Interleaving etc

I was watching another thread and noticed that with Plusnet I'm able to turn interleaving on and off myself. My connection is good but at times my connection speed slows right down to a crawl and I notice the pings go sky high. I'm not intersted in pings really as not a gamer, just an average sort of surfer and a bit of streaming.

Will I get any benefit turning interleaving off? It is currently turned on but the notes on Plusneet website say turning off can cause instability. Or is there anything else I can do myself (I think my problem with slowdowns is contention as I like in a village - no LLU except TT).

Any help appreciated.

Interleaving, and the effect it can have being on or off, is quite complicated. The effect seems to be different on BT Wholesale and LLU networks as well. So as well as my comments below, the actual effect on Plusnet that users report is important. I'm just trying to fill in the background for you.

In theory, on BT Wholesale networks it shouldn't make any difference to connection speed other than in special cases such as the top of ADSL Max. In practice it seems it can do, so my theory knowledge is incomplete.

With regard to throughput, it is important to know, if you don't already, that there is a constant stream of packets between the exchange and your modem when connected, and independent of whether you are using it or not. When not in use the packets are empty of data but still have all the control bits - quite a lot of them!

When you get full speed downloads then the packets are full of data. When there is congestion further back, they are only part full. In both cases there are frequent transmission errors.

Interleaving uses a complex algorithm to correct these errors in flight, you can see how many by the FEC or RS Correction (depending on modem) values. The encoding, decoding and error correction routines are what normally cause a rise in ping/tatency times when it is used.

Without interleaving, any packet that contains even a single-bit error has to re-transmitted. At low error volumes this can be unnoticeable at your end. At high error volumes it can severely affect the throughput, in on top of any congestion issues. Remember, because the packets are always there and always at the connection speed, the amount of data coming down doesn't basically affect the error rate.

Thanks for taking the time to simplify this issue. I think I kind of get it. Is there a degree or percentage of errors when interleaved which indicates it would be risky to turn interleaving off? Or is that a daft question?

Sorry to be so dim.

In my case I found a few thousand a minute made no noticeable difference to performance while interleaved, but I wouldn't risk fast path with that many. A thousand or more per second is a disaster. A million a day is no big deal.

In terms of when it is safe to turn it off, I only ever had interleaved on ADSLx. On FTTC I'm on fast path with no trouble. You need to wait for real-life stuff from other posters. The faster your connection speed, the higher the rate will be anyway, as there are more packets per second. That's one of the reasons why when you are getting disconnections the BT DLM increases the noise margin, to slow the connection.

Obviousy, low rates indicate a cleaner line - noise can come from anywhere - internal phone wiring is the only one you can usually deal with, perhaps also faulty Sky boxes, other step-down power supplies, running machines etc. External can vary from being near to a railway line with dodgy signalling gear, line running above an electricity sub-station, close to a flickering street light outside your house, factories, radio hams in the area, and so on and so on.

The cleaner the line the more scope you have for things like lowering the noise margin to increase speed.

(Edit, first line said would, should have been wouldn't).

Edited by RobertoS (Wed 09-Nov-11 13:20:16)