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[pintosal]

I've been tweaking the SNRM on my DG834GT (DGTEam Firmware) and logging what happens with Routerstats. I have tweaked the SNRM down to 50%.

Usually I switch off my router overnight, and when I switch on in the morning around 8.30am it synchs at ~4.5Mb/s and SNRM of 6~7dB. This is not bad as I am 3.6km from the exchange.

Towards evening the downstream SNRM drifts down, the error rates drift up, and when SNRM hits 3dB the router will re-synch and drop to ~3.8Mb/s.

I have read that the Netgear routers can work down to 0dB SNRM (or lower!). 

Now, I have a couple of questions:

1. Is it the SNRM dropping below 3dB causing the router to resynch, or the error rate, or both?

2. Is the resynch initiated by the my router or by the exchange?

3. Is the error rate and/or SNRM communicated back to the exchange?

4. Is there any way of forcing the router NOT to resynch, ie are there some threshold parameters that set the SNRM level trigger or the error rate trigger which can make the router hang on to the higher synch rate?

Any clarifications or advice gratefully received.

[roseway]

Netgear routers have a bit of a reputation for hanging onto the connection with low or negative noise margins, but I think this reputation came mainly from the older DG834 v2 and v3 which used the AR7 chipset. The DG834GT tends to re-sync when the noise margin drops below about 3 dB.

Actually this doesn't mean a lot. The way that noise margin is defined means that the point at which sync is lost isn't directly comparable between different routers anyway.

1. The trigger for a re-sync is the error rate. As interference increases the error rate increases and the noise margin falls. The noise margin is an indicator of the state of 'noisyness' of the connection, but it's the error rate which eventually reaches an unsustainable level and causes a re-sync.

2. It could be either.

3. Yes.

4. Maybe some routers have the capability to tweak these threshold parameters. But a re-sync is caused by an unsustainable error rate, and when it gets to that point the connection would be pretty poor anyway, and I don't think it would help you at all to hang onto the connection for still longer.

[Weaver]

Roseway wrote:
> Netgear routers have a bit of a reputation for hanging onto the connection with low or negative noise margins, but I think this reputation came mainly from the older DG834 v2 and v3 which used the AR7 chipset. The DG834GT tends to re-sync when the noise margin drops below about 3 dB.

Roseway expressed it perfectly. I'll admit that I used to think that a re-sync was "a bad thing" in the sense that a DSL modem with greater courage which was prepared to tough it out was preferable. But I was mistaken. Re-synching in order to change to a new plan that works better gives an improvement in throughput because of a reduction in packet loss because of the high number of corrupted packets. Changing from a plan that's not working to a better plan is a good thing.

I was influenced by old ways of thinking from the days of PSTN dial-up modems. You certainly don't want a system to react too soon and give up in the face of a purely transient error. But transient noise is one thing and altered line conditions are another entirely.

It would be worth checking how well the router is actually getting packets through at v low reported SNRM. If you use a MS Windows system, the pathping tool will do a throughput test and count packet loss.

From the evidence of the numbers you have posted there is IMO a good chance that you could find a substantial improvement in sync rates by going to a DG834 v3 router. These can be had on ebay for peanuts, but get them while you can.

[pintosal]

Hmmm, Weaver, Very interesting.

The reason that I switched to the DG834GT was because of the Broadcom chipset. This was instigated by my ISP, Zen, when I was having hardware problems with my old Draytek router.

When I discussed my choice of new router with Zen's tech support, they were rather disparaging about the AR7-based routers  and steered me towards Broadcoms, and the Speedtouch 585. I also bought a DG834GT and found that it gave better performance; the clincher was the availability of DGTeam firmware.

I might just take a punt on a DG834v3

[orainsear]

I might just take a punt on a DG834v3

The Sky DG934G is the DG834Gv3 in a different case and can be had very cheaply at well known auction sites.

Before I moved to LLU I was using an unlocked DG934G on a longish line with a TI DSLAM chipset and it worked very well as a modem for me.

[philip_l]

Hi

I have read that the Netgear routers can work down to 0dB SNRM (or lower!).  

You can't put any trust in the reported figures, that 0db might be your current modems reported 4db figure, just calculated differently.

Theory is you can have a connection with 0db margin, as it is only a margin above what is required.  Problem is that is based on background even noise levels and these background noise levels rise due to lots of sources of noise turning on as night approaches.  While this raises noise levels and so reduces your margin, it also increases the likely hood of impulse noise as more things are in use and switching off and on.  Impulse noise is a bit different to normal signal to noise.  Think of a party with lots of people talking and trying to follow someone's conversation, if the noise level stays pretty constant, or even where it slowly rises as more people arrive, you can shout louder and make adjustments to hear.  Impulse noise is like a group of people at the next table suddenly letting off party poppers and so you have a sudden increase in noise levels way above the background noise.  As you are already shouting and struggling to have a conversation, the sudden increase in noise levels means you no longer can hear or be heard.  Even though the noise is short lived, your conversation has been interrupted, and in ADSL terms that requires a resync.

Because impulse noise is short lived, it doesn't usually show up in your SNR reported numbers.  So even where you may still have a few db of noise margin, this simply isn't enough to ride out random impulse noise or sudden events of higher noise.

So you are resync'ing not because your margin has dropped to 3.5db, but because a sudden increase of noise has occurred that goes unseen in the reported figures.  When your modem resyncs, it resyncs on the basis of night time background noise levels, so goes back to a higher margin and a lower sync speed.  The now restored increased margin means bouts of sudden high noise levels are not as problematic.

Regards

Phil