Last update 11 Jan 2013
Interleaving is a process by which the data in TCP/IP data packets are split and shared between packets. A very rough and ready description is that the data in each of (for example) four packets is split into quarters and a quarter from each original package is included in the final packages that get transmitted.
This allows error correction routines to correct most packets that suffer corruption during transmission, so improving throughput rates at any given line speed. (Otherwise the corrupt packet would need re-transmission).
How this works technically is way beyond me and I don’t feel the need to know. But because of the processing involved before transmission, and the reassembly of the data at the receiving end, latency/ping is always increased. Typically around 20ms extra. Which gamers don’t like.
Fast Path is when Interleaving is off.
Connection speed may or may not be affected, but on ADSL Max if the line can connect at 8128kbps on Fast Path this is usually reduced to 7616kbps. A few combinations of modem/router and exchange DSLAM can still achieve the full 8128kbps even with interleaving, particularly TI chipset-based.
On FTTC connections interleaving causes a considerable fall in connection speed.
In the line stats that you may see from your modem/router FECs (Forward Error Corrections) or RS (Reed-Solomon) corrections are counters of such corrected data. If you are thinking of having Interleaving turned off, (not possible on request on FTTC), those figures are useful to know.
There is a detailed explanation of Interleaving on the Kitz site..