DSL alternatives

Written on 1:36 PM by ooe

BT ran SDSL technical trials during 2001 in order to understand the technical implications of supporting symmetric broadband services. These trials of symmetric services were targeted at business users and it is this group that was targetted when the service was launched in September 2003. There is, however, an expectation that eventually services such as 2-way video and multiple derived voice channels will also lead to a demand for symmetric services from the residential market.

SDSL is also being looked at as a way of providing greater reach from the exchange. Two possible techniques could help achieve this. First, the use of multiple pairs between the exchange and the user, and second the use of line regenerators. Although technically possible, these options are a long way from introduction, but may offer a solution prior to VDSL.

The ADSL standard has itself evolved – ADSL2, ADSL2+ and ADSL-ER (Extended Reach). These standards mean that the basic ADSL technology can use a greater part of the spectrum to provide higher bandwidths over potentially greater distances. However, one of the key considerations is how much of the additional available spectrum can be used without compromising either the ANFP or other technologies (e.g. VDSL) that may use that part of the spectrum.

Proprietary DSL techniques may also offer an alternate solution for specific problems such as providing a service to users over long copper lines in enabled exchange areas. The main consideration here is the size of the market for a proprietary solution and if it is big enough to enable an economic solution. There have been small-scale trials and demonstrations of VDSL including one in Greenwich Millennium Village. From these trials it is clear that the only realistic way of providing VDSL is from equipment located in the access network and fed by fibre. This requires a significant uplift to the access network and hence requires considerably more investment than the ADSL or SDSL technologies that simply make use of the existing copper pair access network. However, as the demand for higher and higher data rate increases, it is likely that VDSL will have a significant role to play.

One other advantage of the VDSL solution is the penetration of fibre towards the user. Some of the techniques for feeding the out-stationed VDSL equipment are based on PON technology. Hence, it is still possible that VDSL could be used as a step towards achieving a fibre based access network to the users much as was originally conceived in the early 1990s.