Digital subscriber line (DSL) technology offers high-speed Internet service for homes and offices over telephone lines. It is unique because you can use the Internet and make telephone calls at the same time. A DSL system separates telephone signals into three frequencies bands: lowest band for telephone calls, and other two for uploading and downloading online activities.
There are two types of DSL: asymmetric digital subscriber line (ADSL) and symmetric digital subscriber line (SDSL). If data throughput in upstream direction (towards service provider) is lower, it is ADSL. In SDSL service, both downstream and upstream data rates are equal.
Bit rate of DSL services (downstream) typically ranges from 256kbps to over 100Mbps, depending on DSL technology, line conditions and servicelevel implementation.
Very-high-bit-rate digital subscriber line (VDSL) and very-high-bit-rate digital subscriber line 2 (VDSL2) technologies provide data transmission faster than ADSL. These can also be deployed over existing wires used for analogue telephone service.
VDSL offers speeds of up to 52Mbps downstream and 16Mbps upstream, and is capable of supporting applications such as high-definition television (HDTV), telephone services (voice over IP) and general Internet access, over a single connection.
VDSL2 (second-generation system) uses frequencies up to 30MHz to provide data rates exceeding 100Mbps, simultaneously in both upstream and downstream directions.
ADSL technology covers a larger distance than VDSL. It supports only asymmetric data and plain old telephone (POT) service, while VDSL supports asymmetric, symmetric data and POTs.