Interleave only on downstream

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dinsdale
Posts: 5
Joined: Tue Sep 18, 2012 12:55 pm

Interleave only on downstream

Post by dinsdale » Tue Sep 18, 2012 2:02 pm

Hi,
I recently had a problem with web pages loading very slowly. This was resolved by using different DNS servers but in the course of testing I noticed an unrelated issue with my 7800N on an ADSL2+ line showing interleave only on the downstream. There are no log entries for upstream Reed Solomon errors or corrections and the interleave depth is set to 1, which is 'off'. Downstream has an interleave depth alternating between 32 and 64 with active RS data.

In recent observations I'm seeing roughly four times as many upstream CRC and ES errors as downstream. I'm not getting any sense out of my ISP as to whether upstream interleave can or even should be turned on but they did give me some of their log data which includes a short period today when the router was on but the PC was off:

Date | Downstream Upstream | SNR | Down / Up Errors | Re-syncs / Drops
18th at 07:25 | 6,576 kbps 1,020 kbps | 6.0dB | 6 / 7 | 0 / 0
18th at 06:25 | 6,576 kbps 1,020 kbps | 3.0dB | 0 / 254 | 0 / 0
18th at 05:25 | 6,576 kbps 1,020 kbps | 0.0dB | 0 / 461 | 0 / 0
18th at 04:25 | 6,576 kbps 1,020 kbps | 1.0dB | 1 / 71 | 0 / 0
18th at 03:25 | 6,576 kbps 1,020 kbps | 2.0dB | 0 / 26 | 0 / 0
18th at 02:24 | 6,576 kbps 1,020 kbps | 2.0dB | 0 / 6 | 0 / 0
18th at 01:25 | 6,576 kbps 1,020 kbps | 2.0dB | 0 / 42 | 0 / 0


I don't know yet what type of errors are referred to above and the ratio is very different to what my router log normally shows for CRC and ES errors but they at least demonstrate that large numbers of upstream errors occur even when the PC is not contributing to the data stream. The line attenuation is 48.5dB/26.2dB down/up and with the sync of 6,576 I've currently got SNR of 7.3dB/5.7dB down/up. The router connects to an ADSL Nation filtered faceplate via a 4 metre Cat5e cable with RJ-11 plugs and the problem doesn't improve when connected to the test socket.

Does this sound like a line or router issue?

Regards,
Tom

Tomken
Posts: 467
Joined: Tue Jul 26, 2011 10:31 am
Location: Co Durham

Re: Interleave only on downstream

Post by Tomken » Tue Sep 18, 2012 4:28 pm

Well, your SNRM certainly took a nose dive in the wee small hours and I guess it's down to the Broadcom chipset that the connection held when it dropped to zero.

There is always a signal coming down the line whether you're using it or not and because of varying conditions, the stats for that line can alter as the day progresses - as is clearly illustrated by the stats you have provided.

The two Kitz articles linked below deal with CRCs and conditions that can effect SNR and as the latter can be affected by external sources, then perhaps a channel change could help with that and this program (if you haven't already got it) http://www.metageek.net/products/inssider/ will show who else is around - at least and if you are unable to use channels 1, 6 or 11 without being at least 25 - 30dBm away from the nearest user, then choose one (other than channel 9 if you have a microwave oven) at least 2 (and preferrably 3) away from whoever is showing a RSSI value closest to you and one that will give you a RSSI of about -50dBm
http://www.kitz.co.uk/adsl/linestats_errors.htm
http://www.kitz.co.uk/adsl/lowSNR.htm

Ensuring that your MTU is optimum should also help with browsing - this program will calculate the optimum for your line - read the help links in the article for how to use it and ensure that your computer and router's MTU are set at 1500 prior to running it. http://www.speedguide.net/downloads.php

dinsdale
Posts: 5
Joined: Tue Sep 18, 2012 12:55 pm

Re: Interleave only on downstream

Post by dinsdale » Tue Sep 18, 2012 6:35 pm

Sadly, the exchange equipment is Infineon and there's only one LLU provider at my exchange for now. I live in a rural area about 3.5km from the exchange with no neighbours within wi-fi range. The general performance of the line is good for the distance but I'm just puzzled about the disproportionate number of upstream errors. My ISP decided to turn off interleaving a few hours ago and that seems to have improved things so far although the test will come later this evening. The sync is currently 6,083 and I'll leave it there for at least a day before going higher.

My 7800N doesn't want me to use anything other than 1492 MTU which just happens to be the ideal setting so I've set my LAN interfaces to that value also.

Tomken
Posts: 467
Joined: Tue Jul 26, 2011 10:31 am
Location: Co Durham

Re: Interleave only on downstream

Post by Tomken » Wed Sep 19, 2012 1:15 am

The Broadcom chipset that I was referring to is the chipset of the 7800N which makes it one of the ideal routers for longer lines.

You are very fortunate not to be overpopulated with other broadband users which means you can experiment with the channels, as it's not just other users that can interfere with broadband, it's local interference as well which could just as easily be some RF interference from something within your own home boundary.

I'm not sure what you mean by that the router won't let you use any other MTU setting other than 1492 which is what I think they are default set to - mine was.

As the user, you can change that to whatever you want and the TCP Optimizer will fine tune that for you, providing the router and computer are set to 1500 prior to it doing its thing.

TalkTalk advise setting it to 1432 as optimum for their lines but it isn't a one size fits all.

dinsdale
Posts: 5
Joined: Tue Sep 18, 2012 12:55 pm

Re: Interleave only on downstream

Post by dinsdale » Wed Sep 19, 2012 1:11 pm

My exchange has only had LLU for a couple of months. Up to that point I'd amassed a quantity of different routers with various chipsets in an attempt to squeeze the best performance from my line. The clear winner then was a 2Wire 2700HGV with an Alcatel chipset and exchange equipment also from the same manufacturer. The move to LLU and ADSL2+ was a positive one in all respects but the 2Wire didn't really get on with the Infineon exchange equipment it now had to deal with. I tried a couple of routers with Infineon chipsets but the lucky match which had worked for many years before wasn't there. Broadcom chipsets seemed to work best so as I'd had better results with 6348 than 6328 I took a chance that the 7800N's 6358 chipset would be a useful upgrade.

Since my ISP turned off interleave completely yesterday there were around 1200 CRC errors downstream and 112 upstream so the ratio was back to normal. This seems to suggest that the previous imbalance with four times as many upstream errors to downstream was caused by line configuration. Forty minutes ago my ISP managed to enable interleave for both channels and so far, with a sync of 7,574 and SNR of 6dB, the number of errors has been nominal.

When I began experimenting with MTU I tried to set the 7800N to 1500 as a starting point but even after clicking 'save settings' and 'restart' the value would return to 1492 when I checked. This was so even when I tried an arbitrary lower value. Ping tests later showed that the maximum unfragmented packet size was 1464 so after adding 28 for the header 1492 was the ideal value. Since my LAN interfaces were on the default 1500 I reconfigured those to 1492 also.
Last edited by dinsdale on Sat Sep 22, 2012 12:01 am, edited 1 time in total.

Philip_L
Posts: 111
Joined: Mon Jun 25, 2012 10:06 am

Re: Interleave only on downstream

Post by Philip_L » Wed Sep 19, 2012 9:49 pm

Hi

ADSL is designed to run with errors, that is the only way it can work given the use of old telephone cable. Seeing error counts are quite normal, it is also common for the upstream to not have any interleaving applied as often it isn't as necessary and not being interleaved reduces latency.

If there are errors during transmission then the TCP/IP protocol will mean your computer knows that and sends or requests the data again. The exception is UDP packets but usually the software will cope with lost packets.

When interleaving is enabled then some errors can be corrected, and these then show up under the RSCorr and RsUnCorr counters on Broadcom modems, these counters show corrected and uncorrected errors. RSCorr, no matter how the big the number is not a problem, as these are packets that had errors that were corrected, that's good, the RSUnCorr shows errors that couldn't be correct by interleaving and so caused your computer to have to retransmit them, not so good, but if this percentage is low around a couple of percent or less, it isn't really impacting you at all and is ADSL running as designed.

When your ISP turns interleaving off, RSUnCorr and RSCorr will always be zero, this is because they are only used for counting how well interleaving is working, so without interleaving they stay at zero however you might think you are getting no errors now, but that isn't the case, you will be getting more, they are just not being counted there.

Unless you are a competitive on-line game player, interleaving is the better for reliability, if you play competitive on-line games then getting interleaving turned off means less latency so your reactions are seen quicker, often though you might need to have a slower line speed without interleaving as otherwise there are too many errors going uncorrected which means lots of data is being re-requested, which adds latency by a different door, or makes the line unreliable.

Hope that helps a little.

Regards

Phil

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