XVI. DSL Diagnostics

Vectoring

VDSL Vectoring is a method of increasing the speed of a given VDSL2 (G.993.2/FTTC) line so that it can achieve a higher speed than it would otherwise achieve. It does so by using noise cancelling technology to reduce interference from adjacent lines or other services or plant (equipment). Vectoring is defined by the ITU-T standard G.993.5.

Vectoring is distinct from G.998.4 (formlerly known as 'G.INP') which is another method of increasing immunity to spikes or bursts of EMF (Inpulse Noise) and also Seamless Rate Adaptation (SRA) which adjusts your rate according to current line conditions.

BT are calling vectoring a 'speed enabling' method rather than a 'speed increasing' method, which may seem semantic, but the goal is that they provide connectivity closer to the current 76Mb/s maximum for more users, rather than increase speed for users already running near to those speeds. In tests so far, users reported an increase of 10-15Mb/s when using Vectoring for lines which were previous running at 50-70Mb/s.

It's not yet known what benefits Vectoring be gained to lines which run at much lower speeds due to line quality or length. VDSL is very much dependent on line length for its speed, which is why the cabinet and MSAN must be in your street or a nearby street (compared to ADSL which can run for miles from the exchange). VDSL lines longer than 500M see a significant fall-off in speed. at 1Km the speed can reach approximately 25Mb/s which is not very different to ADSL2+ Annex M. VDSL Vectoring is unliklely to provide much benefit to very long VDSL lines where the issue is attentuation, not just noise or crosstalk, though of course, the longer a line is, the more opportunities it has to suffer interference.

G.INP / G998.4

G.998.4 is a newer alternative to FEC (Forward Error Correction) and interleaving to providing a lower overhead (FEC loses about 12% whereas G.INP only corrects errors when they actually occur) so there is little overhead. G.998.4 was formerly known as G.INP and may also be referred to as Impulse Noise Protection or Physical Retransmission (PHY-R).

G998.4 operates at the physical layer. The transmitting modem keeps a copy of transmitted data in a retransmission buffer. If the receiver detects corruption (by way of a checksum) a retransmission is requested. If the transmitter responds in time, the data is repaired. The round-tip time of a retransmission should be <4ms. If the transmitter doesn't retransmit in time, the corrupt data is forwarded for higher level protocols to sort out. 

What if I have older products which don't support vectoring or G.INP?

If you have an older product or any product which does not support vectoring or G.INP, then you will still be able to connect - it's just that  you just won't see any benefit which you might have otherwise.   In some cases, the telco may switch your line from a 'fast' profile to an 'Interleaved' profile, which has slightly higher latency.

What is latency?

Latency is the time taken for data to be received from a transmitting device - ie. for your data to get from the server you're accessing, to your own PC.  Latency is measured in ms (milliseconds), for example 95ms (which is just under 0.1 seconds). If you access a server on the other side of the world, the latency (transit time) will be higher than a server in the same country or city (depending on the actual route). Higher latency does not affect download or upload speeds (throughput) so you wouldn't notice any effect on web browsing or multimedia, but you might care about it if you were an online gamer or financial trader, where a millisecond can make a material difference (i.e. getting fragged or losing a pip in the dollar/starling rate etc.)

What is interleaving?

Interleaving is a method of error correction used on DSL lines; an interleaved profile can be applied to a line to improve resilience (as opposed to a 'fast' profile) to burst of errors. Interleaving can add additional latency of up to 40ms depending on the maximum interleave depth that has been enabled on the line profile. 

What is INP?

INP (Impluse Noise Protection) is a method of protecting against or correcting the effects of sudden bursts of noise on your line caused by adjacent equipment, lines or other interference. The value of INP (1,2,4 etc.) is the number of DTM symbols (each symbol is a segment of data representing bits as tones) that can be completely corrected for errors. The higher the INP the more resilient a line is to a burst of errors but at the cost of higher overall latency. The INP scheme used could be G.INP (G.998.4) or a traditional scheme based on Reed-Solomon coding and interleaving.

DrayTek Supporting Products 

Various  DrayTek  models  support  Vectoring  and G.INP - please check products' individual specifications on this web site to check. You may also be able to download the latest firmware to add support - check the release notes.

VDSL Beyond 100Mb/s

VDSL does have profiles within the standard which theoretically will run at up to 200Mb/s by using wider bandwith. The current '17a' profile will run at up to 100Mb/s (in perfect lab conditions) but a 30a profile could run at up to 200Mb/s but in reality, even if any telcos introduce 30a operation, speeds are likely to be much lower (for example 140Mb/s on good lines).

FTTP (Fibre to the premises) will run much faster than copper but whereas FTTC (like ADSL) uses existing phone lines which run to almost all premises already, FTTP require each house of office to be newly wired with fibre - a costly exercise. In some countries (for example The Netherlands) new-build homes and offices are pre-wired with fibre and a fibre tray ready for you to drop your modem/router into.