VDSL (Very-high-bit-rate Digital Subscriber Line) is another popular type of DSL (Digital Subscriber Line) Internet connection. As its name suggests, it allows a higher transfer rate than ADSL. Let’s see how it works.
In the United States, ADSL is popularly known simply as “DSL,” which is a misnomer, since VDSL is also a type of DSL connection. Differently from ADSL, VDSL also allows the transmission of TV signals, so to the end user, VDSL is more similar to (and competes with) the cable TV system. Another characteristic that puts this system closer to the cable TV system is the use of fiber optics outside the service provider’s building, as we will see. In the U.S., popular VDSL networks include AT&T’s “U-verse.”
On DSL technologies, the limiting factor for speed is the length and quality of the cables used by the phone company, since it uses regular phone cables (twisted copper pair). VDSL solves this problem by reducing the length of the standard cable by installing an optical node closer to the user’s home, and the connection between this optical node and the service provider is done through fiber optics, while the connection between the node and the user’s home is done using standard telephone wires. This is exactly the same idea used by the cable TV, except that cable TV uses coaxial cables instead of telephone wires.
VDSL goes a step further, and allows the optical node to be installed closer to the user’s point of installation, shortening the regular telephone wires even more, which allows higher speed rates. VDSL even allows fiber optics to be delivered directly to the user’s home.
Depending where the optical node is located, a VDSL network can be classified as:
- FTTN (Fiber To The Node): The optical node is installed in the user’s neighborhood, similarly to what happens with cable TV
- FTTC (Fiber To The Curb): The optical node goes up to the curb of the user’s building
- FTTB (Fiber To The Building): The fiber optics enters the user’s building, but regular cable is used to connect the user’s apartment to the node
- FTTH (Fiber To The Home): The fiber optics enters the user’s home, and no regular telephone wire is used
Currently, there are two types of VDSL connection, VDSL and VDSL2. See table below. Currently, when we say “VDSL”, we are actually referring to “VDSL2.”
|Name||Standard||Maximum Download||Maximum Upload||Bandwidth|
|VDSL||G.993.1||55 Mbps||15 Mbps||12 MHz|
|VDSL2||G.993.2||Varies*||Varies*||8.8 MHz, 12 MHz, 17 MHz or 30 MHz|
* See next page.
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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.