| +---+            /-\
                               | |   | Router    (   ) WDM
                               | +---+ Node       \-/  Node
                               |
                               o----------------------------

  +---+        /-\          /-\           /-\          +---+
  | R1|-------( A )--------( C )---------( E )---------| R3|
  +---+        \-/          \-/           \-/          +---+
                           /   \         /   \
                          /     \       /     \
                         /       \     /       \
                        /         \   /         \
                       /           \ /           \
      +---+          /-\           /-\           /-\          +---+
      | R2|---------( B )---------( D )---------( F )---------| R4|
      +---+          \-/           \-/           \-/          +---+