The example you quote is for a specific application, where the data workflow is along the chain. Chains are considered a bad design for Ethernet in general - Ethernet works better in a tree topology.
There, you'd use two central switches and connect each device to both. In normal operation, the spare switch would use RSTP/MSTP to block all redundant ...
To quote your referenced document:
As shown in Figure 2, a daisy chain device has 2 embedded Ethernet
ports which function as an Ethernet switch, as well as an interface to
the local device. This allows information to flow to the device, or
flow through the ports to other devices in the daisy chain.
Ethernet Daisy Chain allows devices to be cabled together ...
the magic here is your end hosts (nodes that you control with your plc). Each should have two ethernet ports allowing you to go hop-by-hop. See the dotted notation on the right. That is actually a connection that is blocked by the ring algorithm so that a full loop does not occur (standby link). If you lose connection anywhere, the system will detect it and ...
Most switches use multiple buffer queues for each port.
Each traffic class for QoS (frequently controlled by DiffServ) is mapped to one of the buffers. Buffers are then served in some priority pattern, e.g. 4:2:1 for a three-stage queue, aka weighted round robin.
Many switches allow you to configure the number of queues or even the priority pattern.
( As others have already said, PPPoE is literally PPP over Ethernet. And similarly PPPoA is PPP over ATM. )
Ethernet and ATM are oddities in the networking world as they define both a layer-1 and layer-2 component. In the case of ethernet, it's layer-1 has always used it's layer-2; no one ever built it any other way. (Ethernet's layer-2 protocol, however, ...
PPP is designed to ride on top of a byte-oriented, point-to-point physical-layer protocol like a simple modem-style serial link.
Ethernet is no simple serial protocol but it requires frame-level addressing (L2 MAC address), so PPPoE expands standard PPP to take care of discovery and addressing between the link partners.
Why does PPP need a wrapping protocol?
PPP is not a layer-1 protocol, so it needs a layer-1 protocol to carry it. Protocols like ethernet are both layer-1 and layer-2 protocols, so PPP can use ethernet as its layer-1 protocol, but that comes with the ethernet layer-2 protocol that wraps PPP.
Why can't I just use PPP over Ethernet instead of PPPoE?