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Although there aren't 31 varieties of DSL, there are enough flavors to satisfy most customers' needs. Many of these flavors have come about because of continued technological growth that enabled support for new features. Each DSL type falls into one of two categories: asymmetric or symmetric.

Asymmetric transmission methods have a higher bandwidth downstream than upstream, which is good for residential services or enterprises that need more bandwidth coming into an office than going out. In contrast, symmetric setups transmit data at the same rate in both directions--useful for multiple voice lines or offices that need more upstream data bandwidth than is available with asymmetric technologies.

Asymmetric Types

ADSL (asymmetric DSL) uses frequency division to separate lower frequency signals used by POTS from higher frequencies for delivery to a DSL bridge or IAD (integrated access device). Downstream channel throughput can outweigh upstream by a ratio of up to 20:1; ADSL offers downstream rates as high as 8 Mbps.

G.lite is intended as a medium-bandwidth, residential-use version of ADSL (1.5 Mbps upstream and 500 Kbps downstream). G.lite is designed to be plug and play, using existing telephone lines without splitters. This reduces the need for phone companies to install separate lines for home Internet access.

RADSL (rate-adaptive DSL) lines are able to adjust bandwidth depending on distance and line quality. These modems will connect over varying lines at fluctuating speeds at distances greater than 18,000 feet. RADSL devices can be designed to select their connection speeds at train up, during connection or upon signal from the CO.

VDSL (very-high-bit-rate DSL) enables access over 24-gauge twisted pair at data rates from 12.9 Mbps at 4,500 feet to 52.8 Mbps at 1,000 feet. Because of the short distances supported, VDSL is designed to connect fiber optic junction points to nearby networks.

Symmetric Types

G.SHDSL is designed as an industry-standard version of SDSL that will reduce cross talk and allow for multivendor interoperability. G.SHDSL will allow symmetric data rates from 192 Kbps to 2.3 Mbps over one pair of wires.

HDSL (high-bit-rate DSL) provides DS-1 or E-1 data rates over DSL lines using two or three pairs of wires. HDSL is designed to span greater distances than can be achieved without routers for normal DS-1 or E-1 service.

HDSL-2 (second-generation HDSL) is fixed at 1.5 Mbps throughput in both directions and requires only one cable pair.

IDSL (ISDN DSL) uses ISDN lines to carry DSL signals from CPE to DSLAM at 128 Kbps. IDSL is used primarily in areas not serviced by other forms of DSL or when the CPE is too far from the CO to use ADSL or SDSL.

SDSL (symmetric DSL) connections carry the same amount of bandwidth upstream as downstream, but the capacity is lower than ADSL. Downstream and upstream rates vary from 128 Kbps to 2.3 Mbps, depending on the distance from the CO.