This is the high rate link (1Gb/s) on which the first fragment F1 and the two consecutive high priority packets, P2 and P3, are sent back-to-back by the BB-ISA
(BB-ISA->carrier IOM->egress IOM-> out-of-the-LNS).
The remaining fragments (F2 and F3) are still waiting in the BB-ISA to be transmitted. They are artificially delayed by 50ms each.
Additional QoS based on the L2TP header can be performed on the egress port in the LNS toward the LAC. This QoS is based on the classification fields inside of the packet/fragment headers (DSCP, dot1.p, EXP).
The LAC-AN link is not really relevant for the operation of LFI on the LNS. This link can be either Ethernet (in case of PPPoE) or ATM (PPPoE or PPP). The rate of the link between the LAC and the AN is still considered a high speed link compared to the slow last mile link.