Please help interpret line issue

2

21stcn

Well-Known Member
Joined
May 5, 2010
Messages
109
Hi,

I have a 4Mbps line with telkom and am using Afrihost Business Ucapped (so shaping shouldn't be a factor).

My line currently has a downspeed of 0.8 Mbps, and often averages about 1Mpbs during the week in the evenings.

I know this sounds like exchange congestion, but the second hop in pingplotter diagnostics is not particularly high, although the jitter is very high with some packet loss. When the line is running slowly like this, my speedtest ping is ~60ms to local server, and downspeed is consistently ~0.8 Mbps. Please see image for trace routes.

4vx0snij5


Some other stats according to D-Link router:

SNR Margin (dB): 243 down, 130 up
Attenuation (dB): 100 down 65 up
Attainable Rate (Kbps): 11392
Rate (Kbps): 4096 - line is syncing at 4Mbps

The speeds can test at 3.4 Mbps mostly, but poor performance like what I am experiencing currently seems to be happening more and more.

Does anyone have any idea what this problem might be (does it look like congestion)? Thanks so much.
 

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M

Mineer

Expert Member
Joined
Apr 30, 2008
Messages
3,190
which particular D'link router do you have, is it the DSL 2750-U router ?
if so your problem might be DNS

edit* assuming it is the modem

that particular router had its DNS's changed lately
to change the DNS back to normal
login on to the router 10.0.0.2
username: admin password: admin
click advanced
click DNS
select Obtain DNS from a WAN interface
click apply
 
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2

21stcn

Well-Known Member
Joined
May 5, 2010
Messages
109
Mineer said:
which particular D'link router do you have, is it the DSL 2750-U router ?
if so your problem might be DNS
Click to expand...

Thank Mineer. Ja, it is the 2750-U.

I had custom DNS ips in there but have changed them to "Obtain DNS from a WAN interface". Will give it a few days and see if it helps.

Thanks for your assistance.
 
2

21stcn

Well-Known Member
Joined
May 5, 2010
Messages
109
Tim the Techxpert said:
Hi There,
Have you had the line checked lately? The attenuation looks too high for me.
This post gives some guidelines for you http://mydl.itweb.co.za/index.php?option=com_easyblog&view=entry&id=1061655&Itemid=350
This site can give you an idea of the speeds possible.
Click to expand...

I don't think the attenuation is bad. This d-link router has the dB values multiplied by a factor of 10 (as corroborated by this screenshot ). If the SNR was actually 243dB that would be outrageously good, and if the attenuation was 100dB that would be terrible. I don't know why it is 10 times more than conventional values but I've seen on the net before that this is why the attenuation seems so bad (I think it is actually 10).

Would a terrible attenuation result in slow speeds in the evenings and on weekends?
 
R

Robin Hood

Expert Member
Joined
Jun 5, 2004
Messages
3,322
Got this somewhere on the net a while ago...credit to original poster...


SNR or Signal to Noise Ratio:
Describes the ratio of usable data-signals on your line. You can associate the “signal” with the data traveling across your ADSL-line and the “noise” as the unwanted interference affecting the signal. The higher the number the better for this measurement. In some instances interleaving can help raise the noise margin to an acceptable level.

6dB or below is bad and will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with little or no synch problems
20dB-28dB is excellent
29dB or above is outstanding

If your SNR is below 12dB you are pretty much screwed and will not get a consistent level of ADSL-service. You could try to convince Telkom to rewire the cabling – but this is unlikely to happen.

Attenuation:
Is the reduction in signal strength on your phone line. In ADSL this may be reported as “loop loss” and is the natural deterioration of the ADSL signal over distance from the exchange. Attenuation is normally directly linked to the length of your line. Copper is traditionally used in the local loop and the higher gauge of copper will give the best signal, however some lines may have some aluminium or aluminium joints on the line which will increase resistance… as will oxidisation of joints. The lower the dB the better for this measurement.

20dB and below is outstanding
20dB-30dB is excellent
30dB-40dB is very good
40dB-50dB is good
50dB-60dB is poor and may experience connectivity issues
60dB or above is bad and will experience connectivity issues

The standard signal attenuation spread for a given speed is somewhere in the region of 25-30dB for ADSL1 speeds (if we had ADSL2/2+ this would be).

The following guide (distance vs. attenuation vs speed) gives you an guestimate what you can achieve:
<1km should be 23-24Mbit (nice speed, but doesn’t it bug you that Telkom people walk through your bedroom?)
1.0km = 13.81dB = 23Mbit
1.5km = 20.7dB = 21Mbit
2.0km = 27.6dB = 18Mbit
2.5km = 34.5dB = 13Mbit
3.0km = 41.4dB = 8Mbit
3.5km = 48.3dB = 6Mbit
4.0km = 56dB = 4Mbit
4.5km = 62.1dB = 3Mbit
5.0km = 69dB = 2Mbit
>5.0km (you are pretty much poked — sorry for you)

You will not be able to fix attenuation (unless you relocate straight next to the exchange). You will however be able to influence Telkom or your ISPs – use tooling such as MRTG to measure your variances and log incidents against them. It is unlikely that they will move the exchange closer, but you could combat SNR (check your wiring, climb into the manhole and see if your underground wire is of the cheap kind etc).
 
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