G.Fast, Supervectoring & VDSL2

By Michael Spalter
September 2021

About the author

Michael Spalter

Michael Spalter


Michael Spalter has been a networking technician for over 30 years and has been the CEO of DrayTek in the UK since the company’s formation in 1997. He has written and lectured extensively on networking topics. If you’ve an idea for a blog or a topic you’d like explored, please get in touch with us.


VDSL (Very High Speed DSL) covers several technologies all using FTTC (Fibre To The street Cabinet. The street cabinet has a fibre optic feed back to the BT (or other) network and then uses a regular line - copper wire - to the customer premises. The copper part is short, generally up to 400 metres. VDSL2 can run over much longer distances but speed drops considerably (to around 5Mb/s over a 2Km line - slower than ADSL). BT originally had plans to introduce LR-VDSL (long reach VDSL) which used higher power to deliver greater speeds over longer distances but this was eventually abandoned.

FTTC is different to ADSL in that, with ADSL, the DSL signal is added at the telephone exchange (Central Office) and can be carried over several Kilometres to the your home/office (Consumer Premises) over the copper line - ideal for less densely populated areas where installing fibre and a VDSL street cabinet for just a handful of users may not be cost effective.

ISPs use many different marketing terms to describe VDSL technologies (superfast, fibre etc.) and it's not necessarily consistent so always be sure exactly what technology and line type an ISP is offering/advertising.

VDSL2

The most commonly used VDSL technology in the UK is currently VSDL2, which uses a signal bandwidth of up to 17Mhz with up to 4096 carriers and a symbol rate of 4000 bps. This can provide speeds of over 100Mb/s, but to aid reliability, it is capped at 80Mb/s - speed is directly related to line length and quality. The VDSL2 specification does also have a 30Mhz bandwidth profile - however this is not supported/used in the UK, as it introduces too much crosstalk to other services to provide a tangible benefit.

VDSL2 Supervectoring / 35b

VDSL2 Supervectoring uses a higher bandwidth of up to 35 Mhz (hence known as profile '35b') with 8292 DMT carriers of 4.3Khz each and provides speeds of up to typically 200 - 250Mb/s (downstream) in real-world usage. The max line profile used depends on what the specific ISP has found to be feasible in their experience. To benefit from the wider bandwidth, supervectoring has specific methods of noise cancellation to reduce crosstalk from adjacent DSL lines in a bundle which would otherwise provide inductive interference. Supervectoring is not the same as vectoring; vectoring (G.998.4 / G.INP) is widely used on UK VDSL2 lines and uses noise mitigation methods to provide higher connection speeds. Supervectoring is not used by any ISPs in the UK/Ireland (it is available in the Isle of Man) and none have plans to use it as it's incompatible (causes interference) with other telecom services.

G.Fast

G.Fast is only available on FTTC circuits with a copper line length (street cabinet to customer premises) of less than 250 metres (although the specification allows longer, with consequential drop off of speed and reliability). G.Fast is available with a bandwidth of up to 106Mhz (the specification allows for up to 212Mhz).  G.Fast uses similar vectoring methods to VDSL2/35b in order to reduce crosstalk. G.Fast provides real-world speeds of up to 300Mb/s and provides shorter retrain and rate adaptation times than VDSL2.  Compared with VDSL2, a G.Fast line can have its start frequency configured to avoid interference with ADSL lines within the same cable bundle. G.Fast uses an higher symbol rate than VDSL2 and provides lower latency.

G.mgfast

Future plans for G.Fast specifications include bandwidth profiles of up to 424Mhz however this is only at the planning stage currently (not supported by any vendors or planned for by ISPs) and the increasing deployment of full fibre - FTTP (Fibre to the Premises) may overtake the need for new DSL technologies. 

Comparison of DSL Line Types

Line TypeITU-T
Spec.
BandwidthSpeed
(DS)
Bits per
Bin
BinsSubcarrier
Spacing
Symbol
Rate/s
ADSL2+ G.992.5   2.2 Mhz  24Mb/s 15  512 4.3Khz  4000
VDSL2  G.993.2  17 Mhz 80Mb/s 15 4096 4.3Khz  4000
Supervectoring
35b
G.993.2  (A.1)  35 Mhz 250Mb/s 15  8292 4.3Khz 4000
G.Fast G.9701 106 Mhz 300Mb/s 12  2048 48Khz 48000

The diagram to the right compares the maximum downstream rate possible with VDSL2, Supervectoring 35b and G.Fast up to line lengths around 500M. You can see that G.Fast is by far the fastest, but only up to around 300 Metre line lengths.  This is because, the higher the carrier frequency (a DSL signal is made up of hundreds of individual carriers), the more it gets attenuated by the line, so lower frequencies can travel further. If you have a long line, the higher frequencies are not usable, so you have less carriers, thus less speed.

The figure at a given line length are all maximum speeds in perfect environments. As well as the line length, local/ISP implementation, and line quality/wiring will provide lower speeds in the real world.



Photo: Main BT incomer and master socket (NTE5 Mk.3). 1. BT NTE5 Mk3. Main Unit. 2. Removable voice faceplate for phone extension wiring. 3. Incomer junction box. 4. External BT cable from street. 5. Connection into NTE5. 6. To Voice/phone extensions. 7. RJ-15 Cable to modem/router. 8. Telephone cable.

Getting the best speed on your DSL Line

For the best speeds, you want a line with the best conductivity (copper is better than aluminium), adequate conductor diameter (0.5mm or 22AWG is best), good clean, dry terminal connections and as few joins as possible. Always use proper solid-core (not stranded) twisted pair telephone cable.

That will provide the best SNR across all frequencies in use. You can't do much about street cabling on the way to your premises if that has poor connections or multiple joins but, within your premises, connect your CPE (modem/router) into to the main incoming 'master' wallbox if you can and have that master wall box as close to the street (i.e. the shortest internal cabling possible).

It's better to have the modem/router in an 'inconvenient' place but which gets best line speed and then extend the network internally over Ethernet.  Run a cable to a separate wireless access point, best positions for the Wi-Fi client.

Note: BT Openreach do not permit you to move/interfere with the master socket or any junction boxed 'before' the master socket yourself. In the photo, our modem is connected directly into the master socket (1), which is best.

If your socket contains the DSL microfilter (as shown on the right), you cannot extend your DSL modem wiring on the filtered 'voice/phone' connection - you must extend the 'raw' unfiltered signal, but best not to extend your wiring at all. Other sockets provide just a single BT type socket and require the use of a plug-in microfilter.

In part 2, you can read about how does a DSL line actually work.

I hope you found this article useful; please do share the links with others, make comments below and if you have any suggestions for articles, please do let us know.


Tags

VDSL
VDSL 2
Supervectoring / 35b
G.Fast
FTTC