I have been struggling through G.998.4, section 8.1. I remember what
Kitz wrote about overheads bringing G.993.2 throughput down to 91% in the worst case. It seems to me to that that could possibly correspond to DTU type 4 framing
W =
SEQ1 = 8, total overheads = 2 + 8 bytes total (ie 8 CRC bytes) per (maybe) a DTU =
2 × 64 bytes ie
2 units of PTM 64/65 payload. So that would be an efficiency of
2×64/(
2×(64+1)+2+8) = 91.43% which would fit in with
Kitz’s percentage. But then I know nothing at all about VDSL2.
Does that sound correct?
I would be interested to see some examples of the framing parameters from someone who has G.998.4 active. If that reasoning is correct then that would be a 1 byte CRC every 16 bytes of payload data - wow, someone is really struggling if they need such careful checks.
Perhaps some people have lines that are in one sense ok, but they have bad pulse noise / spikes, yet otherwise the attenuation is ok, hf is ok and the noise floor is not too bad. But they must be really in bad shape though if they need to sacrifice 1/11 of their throughput. I am kind-of surprised that this strategy is ever worth doing given the large performance losses due to all that completely unproductive bloat. I would have thought that a large interleave depth is the way to go because there is no throughput reduction overhead, no bloat. Although some people do get very fed up about a small amount of latency.