Types of DSL
ADSL
The most common form of DSL that it available currently is
Asymmetric DSL (ADSL). This is so called because it has a
different (i.e. asymmetric) upstream and downstream bandwidth.
Theoretically, ADSL can offer ~6Mbps downstream and ~600Kbps upstream
(i.e. there is an order of magnitude difference in upstream and
downstream capacities), although in Britain, BT have limited ADSL to
512Kbps downstream and 256Kbps upstream.
This asymmetry is considered acceptable when using the Internet,
because the vast majority of the time that you are online, you are
downloading information, whether it be text, images, programs or
anything else. In general, upload tends to be restricted to requests
for webpages. Another reason for the asymmetry between upload and
download speeds is that it has been observed to reduce crosstalk, although this has not been
mathematically proven.
One restriction of ADSL is that subscribers have to be within a range
of 3.5km (~2.1 miles) of their local exchange. This should not be too
much of a restriction in cities, but could pose a major problem for
spreading high-capacity links out into the countryside.
SDSL
SDSL stands for Sychronous DSL. This is similar to ADSL but, as its name suggests, offers the same bandwidth in upstream and downstream directions. Unlike ADSL, SDSL is not voice compatible. This means that SDSL cannot be used over the same phone line as a telephone, so you will need to get an additional analogue line to be able to use SDSL. SDSL has a distance restriction similar to that of ADSL, i.e. around 3 – 4Km.
SDSL speeds vary from ISP to ISP but in general go from 144Kbps to 2Mbps in both upsteam and downstream directions. Note that SDSL is, in general, slower than ADSL since the bandwidth is being used differently, but unlike ADSL in the UK, the full bandwidth is being offered, so solutions offering a 2Mbps link are available, unlike the 512Kbps limit imposed by BT on ADSL. In the UK, SDSL is currently being targeted at SMEs, rather than home users, a decision that is reflected in the price of the product.
VDSL
VDSL (Very high speed Digital Subscriber Line) is still in early experimental stages but is theoretically capable of delivering 50Mbps upstream and downstream. Of course, there is a penalty to pay for this extreme speed, and this is distance. VDSL is even more restrictive than ADSL, being available only within 300 metres from the local exchange. Again, whilst this may not be a problem in a city, rural areas are likely to miss out on these extremely high bandwidths. Note: The high bandwidths offered by VDSL would make it ideal for transporting HDTV channels.
Return to previous page]Introduction - Modems - ISDN - DSL - cable modems - Mobile comms - Conclusion - Glossary
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