Relationship between line attenuation and latency?

Hi,

I would like to get a better understanding of the difference between latency and transfer speed.

So say we have an ADSL all copper connection, that does not have a very high transfer speed. My understanding is, that this is because the copper cables run a long distance to the exchange (say 3-4 km for example), and on the way the signal gets distorted, meaning you cannot pass as much information through it, which relates to transfer speed. Is this correct?

But my understanding is also that despite the noise, one can have a pretty good latency. So why is that? Because my thinking would be if you had a lot of noise on the signal sometimes packages would not be able to get transferred, and be lost.

Also my assumption when it comes to noise is that it is inherently random. But is the noise predictable, so that the connection can still be quite stable despite these conditions? Or do we simply send the same packages a lot of times, with the idea that if just one of them comes through that is fine enough?

But my understanding is also that despite the noise, one can have a pretty good latency. So why is that? Because my thinking would be if you had a lot of noise on the signal sometimes packages would not be able to get transferred, and be lost.

That does happen, yes. What happens upon data loss depends on the protocol being used. TCP packets are numbered, and if one is missing, the receiver will request it to be sent again. The same packets aren't sent lots of times.

But generally speaking the noise is predictable - think about tuning an AM radio. The background noise between stations tends to be roughly the same volume/strength, especially in the shortwave bands.

"Good" is subjective of course. FTTP is almost always going to have a lower latency than ADSL because it uses light (rather than electricity) for a larger portion of the distance between you and the destination.

Hi and thank you for your answer!

What I meant by good is that the transfer speed between FTTP and copper ADSL, can be vastly different (like 900 Mbps v. 3 Mbps).

Whereas for latency the differences won't be that big, of course the ADSL will probably have a higher latency. But in most cases it should be something one can work with!

Edited by lo22 (Tue 26-Jul-22 11:49:12)

Part of how DSL works is that the transfer speed is set to account for the quality/length of the line. So the modems try to only transmit as much data per second as the line can handle. But as long you aren’t transmitting lots of data the the latency is still about the same as a faster DSL line. It takes about the same time for an individual packet of data to be transmitted, you just don’t get to transmit them as often as on a better quality line.

But I would just think that it was more likely to happen on a noisy line that some of the packages were lost due to noise?

But I guess the answer is that the noise is pretty constant, and this can be accounted for. And as long as you take this into account and transmit less, then it should work.

Longer lines cannot carry the higher frequencies to attain greater bandwidth due to the extra attenuation.
They may or may not be inherently more noisy than shorter lines and this can affect both throughput and latency if packets need to be retransmitted, but interleaving can help maintain or even improve bandwidth and throughput at the expense of latency due to the extra processing load.

Cooll. Thanks for the answer!

Would it make any sense to disable interleaving on a connection with a download speed of 3.0 Mbps and an upload speed of 0.4?

If the primary goal was gaming and thereby having as low of latency as possible.

Or would the connection basically become unusable, because the upload would become so low?

The main issue with a longer line isn't that it is noisy as such, it is that it is quiet, the signal is an a lower level by the time it reaches the other end than on a short line.

So think about the modems having to shout and talk more slowly when on a longer line. So messages still get through without error, just with less information per second.

(and yes, my analogy isn't great, given the way DSL actually works with frequency ranges and isn't really "shouting")

Edited by andynormancx (Tue 26-Jul-22 15:50:45)

In reply to a post by lo22:

Would it make any sense to disable interleaving on a connection with a download speed of 3.0 Mbps and an upload speed of 0.4?

If you have a 3-4 kilometre copper line and getting 3Mbps there isn't really anything that you're going to be able to do to make it significantly better, disabling interleaving is only going to make it more unstable.

You could think about the difference between latency and throughput as comparing a garden hose with a fire hose.

A garden hose will have much less water delivered per second than a fire hose (ie throughput). But, if you turned both hoses on at the same time then the water would start coming out of the end of them at the same time.

If one hose was much longer than the other then it would take longer for the water to get to the end of the hose - this is distance based latency (like if you connect to a new york server from london for Internet) but the width of the hose wouldn't be a big impact.

However, if you had a bit of a kink in the hose then you might find the water comes out in spurts. This is similar to if you have an unstable Internet connection then the data could come through in spurts - that would impact the latency of a specific droplet of water (or packet of data).

Might be pushing the analogy here and mixing them a bit but hopefully you get the idea. Bandwidth and latency are not directly related.

However, if you had a bit of a kink in the hose then you might find the water comes out in spurts. This is similar to if you have an unstable Internet connection then the data could come through in spurts - that would impact the latency of a specific droplet of water (or packet of data).

Like your hose analogy - the last bit though sounds a lot like 'jitter'

The speed of light is still the constant - ADSL is still at the speed of light. The issue is that the bandwidth available for optical signals is much much wides than a copper pair. Far greater than is used. Metal pairs are limited by their ability to retain and carry higher frequencies. These are lost over distances and you need the bandwidth to carry the faster signalling speeds. Lose the higher carrier frequencies and you lose the bandwidth. In optical systems these losses don’t happen in the same way because the light is bouncing along a pipe that reflects the photons back from the edges with almost zero loss. Metal cannot do that. To retain the frequencies for longer in metal we switch to coax rather than twisted pairs. Physics is king here.

Sadly there’s no such thing (unless you go into wierd science and cryo-tech) that gives lossless copper/metal transmission lines.

Switching speeds through the network hardware will also impact latency. Every joint will have an impact, large or small.

In reply to a post by AndrewNi:

FTTP is almost always going to have a lower latency than ADSL because it uses light (rather than electricity) for a larger portion of the distance between you and the destination.

From what I've read, that isn't true. The speed of light in fibre is about 70% of the speed in a vacuum, which is about the same as the propagation speed of a typical electric signal.

In reply to a post by andynormancx:

In reply to a post by AndrewNi:

FTTP is almost always going to have a lower latency than ADSL because it uses light (rather than electricity) for a larger portion of the distance between you and the destination.

From what I've read, that isn't true. The speed of light in fibre is about 70% of the speed in a vacuum, which is about the same as the propagation speed of a typical electric signal.

Its the switching delays, including backhaul and trunk nets, layer 3 routing variability, that contribute mostly to latency "on-shore" though.

Undersea links because of the enormous distances, the speed of light in fibre, it becomes far more of a prevalent factor. Hence the moves towards state of the art hollow-core fibre and highly optimised undersea paths especially for traffic for electronic trading platforms where cost is not an issue but every ns of time saved very much is.

Starlink have promised direct laser links between their LEO birds that have the potential to be faster than terrestrial fibre routes. GEO sats fixed in position above the equator, are too high up to benefit from this technique - but LEO's orbiting at roughly 500 km above sea level are a contender.

Hi, first of all thanks for all the great replies!

I think I have a better understanding of it.

Two things however:

1. I have been told that this ADSL connection has interleaving enabled. I think that adds latency. Would it be worth considering disabling interleaving?
2. Someone told me about buffer bloat, and how that could increase one's latency. As a possible solution that someone suggested getting another router that can handle buffer bloat. Would this be worth considering in your opinion?

Interleaving does add latency but it increases stability. If you ask them to force it to turn off then you may end up with an unstable line.

If you want to learn about buffer bloat go and have a look here:

https://www.bufferbloat.net/projects/bloat/wiki/What...

But, unless you do lots of online gaming or you have problems with VOIP or video calls, just don't worry about it.

In reply to a post by andynormancx:

If you want to learn about buffer bloat go and have a look here:

https://www.bufferbloat.net/projects/bloat/wiki/What...

But, unless you do lots of online gaming or you have problems with VOIP or video calls, just don't worry about it.

Well I do lots of online gaming! So should I just follow the advice from that site?

And do you actually have any issues with unusually high latency ?

I like the analogy. Would you mind if I passed it on to colleagues at work ?

So I got to try out this connection. So it is an copper ADSL, with max speeds of 3.5 /0.5 Mbps Download/Upload.

However when I try speedtesting it with my laptop using a cable I cannot obtain speeds of more than 1.8 Mbps. When I try my phone on WiFi it works better for some reason and I get speeds of 2.5 Mbps.

However the latency and jitter seem to be all over the place. And sometimes you get pings of 200-300 ms when only one computer is connected. It does seem you can get a decent latency under ideal circumstances in a game, but as soon as another program starts using bandwidth like Steam or Windows Updates, there is a massive lag spike.

So can anything be done about this!? Like for instance bufferbloat mitigation? Or is this connection essentially useless, I mean even for surfing you have to wait for things to load, haha

Sorry, been away for a week. Of course you can use the analogy if it works for you.

And btw another question is bufferbloat, only when other devices are hogging the bandwidth?

Because in my case what happens is that another application does something in the background, and then I get a ping spike because my internet is that poor (2.6 Mbps / 0.4 Mbps).

So I guess another router would not help with that when it is only one computer that is connected to the router? Or do I misunderstand bufferbloat?