- BT 21CN Lines
- TT lines - via Adjusting the profile
Interleaving affects the way error detections & correction is applied to the line. These changes take a day to take effect.
With interleaving off the round trip ("ping") time is faster (around 12ms when off, around 28ms when on). However the line is less able to cope with noise, which may cause more frequent retraining and/or a lower sync speed and BRAS rate.
The options are "Set interleave off", "Set interleave on" and "Set auto interleave" (interleave is automatically controlled by the exchange depending on the measured stability of the line).
More about interleaving
On broadband lines there is a key factor that affects the latency. Lines use a technique called Forward Error Correction which allows small errors in the signal to be corrected without having to resend the data. This only works for small errors (e.g. one bit), but sometimes lines have interference (called Impulse Noise) which affects lots of bits at a time (a click or pop on the line, in effect). To fix this a technique called interleaving is used. It spreads each packet out over a longer time, interleaving it with other packets. When there is impulse noise the effect is a small error in a whole series of packets, each of which can be corrected.
The downside is that, whilst the overall speed is the same, interleaving adds latency. Typically around 8ms is added to all communications on the line, though there are different levels of interleaving (or Impulse Noise Protection) which cause even higher latency. The latency added is however consistent, which can, in itself, be an important factor for on-line games.
This is a slight challenge: with interleaving the line has extra latency all the time, but without it you get errors. When there is an error then the packet has to be resent, which adds a lot more latency but randomly and inconsistently. Usually interleaving is a better choice, but it does depend in the interference. In some cases interference is only at certain times of day, and if that is not when you are playing, then interleaving is best turned off.
There is often one other option, which is running the line at a lower speed. This can remove the interference if it is only at certain frequencies, and as we explained above, the speed is not necessarily a key factor.
Good or bad?
Interleaving on lines where is can reduce errors can be a good thing for applications such as VoIP.
Gamers will want to reduce the latency as much as possible.
On a good quality line there should be no need for interleaving, but a longer line may benefit from it.
Errors on lines is usually a problem that needs to be fixed. Masking a fault by enabling interleaving may help, but it may be better to get to the root cause of the errors.
The interleaving is set when selecting the line profile, see: Adjusting the profile
Controlling interleaving on a BT FTTC line is done by Openreach. It is quite hard for us to persuade BT to disable or enable interleaving. We can try though, so do ask.