Regarding Link Layer Adaptation, the documentation (https://openwrt.org/docs/guide-user/network/traffic-shaping/sqm-details) states :
Set the Per-packet Overhead to 44 bytes if you use any kind of DSL/ADSL connection to the Internet (that is, if you get your internet service through a telephone line).
It is especially important to set the Link Layer Adaptation on links that use ATM framing (almost all DSL/ADSL links do), because ATM adds five additional bytes of overhead to a 48-byte frame
Please note that if your IS uses a mandatory VLAN on the bottleneck link, please add 4 additional bytes to the overhead to tell sqm to properly account for it.
Q: What should be the optimal value for a ATM / ADSL line with mandatory VLAN tagging? 48 ?