same time.
  • ADSL 2+ delivers download speeds of up to 20 Mbps and upload speeds of up to 850 Kbps.
  • ADSL Lite or G.Lite offers slower speeds of up to 1 Mbps downstream and 512 Kbps upstream.
  • R-ADSL (rate-adaptive digital subscriber line) delivers the same transmission rates as ADSL, but the transmission speed can be adjusted by the modem.
  • VDSL (very high bit-rate digital subscriber line) is the fastest DSL service. It offers downstream rates of up to 52 Mbps and upstream rates of up to 2.3 Mbps over a single copper wire.

Symmetric DSL

Symmetric DSL connections provide equal bandwidth for both uploads and downloads. Business-class DSL services tend to be symmetric because companies have greater needs for transferring data than residences do.

Symmetric technology works best for simultaneous voice and video communications, which require high speeds in both directions for effective communications.

Forms of symmetric DSL include:

  • SDSL (symmetric digital subscriber line) offers equal upstream and downstream transmission speeds of up to 1.54 Mbps.
  • SHDSL (symmetrical high-speed digital subscriber line) is the same technology as SDSL, but the two standards were approved separately. SHDSL was approved by the International Telecommunications Union, and SDSL was approved by the European Telecommunications Standards Institute.
  • HSDL (high bit-rate digital subscriber line) was developed in the early 1990s, making it one of the oldest forms of symmetric DSL. HDSL offered data rates up to 2.048 Mbps but required multiple phone lines, which eventually made it obsolete.

Other Types of DSL

IDSL (ISDN digital subscriber line) is hybrid DSL/ISDN technology. It was developed with other types of DSL but is rarely used due to its low speeds (144 Kbps maximum data rate). IDSL offers an always-on connection, unlike ISDN.