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DSL/ADSL

Commonplace in many residential and small-business locations (also known as small office/home office or SOHO locations), digital subscriber line (DSL) is a group of technologies that provide high-speed data transmission over existing telephone wiring. DSL has several variants, which differ in data rates and distance limitations.

Three popular DSL variants are

  • Asymmetric DSL (ADSL)
  • Symmetric DSL (SDSL)
  • Very-high-bit-rate DSL (VDSL)

Asymmetric DSL (ADSL) is popular Internet-access solution for residential locations. Note that ADSL enables an existing analog telephone to share the same line used for data for simultaneous transmission of voice and data. The maximum distance from a DSL modem to a DSL access multiplexer (DSLAM) is 18,000 feet. This limitation stems from a procedure that telephone companies have used for decades to change the impedance of telephone lines. A DSLAM acts as an aggregation point for multiple connections, and it connects via an ATM network back to a service provider’s router. The service provider authenticates user credentials, obtained via PPPoE, using an authentication server. Also, the service provider has a DHCP server to distribute IP address information to end-user devices (for example, a PC or a wireless router connected to a DSL modem). The term asymmetric in asymmetric DSL implies that the upstream and downstream speeds can be different. Typically, downstream speeds are greater than upstream speeds in an ADSL connection. The theoretical maximum downstream speed for an ADSL connection is 8 Mbps, and the maximum upstream speed is 1.544 Mbps (the speed of a T1 circuit).

Whereas ADSL has asymmetric (unequal) upstream and downstream speeds, by definition, SDSL has symmetric (equal) upstream and downstream speeds. Another distinction between ADSL and SDSL is that SDSL does not allow simultaneous voice and data on the same phone line. Therefore, SDSL is less popular in residential installations because an additional phone line is required for data. Also, SDSL connections are usually limited to a maximum distance of 12,000 feet between a DSL modem and its DSLAM.

VDSL boasts a much higher bandwidth capacity than ADSL or SDSL, with a common downstream limit of 52 Mbps and a limit of 12 Mbps for upstream traffic.

VDSL’s distance limitation is 4,000 feet of telephone cable between a cable modem and a DSLAM. This constraint might seem too stringent for many potential VDSL subscribers, based on their proximity to their closest telephone central office (CO). However, service providers and telephone companies offering VDSL service often extend their fiber-optic network into their surrounding communities. This enables VDSL gateways to be located in multiple communities. The 4,000-foot limitation then becomes a distance limitation between a DSL modem and the nearest VDSL gateway, thus increasing the number of potential VDSL subscribers.

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