any transport over mpls (and it's special case, ethernet over mpls)
is a point to point layer2 mpls vpn.
control plane involes ldp to verify pseudowire parameters
(type, mtu, etc) between the pe routers and advertise label
for the service for both end. as name implies, you can carry any kind of layer2 frame within
an atom session, including ppp, hdlc, frame-relay, various levels of atm cells, raw ip, ethernet and vlan.
the latter is meaning that the sending pe routers won't strip vlan tag within the
payload ethernet frame. otherwise you can choose to use plain ethernet pseudowire from a vlan subinterface.
the data plane is similar to layer3 mpls vpn,
the inner, service label used from the ldp session
with the remote pe router. the outer, transport label
is taken from the global table looking for an appropirate lsp for the
remote pe router.
here are the captures for
atom with no cw and
atom with cw