How should Interleave Depth be set on an ADSL connection?

For years and years I've always had the Interleave Depth of my ADSL connection set at Level 8 (downstream). This has assisted with error correction and has helped a little with achievable line speed on my otherwise slow and noisy connection. This has been at the expense of some increased latency. But just recently I noticed from my router's stats that (apparently) the downstream ID was only 1. This is paramount to interleaving being off and instead the line running in Fast Mode. This seemed daft, and I wondered why this should be so.

I therefore contacted my ISP and asked for the ID to be changed to 8. They subsequently informed that this had duly been done. But now when I look at the stats, they're showing an ID of 64. I've queried this with the ISP but they insist that their engineers set it to 16 (which isn't what I asked for anyway).

Putting aside for the moment the possibility that the router could be wrongly reporting the values, is there a specific method that the ISP should use when making this change, eg. is it essential that they completely reset the user's profile before applying a new ID value? Do the downstream and upstream ID levels work independently of each other?

My understanding is that these are the options:

Downstream depth: 1 (off), 4, 8, 16, 32, 64.
Upstream depth: 1, 2, 4, 8.

AIUI on BT Wholesale connections the ISP can only request Speed, Stable and Super Stable DLM settings. Not the specific values.

It sounds as if you were on Speed but the DLM had intervened, and has now decided your line is stable enough to put you on Fast Path. The default setting for Stable I don't know, but maybe it is 16.

What makes you think the DLM was wrong to remove interleaving? It is possible some (ADSL) noise source has been fixed or removed.

The line, prior to this, was stable - but with still massive packet errors. So the idea was to improve the error rate. The only reason it was stable speedwise was because I normally operate the SNR facility of my Broadcom-incorporated router to push up the SNR (at the expense of speed, of course). I've been running my line in this way for years and years, with different ISPs, ie. with also an ID of 8, with absolutely no problem.

Since my initial post on this, the router now shows an ID of 16. However, I'm now on a stuck 2M bps profile!!! The router's showing that the SNR has been mysteriously increased to a massive 27dB, which is surely right outside BT's range? Nothing I do - restarting the router, power rebooting, or setting the router's Target SNR to the default - has any effect. The router says my speed's 2048K bps. (I normally sync at twice that).

Why the speed has reverted to fixed 2M profile is beyond me, as adding a modicum of interleaving shouldn't have done that. ID should surely only affect latency?

The original support person at the ISP now refuses to discuss the matter any further but I've e-mailed them again and asked for my case to be referred to a different person. I've suggested that they completely reset my profile, including turning the interleaving off. Otherwise, I'm simply now going to be on 2M bps for ever.

Postscript:

According to what my ISP has subsequently told me, the BT engineer(s) that was instructed to make the change didn't do it properly first time, then later on didn't respond at all to the ISP's request.

Am back on my normal connection speed, I'm glad to say, after the ISP 'gave BT a rocket' over the matter. At the end of it all, though, I've not succeeded in getting some interleaving added. There are rumours flying around that BT have changed how interleaving is applied, it becoming more of a binary thing now: it's either off, or if it's turned on you get just one fixed level of interleaving.