VDSL2 profiles, from 8a to 35b
What 17a and 35b mean on your router, the frequency range and transmit power of each VDSL2 profile, and what vectoring does.
Updated
VDSL2 (ITU-T G.993.2) sends data over thousands of narrow frequency channels, called tones or carriers. The profile sets how high in frequency the line goes, how many tones it uses and how much power it transmits. More bandwidth means more speed, but only when the line is short enough for the high frequencies to arrive.
| Profile | Frequency range | Carriers (tones) | Carrier spacing | Max downstream transmit power | Theoretical max down / up |
|---|---|---|---|---|---|
| 8a | 8.832 MHz | 2,048 | 4.3125 kHz | +17.5 dBm | 50 / 16 Mbit/s |
| 8b | 8.832 MHz | 2,048 | 4.3125 kHz | +20.5 dBm | 50 / 16 Mbit/s |
| 8c | 8.5 MHz | 1,972 | 4.3125 kHz | +11.5 dBm | 50 / 16 Mbit/s |
| 8d | 8.832 MHz | 2,048 | 4.3125 kHz | +14.5 dBm | 50 / 16 Mbit/s |
| 12a | 12 MHz | 2,783 | 4.3125 kHz | +14.5 dBm | 68 / 22 Mbit/s |
| 12b | 12 MHz | 2,783 | 4.3125 kHz | +14.5 dBm | 68 / 22 Mbit/s |
| 17a | 17.664 MHz | 4,096 | 4.3125 kHz | +14.5 dBm | 150 / 50 Mbit/s |
| 30a | 30 MHz | 3,479 | 8.625 kHz | +14.5 dBm | 230 / 100 Mbit/s |
| 35b | 35.328 MHz | 8,192 | 4.3125 kHz | +17 dBm | 300 / 100 Mbit/s |
The rates in the last column are theoretical maxima on very short lines. A real line syncs lower, depending on its distance from the cabinet and the noise in the cable.
Why distance matters so much
High frequencies fade much faster on copper than low ones. On a line a few hundred metres long, the tones above 17 MHz that 35b adds arrive clean and raise the speed. On a long line they fade out on the way, so a 35b line does about as well as a 17a one. 35b promises up to 300 Mbit/s down and 100 Mbit/s up on lines shorter than 250 m.
The families
- 8a to 8d. The same range (about 8.8 MHz) at different power levels. 8b transmits the most power, which helps on long lines.
- 12a and 12b. A 12 MHz range, with the same power and the same theoretical maxima.
- 17a. A 17.664 MHz range. Common on VDSL lines fed from a street cabinet, usually together with vectoring.
- 30a. A 30 MHz range, but with twice the carrier spacing, so it has fewer tones than 17a.
- 35b. A 35.328 MHz range with 8,192 tones. Added to the standard in 2015; ISPs usually sell it as super vectoring.
Vectoring
Inside one cable, each copper pair picks up a little of its neighbours’ signal. This crosstalk is the biggest limit on VDSL speed. Vectoring (ITU-T G.993.5) coordinates the signals of every line in the cabinet and cancels the crosstalk before it reaches your router. For it to work, the cabinet equipment has to control the pairs in the cable, and your router has to support vectoring.
How to see your profile
Your router’s DSL status page usually shows the profile (Profile, VDSL Profile) next to the connection type. The guide to line stats shows where to find these numbers. To help others on your street, share your numbers on your cabinet’s page.