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tohojo / bufferbloat-net

The source repository for the bufferbloat.net web site
https://www.bufferbloat.net
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[cake] Fix vdsl2 overhead description #3

Closed moeller0 closed 8 years ago

moeller0 commented 8 years ago

The old text: Apparently PTM does have a small additional overhead on the order of 1/128, due to HDLC framing which attaches special meaning to 0x7D and 0x7E bytes; I might need to add approximate handling for that, kernel-side. was slightly inaccurate, in that VDSL2 does noy use HDLC on the user visible data bearer, but a deterministic 64/65 encoding. And since no HDLC in byte stuffing mode is used the kernel needs not to learn something new. For VDSL1 HDLC actually is used, but there the PTM overhead is 5-6 Byte compared to VDSL2's 4 Byte, so cake's keywords will not work with VDSL1 anyway.

For reference: VDSL1 (nach G.993.1 Annex H): 1B Opening Flag Sequence (OFS), 1B Address Field (AF), 1B Control Field (CF), 2B TC-CRC (PTM-FCS), 1B Closing Flag Sequence (CFS not used with back-2-back PTM frames)

VDSL2 (IEEE 802.3-2012 61.3): 1B Start of Frame (S), 1B End of Frame (Ck), 2B TC-CRC (PTM-FCS)

Signed-off-by: Sebastian Moeller moeller0@gmx.de

tohojo commented 8 years ago

Your detailed knowledge of DSL is astounding as always. Thanks! :)