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Author Topic: Latency drop and throughput drop - any thoughts?  (Read 6592 times)

AndrueC

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Latency drop and throughput drop - any thoughts?
« on: June 25, 2012, 09:28:11 AM »

I've had FTTC from IDNet for a couple of months now. They regraded me to 80/20 a while back and things were going well. This was a typical Speedtest.net result:



For what it's worth the speed is lower than I got in actual downloads but it was consistent so I accepted it as a useful result for monitoring purposes. I'm glad I did.

Last Tuesday something changed. The latency dropped a lot (actually to a more reasonable value). What was less welcome was a corresponding drop in throughput:



Also noticeable is that instead of being a steady speed the speed is highly variable. Eratic even. Before the change I downloaded the Win8 RC at 6.9MB/s. After the change it's struggling to hit 3MB/s.

A BT Speedtest produces:

"Download speedachieved during the test was - 30.17 Mbps
For your connection, the acceptable range of speedsis 12 Mbps-76.76 Mbps .
Additional Information:
IP Profile for your line is - 76.76 Mbps

2. Upstream Test: -provides background information.

Upload Speed
7.01 Mbps

0 Mbps 20 Mbps
Max Achievable Speed

Upload speed achieved during the test was - 7.01Mbps
Additional Information:
Upstream Rate IP profile on your line is - 20 Mbps "

I've reported it to IDNet but their initial response is that they don't know if they can do anything because I'm above 12Mb/s. I know quite a bit about how broadband works in the UK so I understand that it's reliant on BTw but surely something is wrong here? Surely I don't have to live with 30Mb/s throughput with a 76.7Mb/s profile?

The exchange is Brackley although I believe that we are actually getting FTTC from Banbury.
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kitz

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Re: Latency drop and throughput drop - any thoughts?
« Reply #1 on: June 25, 2012, 10:14:53 AM »

Just to confirm the obvious because you dont make mention of it, but I'm assuming that your sync speed has stayed the same?

Are you aware if the decrease in latency co-incided with any change in DLM..  whats your ping like to somewhere like BBC?
If it was a DLM type change (ie interleaving off) then youd expect more sync speed and downloads to be quicker.

>>> Also noticeable is that instead of being a steady speed the speed is highly variable.

Hmmm..  spiky?   grab a copy of netmeter and use it to graph a largish download to see how variable it is.
Spikiness can be indicative of either congestion or artificial throttling (such as traffic shaping).
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AndrueC

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Re: Latency drop and throughput drop - any thoughts?
« Reply #2 on: June 25, 2012, 10:19:25 AM »

Just to confirm the obvious because you dont make mention of it, but I'm assuming that your sync speed has stayed the same?
To be honest I don't know. I had been putting off unlocking my modem. I will say that I've seen more 'normal' speeds for brief periods though. The graph that speedtest.net displays while running now looks like the Himalayas. Also I thought the profile was based off the connection speed?
Quote
Are you aware if the decrease in latency co-incided with any change in DLM..  whats your ping like to somewhere like BBC?
I'm at work so can't check that right now but will post figures tonight. For what's worth the Thinkbroadband Quality Monitor hasn't shown any disconnects other than when I tried switching the router off for half an hour to ensure I got a new session.
Quote
>>> Also noticeable is that instead of being a steady speed the speed is highly variable.

Hmmm..  spiky?   grab a copy of netmeter and use it to graph a largish download to see how variable it is.
Spikiness can be indicative of either congestion or artificial throttling (such as traffic shaping).
I'll do that. I think it is congestion, or routing myself. On ADSL2 (with Be) it was a rock steady line (13Mb/s at 34db). Also although the speed is naff it seems totally reliable so I think the modem connection is fine.

The irony of this is that I don't really 'need' the speed. I'm only averaging 7GB downloaded a month and most of that is probably speedtest runs, lol. All my machines now have Windows Update turned off as well(*). But it's very irritating to be syncd at 80Mb/s, 500m from the cabinet and apparently unable to get more than 30Mb/s. I might not need the speed, but damnit I do want the speed :)

(*)One two many failures to log back in on restarting.
« Last Edit: June 25, 2012, 10:22:53 AM by AndrueC »
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kitz

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Re: Latency drop and throughput drop - any thoughts?
« Reply #3 on: June 25, 2012, 11:36:38 AM »

>> I will say that I've seen more 'normal' speeds for brief periods though.

So even though its occasional, it shows that your line is still capable.  It does however seem odd that your latency should decrease at a time when speeds reduce.  Saying that if it is congestion, (at least on the BTw side of the network) latency is seldom affected.
The only reason I ask about doing a trace to bbc is that the speedtest.net ones can sometimes be a bit misleading - or at least they are for me personally on this line.

>> I think it is congestion,

Using  a netmeter graph, congestion shows as spikes all over the show with no pattern.   
Traffic shaping spikes are normally more contained within a specific speed band - kinda like teeth on a saw...  the sharpness of the teeth can depend on just how good the SP is at managing their traffic... ie the smoother the better.

>> The irony of this is that I don't really 'need' the speed.

hah I totally understand - its called geekiness  :D
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coolsnakeman

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Re: Latency drop and throughput drop - any thoughts?
« Reply #4 on: June 25, 2012, 04:16:17 PM »

Hey,

Just throwing this one out here but we are seeing customers being upgraded to 80/20 and although the checkers say they can get 75Mb or above they are still only achieving throughput speeds of 30-35Mb and in sync at 80Mb downstream and 20Mb upstream. We have now since then discovered that any customers on copper cable more than 400 metres long can not cope with the high speed from the fibre cable so they are having to then downgrade to 40/10. I have heard nothing as of yet wither BTO themselves have a fix for this yet but i would assume they would be working on a work around for it. There is a possibility you could be encountering the same problems. Perhaps you should get the length of your copper cable checked by your service provider to see if it is over 400 metres.

Regards
Gary
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AndrueC

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Re: Latency drop and throughput drop - any thoughts?
« Reply #5 on: June 25, 2012, 04:50:41 PM »

I don't think it's the connection. In the past (ie;all but the last week) I have seen speeds well above 35Mb/s. I download both of the recent Win8 ISOs and they came down at 6.9MB/s - fifteen minutes in total for both of them. It took me longer to burn them to DVD :)

Connecting directly to the modem makes no difference.

According to Netmeter throughput sometimes hits 70Mb/s but is usually lower than that. Curiously in the several tests I've run (to different speedtesters) the Netmeter output looks like a mountain. It ramps up to a peak (usually at least 50Mb/s, but several times it hit 70Mb/s) then it drops back down again. This is mirrored up to a point by the speedtest.net graph although it shows several other peaks on either side of the main peak.

This now explains what the 'out of XXXMb/s' figure is that the Visual Ware test displays. It's tell you the maximum speed it saw:

http://mcslhr.visualware.com/myspeed/myspeed_line_capspeed.html

Pinging bbc.co.uk is giving a fairly consistent 12ms.
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kitz

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Re: Latency drop and throughput drop - any thoughts?
« Reply #6 on: June 25, 2012, 06:35:28 PM »

Quote
According to Netmeter throughput sometimes hits 70Mb/s but is usually lower than that. Curiously in the several tests I've run (to different speedtesters) the Netmeter output looks like a mountain. It ramps up to a peak (usually at least 50Mb/s, but several times it hit 70Mb/s) then it drops back down again.

So based on that it looks unlikely to be your connection, but pointing more to congestion either with the ISP or BTw.

I still find the coinciding with the latency drop rather strange though. BTw's network has always prioritised latency - even way back 8/9yrs ago when contention first reared its head, and the only BTw side of things that could cause a decrease/increase in latency is a change of backhaul routing (exchange -> RAS).

Would be interesting to see (if you prepared to do a download test at silly-o'clock) if things are better in the wee small hours.
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kitz

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Re: Latency drop and throughput drop - any thoughts?
« Reply #7 on: June 25, 2012, 06:50:53 PM »

Quote
This now explains what the 'out of XXXMb/s' figure is that the Visual Ware test displays.

Oh nice!!!  :clap2: 
Its been a while since Ive used the visualware tests and although I've seen the summary stuff, I hadnt seen or noticed the detailed analysis before.   
Theres some very nice stuff in there..  specifically a graph (TCP pause graph) that can be plotted to help identify congestion.
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AndrueC

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Re: Latency drop and throughput drop - any thoughts?
« Reply #8 on: June 25, 2012, 07:08:15 PM »

Quote
This now explains what the 'out of XXXMb/s' figure is that the Visual Ware test displays.

Oh nice!!!  :clap2: 
Its been a while since Ive used the visualware tests and although I've seen the summary stuff, I hadnt seen or noticed the detailed analysis before.   
Theres some very nice stuff in there..  specifically a graph (TCP pause graph) that can be plotted to help identify congestion.
Yeah. I only got introduced to that recently but it's report on my connection at the moment is pretty disparaging. 26% consistency and a lot of pauses. So it ain't my connection but IDNet is also denying responsibility (which based on their forums I'd have to agree with). So basically somewhere in BT's network there's a blockage. And apparently sod all I can do about it :(
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kitz

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Re: Latency drop and throughput drop - any thoughts?
« Reply #9 on: June 25, 2012, 07:28:49 PM »


Have any of your neighbours got FTTC... who could have a look at how their connection is performing?
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AndrueC

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Re: Latency drop and throughput drop - any thoughts?
« Reply #10 on: June 25, 2012, 07:38:24 PM »


Have any of your neighbours got FTTC... who could have a look at how their connection is performing?
One does, I'm hoping to have a chat at some point. In theory as a newly enabled exchange it shouldn't be a problem but I believe that Brackley exchange isn't actually fibre enabled - we are all attached to Banbury. That's a different dynamic and I can imagine that if demand has been unexpectedly high there might be a problem.
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kitz

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Re: Latency drop and throughput drop - any thoughts?
« Reply #11 on: June 26, 2012, 03:23:35 PM »

I dont know if this helps... but Brackley/Banbury are ok according to BTw.. 
... in fact according to their latest capacity report, theres only 2 exchanges reported as having problems

CAPACITY ISSUES
Date 25/06/12 at 02:38 PM
2 Sites
MDFID
1141
Estimated Relief Date
CHELSEA
WRCHEL
L/FLA
15/09/2012
GATLEY
MRGAT
MR/GAT
30/08/2012
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AndrueC

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Re: Latency drop and throughput drop - any thoughts?
« Reply #12 on: June 26, 2012, 06:46:41 PM »

I dont know if this helps... but Brackley/Banbury are ok according to BTw.. 
Hmmm. It tends to knock one theory out of the water then. It has been suggested that it might be BTw reducing congestion.

Sigh.

It'll be interesting to see what my neighbour says but right now I appear to be stuck. ISP can't/won't help. BT don't see a fault. Just...the way it is.
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AndrueC

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Re: Latency drop and throughput drop - any thoughts?
« Reply #13 on: July 01, 2012, 07:35:59 PM »

It's back to normal now. No sign of a disconnect on TBBQM but all speed tests are 60Mb/s or higher and I got 7.9MB/s downloading. Interestingly I found an advisement on Zen's status page of work affecting my dialling code - 01280 - on the sixth of July. Perhaps they did it early?
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