Other DSL variants

Written on 9:54 AM by ooe

As was noted earlier, ADSL is but one of the flavours of DSL. The other principal flavours are SDSL (symmetric digital subscriber line), VDSL (very high speed digital subscriber line) and HDSL (high-speed digital subscriber line).

SDSL differs from its ADSL cousin in two ways. First, as the name suggests, it provides symmetrical upstream and downstream data channels. Second, the encoding of the data channel utilises the voice part of the spectrum. Hence services offered over SDSL do not support simultaneous POTS (though it is possible to use part of the data capacity to support a derived voice capability). At present it is expected that SDSL-based services will be targeted towards the business rather than the residential community and that service speeds will be up to 2 Mbit/s in each direction to enable a significant penetration of the customer base.

VDSL operates in the spectrum above narrowband services; hence it is compatible with both POTS and ISDN. It also has options of working either in symmetrical or asymmetrical modes and is capable of achieving speeds of between 6 and 25 Mbit/s. However, as with ADSL, there is a trade off between speed and distance. Hence although great speed is possible, it is achievable over distances much more constrained than for ADSL. The only way of achieving a reasonable penetration therefore requires the deployment of equipment in the access network. This will require a significant investment, and hence will probably be deployed only once there is a high confidence in the demand for such capacity.

There is also HDSL which is an older technology that is generally used to provide 2 MHz private circuit tails. This technology differs in a number of ways from the others. First, the service is generally fixed at 2 Mbit/s in each direction, and over longer distances multiple pairs are used to achieve the 2 Mbit/s capacity. Second, there is generally not a DSLAM to aggregate multiple services into a single pipe. For HDSL there tends to be a matching HDSL modem in the local exchange which decodes the users data channel. These are provided on an individual basis.