Interleaving is an extra level of error correction.

ADSL modems are capable of data interleaving, which is a technique used to increase resistance to noise bursts on a line. Interleaving decreases the chance that noise on a line will cause data transmission errors. Interleaving may be necessary in situations where the quality of the phone line is sub-standard or you are approaching the distance limits of DSL service to ensure a stable and reliable connection.

The down-side of interleaving is that it increases latency (ping). This is because a single packet is spread out over several packets before it can be fully sent or fully received.

You can tell if interleaving is active on your line by measuring the first hop ping. If it is under 20ms, interleaving is disabled. If it is above 40ms, interleaving is enabled.

The DSLAM (central office equipment) has interleaving set, and this cannot be changed remotely by the end user. In general, only people who need interleaving have it turned on. However, some technicians will turn it on as a "quick fix" for problems that could be best solved in different ways.

If you would like to have interleaving turned off, please refer to instructions located here. Bear in mind that if your line quality is poor, Sympatico will likely not turn it off. However, if you insist, they will change it, but you must agree to forgo any future support regarding speed or interleaving changes. They will flag your account so that any rep who helps you will see they are not to support you any more.

After all is said and done, your modem should go offline and re-sync.
You will notice lower ping times.

IF after interleaving is disabled, and you experience line quality problems, slow speeds, or connectivity issues, then interleaving is likely required on your line for DSL to work properly.

A more detailed explanation of Interleaving can be found by clicking here
A thank you goes to kevmetric for the information.