DLM and the NTE Modem

Forgive me if this is a dumb question but I've looked around for a basic guide to how DLM operates and can't find one so...

I understand that DLM reacts to errors and adjusts speed accordingly to gain a stable line but I'm trying to get a better understanding of how DLM gets the information (and where it sends it to make changes). Is it correct that it's just the modem itself that measures the errors and sends this info back up the line to some point (to the cabinet or the exchange or further)?

Also does anything downstream from the modem (ie the router, the ethernet cabling, the wifi, the connected PC) get measured? If so would that include the type of traffic passing through? For example if you're trying to download a huge file or some kind of P2P torrent and it's full of corruptions or errors and takes ages because it can't find the bits it wants, would that get picked up by DLM? Would unplugging the ethernet from the modem to the router and connecting a PC direct to the modem be seen as a disconnection? BTW I'm on TalkTalk if that makes any difference.

The DLM is informed of sync events and information about these and other errors via the cabinet hardware, which in turn gets a chunk of the information from the modem in the home.

What you plug into the Openreach modem plays no part in the DLM determination.

When you say full of corruptions, a corrupt file that was corrupt before it got to the cabinet will not affect anything, only errors detected in the transmission ove rthe VDSL element count.

Edited by MrSaffron (Thu 18-Apr-13 11:10:29)

In reply to a post by MrSaffron:

What you plug into the router plays no part in the DLM determination.

Does that mean the router itself can play a part, in which case the modem is detecting stuff downstream, ie from the ethernet out of the modem towards the home network?

In reply to a post by Alexir:

In reply to a post by MrSaffron:

What you plug into the router plays no part in the DLM determination.

Does that mean the router itself can play a part, in which case the modem is detecting stuff downstream, ie from the ethernet out of the modem towards the home network?

No. Only the modem matters. It has nothing to do with anything that happens on your home network.

Having said that DLM is monitoring CRC errors so there might be a relationship between the number of CRC errors generated and the quantity of data being downloaded. I have my doubts though. I think the connection is always pinging back and forth anyway so even if there's no data coming down the connection is never 'silent'. I think the modem and DSLAM are continually chattering between each other even if it's just 'are you still there?' 'yes'. Could be wrong on that though plus some DSL variants can adjust power levels and tone usage to limit cross-talk when the connection is idle so that would impact the number of CRC errors.

But the simple answer is that errors within your network (ie;wifi dropped packets) are not reported to or seen by the DLM.

Edited by Andrue (Thu 18-Apr-13 11:12:54)

Sorry was a typo - corrected it to say Openreach modem

To add some more:

It is only your modem (or the modem part of a hub) that matters in this case.

Within the information sent back to the DSLAM by the modem during speed negotiation is tone specific attenuation and SNR which allows the DSLAM and modem to "agree" the bit loading for each tone. The total bit loading will define the maximum capability of the line and hopefully the DLM monitors the total loading capability over shot/medium time and sets the speeds accordingly.

In reply to a post by MrSaffron:

Sorry was a typo - corrected it to say Openreach modem

Ah makes sense, nothing downstream from the modem has any effect on DLM, thanks. So even pulling the ethernet cable out of the modem makes no difference as long as the modem remains powered up.

I agree with all that has been said above. One extra detail may be useful; where changes behind the modem do affect your speed, indirectly related to DLM.

All FTTC ISP's use the DLM described above; based on cabinet->modem stats, and ultimately setting cabinet->modem sync speed.

Several FTTC ISP's also have a higher level layer of BRAS/IP profile that limits the rate the exchange will send your data to the cabinet.
The IP Profile is set when the exchange->router PPPoE connection is established; unless your package says otherwise this is just a fraction below the sync speed to allow for overheads.
Often, when DLM forces a resync with new sync speed, the PPPoE connection is broken, and so the IP/BRAS profile is automatically set appropriately.

However, quite often, the low level resync is fast enough that the higher level connection does not notice and is not broken.
This can lead to a BRAS mismatch, where the sync speed and IP Profile do not match.
The speed you get will be based on the lower of the sync speed and profile.

If you think that your speed is too low and the DLM may be going to increase it, it is worth disconnecting and reconnecting the modem first thing each morning.
Best done by the web interface, but you can power cycle or reset the router.
Do it early morning as DLM usually schedules speed increases overnight.

In reply to a post by Andrue:

I think the connection is always pinging back and forth anyway so even if there's no data coming down the connection is never 'silent'. I think the modem and DSLAM are continually chattering between each other even if it's just 'are you still there?' 'yes'.

Yes - there are still synchronisation bits that are transmitted as framing data, and that the modems watch out for. If those go missing, it causes a "loss of signal" event and a resync (under the assumption that the noise is now too high, and masking the signal).

The blocks of data transfer continuously, so CRC and FEC stats are updated even when no user data transfers.

In reply to a post by Andrue:

Having said that DLM is monitoring CRC errors so there might be a relationship between the number of CRC errors generated and the quantity of data being downloaded

Eh? So there is possibly a connection between high data volumes, errors and DLM? The reason it came to mind is that for a year I've had a pretty stable 38meg download, then about a month ago it started dropping off to 32 meg and then to 24meg with a 0.67 upload. These were firm ceilings so I guessed were set by DLM but I didn't know why. Around the same time my new flatmate had arrived and (I just found out) has be running P2P 24x7, downloading up to 20gig per day. Could this be connected to DLM setting caps or am I barking (up the wrong tree)?