Hi
G.INP does two things:
1) Corrects errors on UDP packets that otherwise would be lost if noise on the VDSL line caused the packet to be corrupted. This is important for voice/video services that use UDP. UDP is a light weight protocol and there is no way to re-request a UDP packet if one is missing due to an error, and if there was a way, by the time the request has gone all the way back to the originating server and been resent, in real time applications that means it arrives back two late and is of no use anyway. Interleaving of course helps here anyway by adding error correction, but packets can still be lost if the error can't be corrected. G.INP is simply a buffer at the VDSL cabinet modem that holds on to frames for a short while after they have been sent, and a low latency data exchange between the modems is able to signal back to the cabinet and say "this frame is corrupt, send again", and its sent with very low latency from the buffer so can work with real time applications.
2) For all other data exchanges using TCP/IP then corrupted packets aren't lost, as there is a mechanism to request the packet again, this would go back to the originating server which then re-transmits the packet. The packets are reassembled in the correct order when received even if arriving out of sync. This is why web pages and downloads are not corrupt even though our modem stats are showing errors. The problem with this is, over millions of connections re-transmitting errors adds up to a lot of extra bandwidth and overheads. By using G.INP, the frame containing the corrupt packet is re-requested from the cabinet modems buffer and the network stack never sees the error, so doesn't have to send a re-transmit request back to the originating server, so removing those overheads of re-sending data from source to recipient.
For 2, this is why interleaving is turned on even when the errors aren't particularly noticeable or causing a problem for the end user, as it helps remove the extra overheads by reducing the errors as much as possible. As G.INP does the same thing but better is the reason why the DSLAM is allowed to turn interleaving off when G.INP is available. The same error rate exists whether G.INP is on or off, but as G.INP stops those errors causing overheads on the network, BT are quite happy disabling interleaving when G.INP is available unless things are so bad both are required.
Regards
Phil