Sigh. Yes I do understand that, but as far as I can see, it's all missing the point.
Reference Ethernet in the First Mile whitepaper - link
This whitepaper is so old it only talks about PTM using HDLC, and not this "64/65 octet" encapsulation. It's talking about VDSL1, which could have used PTM using HDLC, VDSL2 did not exist at that time.
DSL modems most often rely on ATM as
their data link protocol, a choice that was inspired by the
vision of DSL as a multi-service multi-media platform.
ATM is indeed very well equipped to handle multiple
streams of data, each with different bandwidth and quality
of service (QoS) requirements.
But we never used DSL like that. There aren't different classes of ATM cells carrying voice data and Internet data. Everything sent over the ATM used over an ADSL link goes into the same PPP / Internet data traffic, there's no QoS at the ATM level.
If you are sending a 1500 byte packet over ATM over AAL5, it'll still take the same amount of time to transfer all those 1500 bytes due to the bandwidth of the link! If you are sending a 100 byte packet over ATM/AAL5, it'll take three ATM cells then the end of the AAL5 trailer. Sending the 100 byte packet over PTM, it'll take two of those 64 byte PTM codewords, and the sync byte will indicate it's the last codeword of the packet.
ATM broke up all packets, data, and voice streams into 48-byte chunks
But going over the ATM used over an ADSL link, all there is is data packets. A lot of the information about ATM in general, is, as far as I can see, not relevant for how it's used on ADSL. When it's talking about low delays for voice traffic, that'll be from putting the voice traffic in separately classed cells from the bulky data traffic.
And the PTM used on VDSL2 is breaking up everything into 64 byte chunks anyway!
When I'm referring to this 64/65 byte encapsulation, that's how it's written in the documentation, it's not that I don't know whether it's 64 or 65 bytes. Most of the documentation about general ATM pre-dates this 64/65 byte encapsulation used on VDSL2.