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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. Interleaving may be necessary to ensure a stable and reliable connection in cases where the line quality is poor.
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 35ms, interleaving is enabled.
The DSLAM (central office equipment) has interleaving set, and this cannot be changed remotely by the end user. Qwest turns on interleaving for all ADSL customers. The interleave value is 16ms in each direction which adds 32ms total to first hop latency. This is system wide and to date there are no reports that any Qwest ADSL users have successfully convinced Qwest to turn off interleaving and run Fastpath. [Correction: There are isolated reports of people managing to get it turned off, however this appears to violate stated company policy.]
Also see: »More On FEC and Interleave