Any advice on this interesting ADSL problem?

[phil1982]

I have an interesting scenario/problem: I am now on 4Mb DSL line (upgraded from 2Mb by Telkom) with a 4Mb OpenWeb Gold/Leon account, but the max speeds I get even when testing at 12:30am is about 365KB/s. I should be able to get about 440KB/s or more not so (even taking into account non optimal conditions, i.e. not just theoretical values)? The thing is that I have tested some other backup ISP (capped/prepaid) accounts I have and the same thing happens so it does not appear to be OpenWeb related; my download speed just cannot seem to go beyond about 365-380KB/s max. Anyone have any idea what could cause this or have had a similar experience? For reference, my line stats (all downstream):

SNR Margin (dB): 7.9
Attenuation (dB): 57.0
Output Power (dBm): 5.0
Attainable Rate (Kbps): 5526
Current Rate (Kbps): 4096

My DSL is very stable, doesn't randomly disconnect or anything and the download speed is very stable at about 300-360KB/s (when not shaped of course), so it does not seem a stability problem. I know my attenuation is quite high (a bit far from exchange), but still...the modem shows that I can go till about 5.4Mb and the connection is stable. Could the attenuation/distance really be the problem? If so (and this is my core question) how is it then possible for me to consistently and stably connect at 4Mb, but the actual data throughput cannot reach beyond 370KB/s given that shaping and congested exchange problems are not relevant; tested for exchange congestion and shaping cannot be applicable since it does this on 3 different backbone/ISP accounts at the same time, surely it cannot be that coincidental? Should I try a port reset at Telkom? Is it possible for failed packets (crc errors, etc.) to make up the difference in what data throughput I should be getting versus what data throughput I am getting?

Either way, any advice is welcome. It's not a life or death scenario, but it does bother me a bit. Thank you in advance.

-phil1982

 

[madrynn]

Rule of thumb, divide your line speed in kbits by 10 (not 8, to compensate for overhead) to get an idea of what your speed should be. But to tell you the truth I have not notice my downloads exceeding 400kB/s since the upgrade to 4mbit either.

Your attenuation and SNR isn't great at all, but that would give you a lower sync and probably disconnect from time to time (which I would be more troubled about).

To make sure you do not get packet loss you can maybe try the pingtest.net website.

Else I think you should be happy with what you get!

Good luck!

 

[DeatheCore]

I have a feeling that your low SNR Margin is negatively impacting your throughput.

I've tested my own ADSL account on multiple 4Mbps lines, and have seen the most performance on lines with higher SNR Margins, i.e. 14dB+, compared to my 9-10dB @ home, in which case the highest speed I can achieve is around 405KB/s.

Just a theory.

 

[JonesY]

Is it possible for failed packets (crc errors, etc.) to make up the difference in what data throughput I should be getting versus what data throughput I am getting?

Modem errors cause retransmission and reduce throughput, even though said modems report a higher sync speed. AFAIK the only fix is to get Telkom to increase SNR, which will lower your sync speed. Catch 22?