How the VDSL Connection Works

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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Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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