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Router connection speed vs Download test speed

stevebirch
Newbie
Posts: 4
Registered: 09-10-2014

Router connection speed vs Download test speed

Hi,
I'm just looking for some advice about what ADSL connection speeds I should expect.
Several months ago I was on Virgin ADSL and generally achieving download speeds at approx 3.5Mbps.  I changed to Plusnet and found that my download speeds were constantly at 1.0Mbps.  After a couple of months I found that my speeds were constantly at 1.3Mbps (this appeared to be an overnight leap in performance).  A few weeks ago I replaced my Plusnet router with a new Billion router in the hope that I could get back to my previous speeds.  There was no immediate performance improvement.  However, a few days later my speeds leapt to a consistent 2.3Mbs.
So, it seems that I'm slowly moving closer to my old speeds (in leaps).  However, I'm still a reasonable way off what I used to get.  Additionally, my router always appears to think its connected at approx 3.4Mbps
I would like to understand what speed I should consider a reasonable expectation.  If my router is connecting at 3.4Mbps, is it reasonable to expect download speeds at that level?  The bit I'm struggling to understand is why my achievable download speeds are lower than my router connection speeds.  Does this imply some sort of throttling, or am I miss-understanding what this means?
Thanks in advance for any advice.  My router connection stats are shown below.
Thanks,
Steve
DownstreamUpstream
Line Coding (Trellis)OnOn
SNR Margin (dB)7.111.0
Attenuation (dB)56.531.5
Output Power (dBm)18.612.5
Attainable Rate (Kbps)4308968
Rate (Kbps)3488736
K (number of bytes in DMT frame)11024
R (number of check bytes in RS code word)1616
S (RS code word size in DMT frame)2.002.00
D (interleaver depth)84
Delay (msec)4.004.00
INP (DMT symbol)0.270.20
11 REPLIES
kitz
Rising Star
Posts: 817
Thanks: 46
Registered: 08-06-2007

Re: Router connection speed vs Download test speed

I'd expect a 57dB line to be connecting at 3488 kbps.
I havent just said that either - check out the Maximum adsl speed calculator and put in 57dB.
That said - the calculator is based on a standard 6dB SNR and no interleaving.  Your line is interleaved so its actually performing slightly better than anticipated.    
Interleaving & error correction adds additional overheads - which is why your router is reporting 4308 in the attainable rate.  
As you can see from the speed calculator,  if you connected at 3488, then you'd expect your IPprofile to be at 3076 if on 21CN (or 3000 on 20CN) ..  so throughput speeds should be just slightly less than that.

Quote
I'm struggling to understand is why my achievable download speeds are lower than my router connection speeds.  Does this imply some sort of throttling,

No its not throttling.  When a line is interleaved it carries extra [redundant] data packets to allow any lost data to be recovered.   Its usual for most routers to display a much higher attainable rate on interleaved lines as it makes an adjustment for the redundancy.   DSL uses the Reed Solomon encoding
A point to mention is that if interleaving was switched off..  it doesnt mean that you will get a sync of 4308.  You may get a bit more, but not that much.   Its perhaps easier to explain it the other way round with some example figures which arent accurate, Ive just plucked them out of air to use as the rate can be affected by interleaving depth.
You may have a line that syncs at 3800 kbps, with an attainable rate of 3800, but its unstable and getting lots of errors and packet loss so the DLM applies interleaving.
Once interleaved, the line may sync at 3500 kbps, but the router now shows an attainable rate of 4100 kbps which is based on the redundancy overhead.

Although you never physically 'see' the redundant data in speedtests etc  it will be there in the background so the router can use it to reconstruct data packets when needed.   Your FEC count will show you when your router has needed to reconstruct data packets from that redundant data.
HTH
kitz
Rising Star
Posts: 817
Thanks: 46
Registered: 08-06-2007

Re: Router connection speed vs Download test speed

Quote
if you connected at 3488, then you'd expect your IPprofile to be at 2500 3076 .


hmmm  I think I need to look at that calculation again & check the code..  I think it may have got muxed up with 20/21CN  Undecided

Can you check what your IP profile says on a BT performance test?

------
No it was me having a blonde moment and reading the figure wrong.  I dont know if youre on 20CN or 21CN -  Crazy  I'll edit my post with the correct figures in a moment.
Community Veteran
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Registered: 15-06-2007

Re: Router connection speed vs Download test speed

I don't wish to argue with the experts but as the OP is on 21CN a rate of 3488kbps would giver a IP profile of 3076kbps and a Plusnet profile of 3Mbps
kitz
Rising Star
Posts: 817
Thanks: 46
Registered: 08-06-2007

Re: Router connection speed vs Download test speed

Yep OJ..   I realised myself as soon as I'd hit 'post' that I'd had a blonde moment and looked at the IPprofile figures the wrong way.  
Corrected now.  You can see my thought process on this.  Post.. look at the figures and go.. no that cant be right..  think that my code must be wrong.. then realised I was being an idiot and had taken the 2500 from a 20CN sync on the calculator.
Okay.. blonde moment aside with the IPprofile calculation..  I think the rest of the post is ok..  I shall now go beat myself with a stick for being an idiot Crazy
Community Veteran
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Registered: 10-06-2010

Re: Router connection speed vs Download test speed

I have a similarly long line (maybe slightly longer), and I found that the sync speed was about 1Mb lower during the week or so when interleaving was off. I think interleaving essentially increases the effective SNR, taking into account the coding gain from the interleaving.
kitz
Rising Star
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Registered: 08-06-2007

Re: Router connection speed vs Download test speed

Ive never actually looked into what the actual overheads are and how much the coding gain is.  I just know its there.  The only calculation that I can recall off the top of my head is S= 1/2 mode.
It may be something of interest for someone to look into should they ever feel so inclined.  I think I shall pass though.  Wink
stevebirch
Newbie
Posts: 4
Registered: 09-10-2014

Re: Router connection speed vs Download test speed

Thanks for the info.  That's a help.  It can be a bit hard getting started on this subject, but I'm learning...slowly.  Smiley
The BTWholesale checker says my IP profile is 3.08Mbps.  So, the router is connecting at at a given speed, but the download speed test results I see appear to be lower due to the redundancy in the actual data sent.  That all seems to make sense.
Why would I see such a large difference in speeds when switching from Virgin to Plusnet?  My download test speeds drop from ~3.5Mbps to ~1.0Mbps (I'm sorry, I don't have any router details from when I was on Virgin).  Since then I've noticed two upward increments in speed.  I have a SamKnows whitebox which is where I'm seeing the increment (see graph attached).  Whenever I do a download speed test it matches what SamKnows is telling me, so I assume the values are reliable as indication of download speeds (i.e. the effective speed after redundancy is accounted for).  I wonder why the speed steps up and stays around that level.
One of the reasons I bought this new router was to be able to tinker with the SNR values.  I haven't done much with this yet as I need to understand more first (I set the SNR value briefly just to have a go, then put it back to default for now).  Am I being unreasonable to expect download test speeds of ~3.5Mbps after a little tinkering (given that's what I used to get)?
Thanks.
Community Veteran
Posts: 5,062
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Registered: 10-06-2010

Re: Router connection speed vs Download test speed

No. The speedtest result should be fairly close to the IP Profile.
One easy thing to check is the plusnet current line speed: https://portal.plus.net/my.html?action=data_transfer_speed
It should be 3 (same as the BT IP profile, but rounded down).
Are you doing the speedtests over wireless or with an ethernet connection to the router?
You could also find the DSL statistics with the error counters (FEC, CRC or alternatively called corrected and uncorrected errors). It might also be worth seeing if the line is running as ADSL1 or ADSL2. I wouldn't recommend tinkering with the target SNR margin, your DSL line rate is already at about the expected rate for the attenuation figure.
stevebirch
Newbie
Posts: 4
Registered: 09-10-2014

Re: Router connection speed vs Download test speed

Recently, I've been doing the speed tests over WiFi, however I have previously been doing them on wired LAN and I've had the same results.  Also, the SamKnows whitebox is sampling download speed approx 6 times a day and that operates over wired LAN so I'm happy to use the speeds shown in the graph I attached.
stevebirch
Newbie
Posts: 4
Registered: 09-10-2014

Re: Router connection speed vs Download test speed

I'm a little unsure how to identify the ADSL type.  Would it be "ADSL_G.dmt"?
I'm also unsure about the error information.  This is what I'm seeing:
DownstreamUpstream
Super Frames5150050651500506
Super Frame Errors8912445
RS Words1377774919688895420
RS Correctable Errors25474060
RS Uncorrectable Errors351860
HEC Errors83088500
OCD Errors3580
LCD Errors3580
Total Cells28815050670
Data Cells10402408330
Bit Errors00
Total ES35520
Total SES880
Total UAS5555
Superuser
Superuser
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Registered: 22-08-2007

Re: Router connection speed vs Download test speed

Steve,
Yes it would - that indicates that you are on ADSL(1) modulation and the US speed tells us you are on a 21CN circuit.
I hope this will help you to understand a little better how things work...
1. The router negotiates a bit SYNCH speed with the exchange which will achieve the target SNRM
2. The PROFILE speed is then set at 88.2% of the synch speed - this is the MAXIMUM DATA speed and is intended to ensure that the line is not "over-run"
3. The MEASURED DATA SPEED is the usable transfer speed AFTER errors and retransmissions have been handled - it is the EFFECTIVE USABLE speed
Trying to push the bit rate (by lowering the SNRM) can simply result in a higher error rate, leading to higher retransmissions and a lower effective data rate.
HTH,
Kevin