Speed Issues (Poor S/N Ratio)

Connection issues, drop outs or speed related faults for ADSL and ADSL2+ services
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aphasia
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Speed Issues (Poor S/N Ratio)

Post by aphasia » Sun Apr 11, 2010 3:09 am

been noticing some speed issues on my connection lately. not sure you guys can help with this as my S/N ratio is horrible.

UpStream;
-speed: 1000Kbps
-output pwr: 20.5dBm
-S/N Ratio: 13dB
-attenuation: 15dB

DownStream;
-speed: 18.07Mbps
-output pwr: 0 dBm
-S/N Ratio: 6 dB
-attenuation: 26 dB

as you can see my S/N is rubbish especially on the downstream. attenuation is good for both and i'm syncing just fine. i've done the usual trouble shooting steps (iso test & 3 different phone leads, power cycle modem) none of which made much discernable difference. i only have the 1 phone socket in the house and in the recent past i've had very good connection speeds (btw 1.5 to 2MB/s). i know my phone exchange is 1.3 to 1.5km from my house via direct street route.

i'm not sure what’s happened to degrade my connection so, whether its some environmental issue, problems at the exchange or damage to the phone lines themselves. any help or suggestions would be appreciated though.

cheers.
"Arguing on the internet is like running in the Special Olympics. Even if you win, you're still retarded".

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Dazzled
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Re: Speed Issues (Poor S/N Ratio)

Post by Dazzled » Sun Apr 11, 2010 7:42 am

Apart from a lowish SNR margin (most modems report the margin) does the modem report transmission errors?

jubba
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Re: Speed Issues (Poor S/N Ratio)

Post by jubba » Sun Apr 11, 2010 9:58 am

unless your getting drop outs your snr margins are fine.
When you say speed issues what do you mean? Constantly low speeds or low speeds at certain times of the day? Is it low speed on all websites or only some?

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CoreyPlover
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Re: Speed Issues (Poor S/N Ratio)

Post by CoreyPlover » Sun Apr 11, 2010 12:57 pm

aphasia wrote:as you can see my S/N is rubbish especially on the downstream.
Your S/N margin is wonderful.

Low S/N margin means that your line has low fluctuations in noise, therefore only a 6dB buffer is required to maintain sync and prevent drop outs. Typical margins are 9db, 15db would be considered bad.
aphasia wrote:in the recent past i've had very good connection speeds (btw 1.5 to 2MB/s). i know my phone exchange is 1.3 to 1.5km from my house via direct street route.
S/N margin does not affect speed, it affects drop outs (even the sync speed listed is *after* S/N marin is deducted so you are syncing at full 18Mbps speeds). Post some indication of your speeds for further comments (http://speedtest.net, or www.exetel.com.au/speed/ or download some files from microsoft.com)
I am a volunteer moderator and not an Exetel staff member. As with all forum posts, mine do not constitute any "official" Exetel position. Support tickets may be logged via https://helpdesk.exetel.com.au or residentialsupport@exetel.com.au

aphasia
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Re: Speed Issues (Poor S/N Ratio)

Post by aphasia » Sun Apr 11, 2010 3:28 pm

howdy

thanks for the replies guys.

'Dazzled' - there are some rx bad packets (total rx packets = 4.6 million / rx bad packets 130,000)

'jubba' - browsing and downloading has been slow since about 7pm last night till approx 4am this morning. i did some testing on 'ozspeedtest.com' (several mirrors - exetel, optus & telstra) & 'speedtest.net'. theu all reported between 40KB/s to approx 300KB/s. have been trying to download a game via steam. most of the australian servers (only tried vic, nsw & sa) peak at around 200KB/s. i did get slightly better results from the NSW (telstra) and VIC (gamespace). obviously i conducted these test independant of each other as to not sour the results.

'CoreyPlover' - the higher the s/n ratio the better was my understanding utill i did some digging. i have a billion 7402vgp modem so i've been using the billion SNMP viewer. great little app. in the SNMP viewer it reports the 'S/N ratio' however on the modem webpage it reports 'SNR Margin' (both = 6dB) . if the modem is correct, a S/N of 6dB, as you stated is good. if the SNMP viewer is correct then 6dB is real bad.
if i would accept your suggestion that SNMP was infact displaying S/N margin then my upstream result of 13dB is pretty bad.
i'm probably inclined to agree with the modem's readings though it's still confusing.
'S/N ratio' will affect speed from my understanding.

i post some actual results from the speed tests sites shortly.

cheers.
"Arguing on the internet is like running in the Special Olympics. Even if you win, you're still retarded".

aphasia
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Re: Speed Issues (Poor S/N Ratio)

Post by aphasia » Sun Apr 11, 2010 5:50 pm

howdy

my connection details with exetel;
============================================
Your ADSL Account Plan Details
Service Number: 03xxxxxxxx
Plan Type: NF/23BYOLINE
Monthly Access Charge:
Peak Download Limit (Mbyte): 70000
Peak Period Option: 12:00pm - 12:00am
Excess Downloads per Gbyte: $6.00
OffPeak Download Limit (Mbyte): 60000
OffPeak Excess Downloads per Gbyte: $6.00
Service Line Speed: ADSL2+
Service Activation Date: 2008-08-11
Contract Period: 6 Months
Initial Contract End Date: 2009-02-11
============================================


and my speed test results (all tests were complete with no other internet activity on connection. 1 pc connected, SNMP viewer reported 0 activity on line)

OPTUS - VIC MELBOURNE
[`[*Test Results from <a href="http://www.ozspeedtest.com/bandwidth-te ... 102855">Oz Broadband Speed Test</a>*]`]
[(----------------------------------
Test run on [*11/04/2010*] @ [*05:23 PM*])]

[(Mirror: [*Optus*]
Data: [*3 MB*]
Test Time: [*21.58 secs*])]

[(Your line speed is [*1.14 Mbps*] (1135 kbps).
Your download speed is [*142 KB/s*] (0.14 MB/s). )]

TELSTRA - NSW SYDNEY
[`[*Test Results from <a href="http://www.ozspeedtest.com/bandwidth-te ... 102873">Oz Broadband Speed Test</a>*]`]
[(----------------------------------
Test run on [*11/04/2010*] @ [*05:25 PM*])]

[(Mirror: [*Telstra Bigpond*]
Data: [*3 MB*]
Test Time: [*20.89 secs*])]

[(Your line speed is [*1.17 Mbps*] (1173 kbps).
Your download speed is [*147 KB/s*] (0.14 MB/s). )]

EXETEL - NSW SYDNEY
[`[*Test Results from <a href="http://www.ozspeedtest.com/bandwidth-te ... 102906">Oz Broadband Speed Test</a>*]`]
[(----------------------------------
Test run on [*11/04/2010*] @ [*05:27 PM*])]

[(Mirror: [*Exetel*]
Data: [*3 MB*]
Test Time: [*68.88 secs*])]

[(Your line speed is [*356 kbps*] (0.36 Mbps).
Your download speed is [*44 KB/s*] (0.04 MB/s). )]

AAPT - VIC MELBOURNE
[`[*Test Results from <a href="http://www.ozspeedtest.com/bandwidth-te ... 102941">Oz Broadband Speed Test</a>*]`]
[(----------------------------------
Test run on [*11/04/2010*] @ [*05:30 PM*])]

[(Mirror: [*AAPT*]
Data: [*3 MB*]
Test Time: [*35.04 secs*])]

[(Your line speed is [*699 kbps*] (0.7 Mbps).
Your download speed is [*87 KB/s*] (0.09 MB/s). )]

Image

also downloaded a driver from microsoft.com (IntelliPoint 7.0 Mouse Software for Windows x64) the speed sat between 40KB/s & 60KB/s
================================
Upload Test
Results

You uploaded 1 MB bytes in 9.97 seconds.

kbps: 841.85
KB/s: 105.23
Mbps: 0.84
================================

cheers.
"Arguing on the internet is like running in the Special Olympics. Even if you win, you're still retarded".

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Dazzled
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Re: Speed Issues (Poor S/N Ratio)

Post by Dazzled » Sun Apr 11, 2010 5:53 pm

It's worth looking at this and the next 6 or 7 pages - http://whirlpool.net.au/wiki/?tag=ADSL_Theory_Basics. Have the modem line stats changed?

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CoreyPlover
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Re: Speed Issues (Poor S/N Ratio)

Post by CoreyPlover » Sun Apr 11, 2010 6:06 pm

aphasia wrote:If i would accept your suggestion that SNMP was infact displaying S/N margin then my upstream result of 13dB is pretty bad.
Upstream is a different frequency, hence different scales and db readings for attenuation and margin. In general, you ignore upstream stats unless your upstream sync is low (100-300kbps say)
aphasia wrote:'S/N ratio' will affect speed from my understanding.
Yes it will, but not in the way (or the direction) you are implying. Your line has an attenuation reflecting how noisy it is (higher dB = more noise). This attenuation determines the potential ADSL speed. This total speed is split between a margin and the data stream. The margin acts like a buffer against noise fluctuations. So, actually, higher SNR margin = lower speeds, not the other way around.

What you might be getting confused with is how the situation is different under ADSL1. With ADSL2+, you use the entire spectrum (up to 24Mbps), so you want a low noise margin to get the maximum speeds. But with ADSL1, speeds are capped at 1.5Mbps (or 8Mbps), so once you've hit this speed limit, the extra dB may as well go towards the noise margin and increase the stability of the service. Therefore, under ADSL1, as long as you get the maximum sync, you want SNR margin as high as possible to get maximum stability. But under ADSL2+, you want low SNR margins so that you get maximum speed.

In summary...you are syncing at 18Mbps (average is 8-10Mbps, higher better) have a 6db noise margin (average is 9db, lower is better so long as you don't experience drop outs) and a 26dB attenuation (average is probably 35db, lower is better). So your connection is unequivocally better than average, your SNR margin is great, and your SNR margin is *not* affecting your speeds.

The most likely explanation for your speeds is
1. That ozspeedtest is, in my opinion, incredible unreliable for speed testing, so should not be relied on one bit.
2. However, speedtest.net is, in my opinion, quite good for speed testing and it backs up your slow speed experience. But what can sometimes happen is that speeds on individual streams (like speed tests and some downloads) get capped. If you have a download manager that supports simultaneous streams, try it and see if speeds increase. Or alternatively, download several files from sites like microsoft.com, apple.com, etc simultaneously to test
3. I suspect that even the above won't dramatically improve your throughput. If so, the most likely suggestion is general congestion. This is a regular occurrence, but generally gets resolved over time as exchanges upgrade their capacity, etc
I am a volunteer moderator and not an Exetel staff member. As with all forum posts, mine do not constitute any "official" Exetel position. Support tickets may be logged via https://helpdesk.exetel.com.au or residentialsupport@exetel.com.au

aphasia
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Re: Speed Issues (Poor S/N Ratio)

Post by aphasia » Sun Apr 11, 2010 6:58 pm

howdy

thanks for your input corey. i understand the basic of S/N and attenuation. just wanted to point out that i was talking about both 'S/N margin' & 'S/N ratio'. my modem and SNMP viewer seems to use the terminology interchangeably which is obviously incorrect and confusing.

i've been with exetel for some time now and from having used the services of several other isps, telstra, optus, netspace & dodo (the last i worked for for several years in both the tech & provisioning depts, is probably the worst isp i've encountered both service, management & ethically), exetel is head and shoulders above the rest. generally i've ignored speed issues on my line with exetel as i accept there are going to be congestion as you said. i also understand it’s a balancing act to purchase enough bandwidth for users while maintaining a feasible fee structure.

again it doesn't bother me when browsing slows and normally i'd do as you suggest to use a DL manager (i use firefox's addon 'downthemall!) to start several threads or chunks to complete a file faster. unfortunately i can't do the same via steam which was how i noticed the problem in the first place. the best you can do via steam is pick the closest sever with the most capacity. i don't doubt the general congestion will be resolved just frustrated as steam DLs are generally lightning quick. course that doesn't help if your link to the net is bogged down.

i'm still confused on the conflicting reading my modem & SNMP viewer are displaying regarding 'S/N ratio' and ‘S/N margin’.

i just finshed downloading the lastest ati/amd display driver via the download manager with 10 streams and it averaged btw 300 & 360KB/s. still not great, guess i'll just have to be patient when i restart my steam DL.

cheers.
"Arguing on the internet is like running in the Special Olympics. Even if you win, you're still retarded".

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CoreyPlover
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Re: Speed Issues (Poor S/N Ratio)

Post by CoreyPlover » Sun Apr 11, 2010 8:05 pm

Similar intermittent slow speeds have happened on past occasions with Exetel. Generally speaking they don't persist very long (by this I mean within a month or so congestions clears a little and it seems to oscillate between congested and clear with about 3-6 month cycles) with capacity continually upgraded both at Exetel's end and at exchanges. Many connections don't experience congestion issues at all though

Closing out, I believe the terms "margin" and "ratio" are interchangeable with respect to signal noise. For instance:
http://en.wikipedia.org/wiki/Noise_margin wrote:In communications system engineering, noise margin is the ratio by which the signal exceeds the minimum acceptable amount. It is normally measured in decibels.
.
The trick is that "decibels" is a logarithmic (base-10) scale, therefore a 3db margin is about equal to a ratio of 2.
I am a volunteer moderator and not an Exetel staff member. As with all forum posts, mine do not constitute any "official" Exetel position. Support tickets may be logged via https://helpdesk.exetel.com.au or residentialsupport@exetel.com.au

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Dazzled
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Re: Speed Issues (Poor S/N Ratio)

Post by Dazzled » Sun Apr 11, 2010 8:34 pm

Aphasia, your line sounds a lot better than mine, but if you enter the modem on it's CLI interface, normally by telnet, you can read out most aspects of the ADSL connection, and satisfy yourself concerning the line.

Please forgive me for getting geeky, but it's no worse than snmp viewers. Most modems are embedded Linux boxes. The ls command is usually omitted from the abbreviated shell, so reading the /proc interface can involve some exploring, but (cat is usually available (the system has no need for ls). There is usually an adsl info or similar command, with extra arguments available, which should give you everything, including at least the following, which gives you something to research further:
Hlin (i · ∆f ) Channel Characteristics per subcarrier, linear X + jY
Hlog (i · ∆f ) Channel Characteristics per subcarrier, 20 · log10 (|X + jY |)
QLN (i · ∆f ) Quiet Line Noise per subcarrier, [dBm/Hz]
SNR(i · ∆f ) Signal-to-Noise Ratio per subcarrier
Attenuation
Signal-to-Noise Ratio Margin
The command ? will usually give a list of the available CLI commands.

aphasia
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Re: Speed Issues (Poor S/N Ratio)

Post by aphasia » Sun Apr 11, 2010 10:00 pm

howdy

again thanks for the replies (Dazzled & corey).

forget about everything else for the moment, i want to investigate the Signal to Noise issue again.
not to be difficult, but the 'S/N ratio and S/N margin' are related but not the same thus not interchangable.

'S/N ratio' as applies to communications and audio equipment states the higher the ratio in dB's (more signal units to every unit of noise if you like) the better. when looking at amps, mp3 players & sound cards etc, all things being equal you'd purchase the one with the higher S/N ratio yes? the same applies to a signal transmitted from a telephone exchange to your modem. the 'S/N margin' is a calulation done on the base decibel 'S/N ratio' value to attain a specific line speed.
The SNR Margin is the difference between the actual SNR and the SNR required to run at a specific speed. For example, if your line needs 35dB of SNR to run at 8Mbps, and the actual line SNR is 41dB, then the SNR Margin would be 6dB.

Read more: http://www.kitz.co.uk/adsl/linestats.htm#ixzz0kn15sXBO
now going back to my modem spec of 6dB downstream, despite my modem and SNMP viewer represented this value differently (modem as 'S/N margin' and SNMP viewer at 'S/N ratio') i can safely assume that this 6 dB value is the S/N margin, yes? for for my modem to attain the 18.07Mbps, my line is exceeding the required S/N ratio by 6dB's.

thanks dazzled for unleashing the 'geek', i'll have a look at the modem cli a little later to confirm my line characteristics. i believe steam has almost completed my game DL (@ 99%).

cheers.
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Dazzled
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Re: Speed Issues (Poor S/N Ratio)

Post by Dazzled » Sun Apr 11, 2010 10:39 pm

Every discipline seems to have a slightly different meaning for the term SNR, usually for reasons of practicability. In my work it usually involved complex conjugates, Fourier space and Wiener filters. Modems on the other hand live in a world more readily understood by humans, and nearly always use SNR to mean SNR margin, even if complex numbers lurk in the theory.

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CoreyPlover
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Re: Speed Issues (Poor S/N Ratio)

Post by CoreyPlover » Sun Apr 11, 2010 11:09 pm

aphasia wrote:now going back to my modem spec of 6dB downstream, despite my modem and SNMP viewer represented this value differently (modem as 'S/N margin' and SNMP viewer at 'S/N ratio') i can safely assume that this 6 dB value is the S/N margin, yes?
Ah. Yes I see now.

Yes. 6dB is your margin. Modems refer to it as the "SNR margin" (signal-to-noise ratio margin). It is made confusing because some call it "S/N margin" which can be misread as "Signal Noise Margin" but is actually "Signal, divided by Noise, margin".

For your modem to attain the 18.07Mbps, it needs a high signal-to-noise ratio (i.e. low attenuation) in order to achieve a, say, 19Mbps sync. Then it allocates 6dB of buffer (signal to noise ratio margin) which brings the actual sync down to 18Mbps.
I am a volunteer moderator and not an Exetel staff member. As with all forum posts, mine do not constitute any "official" Exetel position. Support tickets may be logged via https://helpdesk.exetel.com.au or residentialsupport@exetel.com.au

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