Vectoring technology has emerged as a means of delivering high bandwidth over existing copper infrastructure by extracting more value from it, without investing in an extensive fiber roll-out.
What’s the idea?
Phone lines that carry VDSL2 signals are part of cables that contain from 10 to a few hundred lines positioned very closely together. This close proximity results in cross-talk (FEXT, undesired effect of a signal transmitted on one circuit to another one) and the higher the number of lines in a bundle, the more cross-talk is generated. Cross-talk is the one of the biggest impediments to deliver good VDSL2 performance. With no interference, every VDSL2 line can operate as if it were the only line in the cable, and, as consequence, deliver higher bit rates. So basically, vectoring is not a method for raising the theoretical maximum speeds, but for decreasing the gap between the theoretical upper limit of performance and the speeds that can be delivered in variable field conditions.
Opposite to Dynamic Line Management (DLM), for which there is no guarantee that the improved speed once awarded by the DLM process on a specific VDSL2 line will always remain, the vectored lines will keep their new higher rates based on a recurrent process of measurement, processing and correction.
Vectoring technology can be compared to the noise-cancelling technology used in headphones: it works by 'listening' to unwanted noise and producing a sound that is exactly like it but with an inverted phase—this way two sounds can cancel each other out.
How does it actually work?
In the Proximus network, vectoring technology has been implemented at street cabinet (ROP) level where the cross-talks of the lines of the same bundle can be properly measured and neutralized by generating appropriate anti-phase signals, which is done by DSLAM.
Surely these calculations needed for vectoring requires enormous computing power. In order to fulfill requirements mentioned above, there have been two things that have been implemented in order to bypass it: use of the new VDSL2 line cards in the ROPs and an upgrade of all installed CPE (Customer Premises Equipment) to vector-friendliness or its replacement by vector-compliant hardware.
In February 2014 Proximus started to replace VDSL2 cards, street cabinet per street cabinet. By now, almost all street cabinets do support vectoring. As for second requirement, the efficient estimation of the cross-talk between VDSL2 lines, the additional functionality at the CPE side is defined by the International Telecommunication Union (ITU vectoring standard, G.993.5 (G.vector).
On the hardware level, VDSL2 line can only be vectored if two following requirements are fulfilled: modem installed on it should be vector-compliant and, at the same time, each modem installed on every other line of the same bundle is vector-compliant or at least vector-friendly. Otherwise, a modem, which is not at least vector-friendly, is installed on any other VDSL2 line, will interfere with the calculation of the cross-talk between the lines. Thus, the cross-talk from some line will remain uncancelled, resulting in an unpredictable negative impact on all other lines of the bundle.
In order to prevent it, all modems that are not at least vector-friendly will synchronize on a fall-back profile (7 Mbps down and 512 Kbps up) that will not create more cross-talk than the one coming from ADSL2+ lines and will not disturb the continuous cross-talk estimation. On the contrary, a vector-friendly VDSL2 CPE does not disturb any other lines in the cable, but will not allow to benefit from the advantage of vectoring, being a higher speed rates.
What speeds can vectoring achieve?
No matter how far from the street cabinet VDSL line is, vectoring significantly increases its maximum speeds (by from around 30 to more than 100 per cent depending on the provisioning profile). Vectored VDSL2 lines can reach downstream speeds of 70 Mbps (and that’s not considering DLM!) at distances of up to 900 meters, with the attenuation of the cable not exceeding 0,4 dB, while 50 Mbps can be supported with loops as long as 1200 meters.
The speeds will also depend on the type of vectoring the street cabinet supports. Vectoring as it was originally launched was operating in the frequency bands above 2.2 MHz. As ADSL is currently being phased out, on ADSL-free street cabinets the vectoring can be deployed in the ADSL and ADSL2+ frequency bands (above 1,1 MHz and 552 KHz, respectively), which results in a more accurate cross-talk estimation. Learn which speed profile you are eligible for.