reply to Kirill
Re: Interleaved vs Fast Path,help to check it. DSL protocol includes forward error correction. If noise corrupts a few bits the receiving modem is still able to perfectly recover the data. However if the burst is long so many bits get corrupted that FEC cannot recover and the frame needs to be retransmitted.
Interleave achieves improved noise resistance by interleaving the data from multiple frames. If an extended noise burst corrupts a lot of bits once the data is deinterleaved only a few bits in each frame are corrupt allowing FEC or recover the data.
The downside of interleave is slightly increased latency. Look at first hop latency. If you are actually on fast path latency should be reduced by a few milliseconds. The precise improvement is depending on modem speed. In general the effects of FastPath are often swamped out by other network latency issues. We are talking about improvements on the order of 10 ms or so.
DSL is a master/slave implementation. I very much doubt you are able to enable/disable FastPath at the modem. Probably the best you can do is enable the modem's ability to support it, if requested to do so by the DSLAM.