If like me you need to set a lower sync rate to achieve a stable line, there is no point in trying to get the absolute highest sync value you can, this is because BT set the BRAS profiles for sync speeds within a given range, and they are a fixed set of download rates. You only need to set the maxdnrate to the lowest sync speed for the BRAS vs sync range you fall into for a stable line to achieve the given BRAS profile. Setting a maxdnrate that matches the highest value for a BRAS profile sync range only worsens your snr, it won’t improve throughput in any way as you will still get the same BRAS as the lowest sync in that range.

If you own a Draytek Vigor2800 and have firmware 2.7 or above then you can force a max sync speed (As also posted on Draytek forum by me).

The BRAS profile vs sync ranges can be found here
http://www.zensupport.co.uk/knowledgebase/article.aspx?id=10550

And these are the maxdnrate settings for the min sync speed for each of the BRAS profiles.
sync speed(BRAS profile)

8128(7150) - adsl maxdnrate 255
7968(7000) - adsl maxdnrate 250
7392(6500) - adsl maxdnrate 232
6816(6000) - adsl maxdnrate 214
6240(5500) - adsl maxdnrate 196
5696(5000) - adsl maxdnrate 179
5120(4500) - adsl maxdnrate 161
4544(4000) - adsl maxdnrate 143
4000(3500) - adsl maxdnrate 126
3424(3000) - adsl maxdnrate 108
2848(2500) - adsl maxdnrate 90
2272(2000) - adsl maxdnrate 72
1728(1500) - adsl maxdnrate 55
1152(1000) - adsl maxdnrate 37
576(500) - adsl maxdnrate 19
288(250) - adsl maxdnrate 10
AUTO - adsl maxdnrate 0

Sadly the fixed profiles of 2M, 3M etc. that maxdnrate also offers don’t match the minimum settings for the BT BRAS profile ranges, would be nice if they did.

Rob
Zen8000 Active, 2800VG, 2.7 firmware, maxdnrate 232, and a taxi cab base transmitter close by that regularly corrupts an otherwise perfect 8128 sync.

An interesting listing, Rob! With 2 exceptions, the settings you give for adsl maxdnrate match the rule

  mxdnrate = syncrate/32 + 1

the same as for the BT Voyager 205 router. The exceptions in your list are

4544(4000) - adsl maxdnrate 142 (4544/32+1 = 143)
1728(1500) - adsl maxdnrate   54 (1728/32 + 1 = 55)

Were these typos or are they as displayed by the router?

Ooops - typos. Post updated

Indeed the rule is the same for the Vigor, but the 2800 also has text versions of sync rates 3M, 4M etc., but these don't match the BRAS min limit.

Rob

Just one thing though, I found that when I had 2Mbit sync and 1Mbit CAP I could max out at 1Mbit and my ping times would still be as low as if I was idle. So it seems it IS worth having a "little" higher sync than the actual cap (of course maybe my cap was not a BRAS one, though I suspect it was) as in some way that leeway drastically improves your pings when you are downloading.

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Ex-Zen Home2000 Customer