Protocol overheads.

Thought so.

It doesn't answer why 9%
The protocol overheads on Openreach FTTP aren't 9%. They are quite a bit less than that, even with PPP.

The 9% was plucked from thin air because it roughly matched their quoted averages.
The advertising rules are based on real world throughput at peak times and it's nothing to do with 9% anything.

In reply to a post by hunnymonster:

Protocol overheads.

No. This is BT Business deciding how to label their tier offerings, rather than anything to do with protocol overheads more generally.

Openreach FTTP tiers are net of all protocol overheads (associated with GEM / Ethernet framing, FEC, QinQ, etc, etc.). Its the nominal peak *(net) data rate effective form the RJ45 port on the ONT.

The only "ISP" related protocol overhead - for BT and BT Business - is PPPoE which as said is far less than 9% and in reality is closer to ~1-2%.