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Home » Networking
How the VDSL Connection Works
Author: Gabriel Torres 42,959 views
Type: Tutorials Last Updated: September 17, 2013
Page: 2 of 2
How it Works

VDSL works similarly to ADSL, dividing the available band into channels, and testing the signal-to-noise ratio of each channel to determine its maximum speed, process known as DMT (Discrete Multi-Tone). Read our ADSL tutorial for a more in-depth explanation of this process.

The main difference between ADSL and VDSL is the bandwidth available. While ADSL and ADSL2 have available a 1,104 kHz band, which is divided into 256 channels, and ADSL2+ has available a 2,208 kHz band, divided into 512 channels, VDSL can use a band of either 8 MHz, 12 MHz, 17 MHz, or 30 MHz, as shown in the table below. The use of these wider bands allows far higher transfer rates.

Profile

Bandwidth

Channels

Channel Size

Max. Downstream Speed

Max. Upstream Speed

Downstream Power

Upstream Power

8a

8,832 kHz

2,048

4.3125 kHz

50 Mbps

NA

+17.5 dBm

+14.5 dBm

8b

8,832 kHz

2,048

4.3125 kHz

50 Mbps

NA

+20.5 dBm

+14.5 dBm

8c

8,832 kHz

2,048

4.3125 kHz

50 Mbps

NA

+11.5 dBm

+14.5 dBm

8d

8,832 kHz

2,048

4.3125 kHz

50 Mbps

NA

+14.5 dBm

+14.5 dBm

12a

12,000 kHz

2,783

4.3125 kHz

68 Mbps

NA

+14.5 dBm

+14.5 dBm

12b

12,000 kHz

2,783

4.3125 kHz

68 Mbps

NA

+14.5 dBm

+14.5 dBm

17a

17,664 kHz

4,096

4.3125 kHz

100 Mbps

50 Mbps

+14.5 dBm

+14.5 dBm

30a

30,000 kHz

3,479

8.625 kHz

200 Mbps

100 Mbps

+14.5 dBm

+14.5 dBm

The maximum speeds are theoretical and actual speeds will depend on the length of the copper wires. Initially, 100 Mbps speeds could only be achieved on FTTH deployment, however, with a noise-cancelling technology called “vectoring,” now it is possible to achieve this speed with regular copper wires.

What is interesting about VDSL is that the division of the band is done in a way to keep VDSL 100% compatible with ADSL. The beginning of the band is divided just like ADSL2+, and the rest of the band is divided into several intercalated upstream and downstream bands. The division of the available spectrum varies depending on the VDSL2 profile and also on the band plan followed by the service provider. On this link you can see detailed diagrams of all possible deployments.

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