DSL at 100Mbps?!

DSL (Digital subscriber line) is a family of technologies that provide internet access by transmitting digital data over the wires of a local telephone network. The data bit rate of consumer DSL services typically ranges from 256 kbit/s to 40 Mbit/s, contrasting to a 100Mbit/s of an optic fiber transmission. While running fiber to millions of individual homes is one expensive construction project, Alcatel-Lucent() announces commercial rollout of a latest DSL technology, called VDSL2, with vectoring. VDSL2 vectoring can use legacy telephone access networks with a data rate 3 times of conventional DSL i.e; 100Mbps!

Michael Peeters, chief technology officer of Alcatel-Lucent’s wire-line division, says "Cross talk limits bandwidth by corrupting the signals that are transmitting information.Cross talk can be eliminated by estimating how much one line is leaking into another and then cancelling out that interference with calculations performed by hardware installed in the street cabinet. Cross talk estimation uses what’s called an error vector, which is sent from a user’s home equipment back to the cabinet. The receiving equipment there knows what the signal should look like, and the difference is a good measure of the amount of cross talk between a wire and all the wires around it."

A basic vectored DSL system.

The term vector is used because the DSL’s individual physical layer voltages can be viewed as a coordinated set or vector of voltages. The group or vector is processed by a common signal processing device for downstream transmission and also for upstream reception as shown in Figure. Essentially, the vector/MIMO processor performs pre-processing of the transmitted signal in downstream transmission via pre-coding or linear pre-filtering, and joint processing of the received signals in the upstream via receive filtering and successive cancellation. This group processing allows cancellation or removal of crosstalk. The gain from the vectoring is largest when all the lines in the binder are processed simultaneously, but even partial vectoring or independent cancellation by different operators provide significant improvement over non-vectored systems.

Level 3 DSM is known as vectoring (Ginis and Cioffi, 2002) and allows for special processing methods in a DSLAM to essentially eliminate all crosstalk between lines, leading to 100s of Mbps speeds in DSL.