xDSL Variants

RADSL, like ADSL, operates faster in one direction than in the other and has the same transmission limits as ADSL. But as its name suggests, it adjusts transmission speed according to the length and quality of the local line. Connection speed is established when the line syncs up or is set by a signal from the central office.

HDSL technology is symmetric, meaning that it furnishes the same amount of bandwidth both upstream and downstream. The most mature of the xDSL approaches, HDSL has already been implemented in telephone company feeder plant (the lines that extend from central offices to remote nodes) and also in campus environments. Because of its speed – T1 over two twisted pairs of wiring, and E1 (2.048 Mbit/s) over three – HDSL is commonly deployed as an alternative to T1/E1 with repeaters. At 12,000 feet, HDSL’s operating distance is shorter than ADSL’s, but carriers can install signal repeaters to extend its useful range (typically by 3,000 to 4,000 feet).

SDSL is essentially the same as HDSL with two notable exceptions: it uses a single wire pair and has a maximum operating range of 10,000 feet. In the autumn of 2002, amidst claims that it had been deliberately delaying bringing SDSL services to market so as to protect its lucrative leased line business, UK telco BT announced trials of an SDLS service that would provide 2Mbit/s of bandwidth in both directions over a dedicated copper line.

VDSL is the fastest DSL technology. It delivers downstream rates of 13 to 52 Mbit/s and upstream rates of 1.6 to 2.3 Mbit/s over a single wire pair. However, the maximum operating distance is only 1,000 to 4,500 feet and VDSL and ADSL equipment aren’t compatible, though sharing many compression and modulation technologies.

The table below summaries the speed and distance characteristics of the other principal xDSL variants:

Type Upstream Downstream Distance (feet)
RDSL 128 Kbit/s-1 Mbit/s 600 Kbit/s-7 Mbit/s 18000-25000
HDSL 2.048 Mbit/s 2.048 Mbit/s 12000
SDSL 1.544-2.048 Mbit/s 1.544-2.048 Mbit/s 10000
VDSL 1.6-2.3 Mbit/s 12.96 Mbit/s

25.82 Mbit/s

51.84 Mbit/s