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[IanG]

I am in a hopefully brief investigative phase that involves fairly frequent swapping of ADSL2+ modems. Is this liable to confuse the DLM system? And if so, is there any advantage to asking a modem to power down, thereby giving it a chance to talk to the exchange first, rather than just pulling out the power plug? The latter is not something I would do to any computer, but assumed that a modem with a read-only file system would not be harmed.

[Weaver]

> Is there any advantage to asking a modem to power down.

They may have something ‘dying gasp’ whereby there is a little bit of stored energy that powers the modem for enough time to allow it to throw out a short message to the exchange. Yip would have to ask about this as memory fails me.

However if you pull the phone line out, then this cannot happen obviously.

So it cannot hurt to let the modem power down in an orderly fashion, seeing as we may or may not know whether the dying gasp mechanism is present we might as well assume that it it.

Iirc kitz says that dying gasp is not taken into account by DLM anyway and she is the expert. BT had to handle the case when there is no dying gasp anyway, either because of pull-outs or possibly because the mechanism is not present, but I don’t know about the latter.

Someone knowledgeable will I hope chip in?

About DLM. From experience you can upset OR by doing swapping of modems. Iirc it might be less if you are on ADSL2+/21CN. In the old days you certainly could get into trouble, from experience, and kitz recommendations implied that you when you turn a modem off you should wait leaving it modem switched off, but think about this in terms of what the exchange sees one each particular line, and not turn it back on or substitute an alien, until the next but one fifteen minute slot. Slots are 00, 15, 30, 45 mins past the hour, so you leave it off for at least 15 mins so there is a whole slot during which it is off for the entire time. If it is switched off at xx:01 you have to wait until xx:30 according to this idea, so that xx:15-xx:30 is covered.

I used to follow that and it saved me. Not observing it did get me into trouble and modem swap outs got me into trouble with DLM in the old days. But in recent years I have just found that nothing matters at all.

I am not a good guide in this thought seeing as I can reset the target SNRM values both up/down and restart the training period (supposedly) using my ISP’s website myself by hand whenever I need to and others may not be so lucky.

[kitz]

As already stated by Weaver - both BTw and Openreach's DLM does not take notice of the dying gasp messages.    Instead it uses something called MTBR to count the number of retrains per 24hr period.     There is more info about how it works here here.

Theoretically it should be possible to trick the DLM into classifying the retrain as 'unforced' by leaving the modem powered down for 30mins (ensuring there is a full 15 min bin with no activity).

[PhilipD]

Hi

I am in a hopefully brief investigative phase that involves fairly frequent swapping of ADSL2+ modems. Is this liable to confuse the DLM system? And if so, is there any advantage to asking a modem to power down, thereby giving it a chance to talk to the exchange first, rather than just pulling out the power plug? The latter is not something I would do to any computer, but assumed that a modem with a read-only file system would not be harmed.

Options in order of what is best:

1) On Broadcom based modems you can issue from Telnet: xdlsctl --connection down  and this will bring down the connection.
2) Turn off the power, doesn't matter if using the switch on the back of the modem or at the mains
3) Pull the cable out.

With 1 it is the most graceful shutdown of the link and if not possible using option 2, a dying gasp can be sent.

This makes some assumptions that may be false.  That using option 1 makes any difference to the DLM, it may treat the drop request no differently to just yanking out the wire.  Option 2, this assumes the modem supports a dying gasp, and if it does, again does the DLM treat that any differently to just yanking out the plug.

Regards

Phil

[IanG]

Many thanks for all your replies. The link to the Good/OK/Poor criterion was particularly useful.

Ian