It is likely to be errors in the data being received at either end, caused by interference or crosstalk on the line.
In the cabinet, a DLM process monitors the error statistics on your connection, and works out the best profile (not the IP profile) to assign to your connection. The default profile allows connections to run at full speed without interleaving.
If errors are detected at either a high rate for a short period, or a low rate for a long period, or disconnections occur, the DLM will drop your profile. This may restrict your speed, or turn on interleaving (which can affect your speed too) or both. When DLM chooses to do this, you will usually see a resync between 4AM and 8AM next morning. The resync will then show the new (lower) sync speed, and consequently a new IP profile.
When interleaving is turned on, it also turns on a "forward error correction" process. This allows the modems to be able to correct errors (up to a certain rate), but uses up some of your bandwidth by carrying extra parity data. The extra parity data is an overhead not seen in the sync speed (or IP profile), but is data that must be transferred over your line. In essence, the parity bits steal from your sync speed.
The FEC process *also* adds latency to transfer times (to receive & rebuild the data packets). So turning on interleaving tends to reduce your speed *and* add latency.
If the errors continue, DLM will drop the profile again - either adding more interleaving, or reducing speed further, or both.
If the errors disappear, DLM will raise the profile level. However, it can take anywhere from 2 weeks to a month for this to happen. Between 4AM and 8AM there will be a new resync, at a higher speed, and with a new IP profile.
Plusnet Customer
Using FTTC since 2011. Currently on 80/20 Unlimited Fibre Extra.
