Unusually low latency with interleaving

[roseway]

This isn't a problem, just a curiosity. My ADSL Max connection is interleaved and always has been. In the past, when I've pinged a responsive UK site such as bbc.co.uk I got a value of around 40 ms. But now, if I ping bbc.co.uk I get a result averaging less than 30 ms, with some results as low as 25 ms. It's normally expected that interleaving will result in an increase in latency of 20-40 ms, so this seems unusually low. I don't know if this is relevant, but i see in my router's connection data the line:

Delay of latency path:     4.00 ms

but i don't know if this relates to interleaving.

It appears that this router, on this connection, delivers surprisingly low latency. It's an unlocked BT 2700HGV Business Hub. Can anyone shine any light on this?

[orainsear]

I think that 4.0 ms figure correlates to the interleaving depth (higher delay = higher depth) and is the delay seen between 2Wire and the DSLAM.

Do you know the chipset in the DSLAM?  When testing various configurations I found that the TI chipsets (in the modem) regularly gave good results for latency, and even better if the chipsets matched.

There may also have been routing changes, changes in the backhaul system which could account for something.

I'd also be cautious about using the BBC for ping tests.  I found recently that I was losing packets to bbc.co.uk - basically they were being dropped by their servers (gets worse in the evening), and I think is probably related to congestion on their part.

[roseway]

Thanks for your comments. The DSLAM has got an Alcatel chipset.

I note what you say about the BBC, and on the rare occasions I do anything like this I check out a couple of other UK sites too, and I got very similar results when I did this.

[Azzaka]

Try using echo.zen.co.uk. This is setup specifically for PING.

On the latency note, could be your line quality has somehow increased or a high load has been removed from you VP/Line Card.

[roseway]

Oddly enough, bbc.co.uk now gives me an average of 25 ms, but echo.zen.co.uk gives 30 ms. I guess that the difference is in routing, so when I've got a moment I'll check that out.

[waltergmw]

Hi Eric,

The three long line HGV 2700 modems I look after all give an 8 ms latency here. They are on different DSLAMS and have attenuations between 75 and 81 dB.
I've put one picture below.

Just for reference, here are my pings on Virgin Media to the zen site

64 bytes from 212.23.5.209: icmp_seq=0 ttl=54 time=18.018 ms
64 bytes from 212.23.5.209: icmp_seq=1 ttl=54 time=17.880 ms
64 bytes from 212.23.5.209: icmp_seq=2 ttl=54 time=34.064 ms
64 bytes from 212.23.5.209: icmp_seq=3 ttl=54 time=17.597 ms
64 bytes from 212.23.5.209: icmp_seq=4 ttl=54 time=18.350 ms
64 bytes from 212.23.5.209: icmp_seq=5 ttl=54 time=18.552 ms
64 bytes from 212.23.5.209: icmp_seq=6 ttl=54 time=18.464 ms
64 bytes from 212.23.5.209: icmp_seq=7 ttl=54 time=18.426 ms
64 bytes from 212.23.5.209: icmp_seq=8 ttl=54 time=18.232 ms
64 bytes from 212.23.5.209: icmp_seq=9 ttl=54 time=18.785 ms

--- echo.zen.co.uk ping statistics ---
10 packets transmitted, 10 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 17.597/19.837/34.064/4.754 ms

[roseway]

Thanks Walter, that rather confirms that the latency which I see is a function of routing, or with my own ISP. Not that it's a problem of course (my ping values are still quite low) but it's interesting to try to find where the limits are.

[kitz]

delay does relate to interleaving, but probably not in the manner which seems most obvious as being the amount of time that would be added on to the existing latency.

Delay in this case is the time spacing between associated segments of the interleaved stream.
The explanation at the bottom of the page and the corresponding diagram will probably explain it better.

http://www.kitz.co.uk/adsl/interleaving.htm


---
As regards to the changes in latency times... could be a few reasons

Its possible that the depth of interleaving in your profile may have been amended in your BTprofile - theres several different levels.

A common one is change of routing on the BT backhaul such as what may occur after a reRAS or BTw mapping.  A large portion of the 'time' is often on the ATM part of their network between exchange and RAS.

[roseway]

Thanks for that explanation.