Latency on gbit

[snozski]

Hi guys 

Just had Virgin installed yesterday, pretty happy with it the speeds are great however I'm a little disappointed in the pings I get now,  on my VDSL line before I had 9ms to bbc.co.uk  on this I'm at 27ms.  Is this normal?   Are there any settings I could ask virgin to change that could improve this ?  (like fastpath vs interleaved on DSL)? 

Thanks ! 

[Andrew-G · Accepted solution]

You can check the performance of your by setting up a BQM and that'll show what's going on with your VM connection.  

Unfortunately VM's DOCSIS technology normally has poorer latency than VDSL, with both a higher minimum latency, and greater latency spiking.  Normally this is not enough to interfere with gaming, video calls and live streaming.  A further aspect of DOCSIS is that when line conditions deteriorate, as often as not speeds hold up but latency suffers, whereas on VDSL speeds drop more readily than latency (some grand and not universal generalisations in there).  VM also try and run their network at higher contention rates than Openreach, so a small proportion of users suffer from "over-utilisation" which destroys latency.  A further DOCSIS peculiarity is that the performance can vary day to day for no obvious reason - see charts below.

Example BQM from a particularly good day:

And here's a mediocre day:

Obviously if latency performance is critical you may need to quickly consider your cooling off rights.  Sometimes VM can fix power or noise problems to resolve latency issues, sometimes it's just an inherently noisy circuit, and in that case you'll run into the company say there's no fault, and it won't ever get fixed.  

[snozski]

Hi Andrew

Thanks for the well written and thought out reply. 

I had heard a lot of people have terrible connections with cable and it never gets fixed.  Guess I was hoping I would not be one of them.

On the other hand, I clearly remember way back in the day when I played quake 3 all-time (circa 2000) the people with the best pings in the UK were all people on telewest/blueyonder cable (we used to call them London cablers 🙂 ) 

I'm not that bothered these days as I don't take gaming seriously anymore but yes I was a little surprised.  I've seen that adslguide tool before and will give it a go see whats going on ! 

Do you have any links for reading on understanding how  DOCSIS works in regards to SNR and power.   I'm pretty clued up on DSL having had it since it was first in beta 512k adsl1 then adsl 2+ then VDSL etc but I do not really know how cable works.   

The main thing really I wondered is if virgin can put interleaving or fastpath on like with DSL  IE if turning on/off error correction had an effect on latency

[Adduxi]

Have a read through of these two articles written by two Forum users.  They are a great source of information around VM Hubs and what to look out for.

https://community.virginmedia.com/t5/QuickStart-set-up-and/POWER-LEVELS-amp-SNR-A-TECHNICAL-PRIMER-u...

https://community.virginmedia.com/t5/Speed/Hub-status-data-understanding-network-log-messages/td-p/4...

As for Fastpath and Interleave, they are not used on cable broadband.

VM is a bit hit and miss, depending on where you live.  Some areas like mine are pretty good, but some areas are not so good.  It's a bit of a lottery at times, but you really need to setup a BQM within your 14 day cooling off period.  This will monitor and record the broadband circuit and give a great pictorial overview on the state of the local circuit.

[Andrew-G]

@snozski Do you have any links for reading on understanding how  DOCSIS works in regards to SNR and power.

There's an excellent pink sticky post of a DOCSIS primer authored by forum demi-god Sephiroth.  Some of the numbers on power levels are a bit out against the values that apply with VM (since these vary according to the network equipment used), but if you want to know how DOCSIS works that's the starting point.  As a broad rule, downstream power levels need to be no lower than -5 dBmV, no higher than +10 dBmV, with a range across all channels no greater than 4 dBmV.  On a gigabit line about half your traffic passes over a DOCSIS 3.1 channel (which can be considered as bonded frequencies used in conjunction with the bonded single frequency channels of DOCSIS 3.0), there's a few good summaries to read on that via the web since Seph's masterpiece pre-dates DOCSIS 3.1. 

The more I've learned about DOCSIS the more amazed I have become that it works at all.  The complexity is absolutely staggering, and whilst it is a testament to very clever engineers and technologists (and mathematicians), it is now an analogue technology that has been almost perfected as it comes to the end of its days.  VM have said they're going to transition from radio frequency DOCSIS to true optical FTTP (XGS-PON) which is a completely different technology, but for the vast majority of VM customers that's 4-8 years away yet.   

There's no fine tuning that can be done to make a DOCSIS connection work better - it can be made worse if there's noise or power problems, once in spec there's nothing more.  Also worth knowing the vast majority of VM staff and technicians don't know how DOCSIS works - they're trained to use some occasionally very ropey automated diagnostics, and various rules of thumb when it comes to fault finding and fixing.  That works most of the time, but means the concept of the Completely Beyond Us (CBU) fault is a risk for a very few VM customers.  I seem to be in that place at the moment, with appalling reliability of my 200 Mbps connection. 

Problems with gigabit connections seem proportionality more common than for other speeds, but if everything is working as you want, then don't get wrapped up in the technology and the numbers.

[snozski]

I really appreciate you taking the time to write this for me.   I will look into it all over the next few days.