There are two parts to SONET: framing and switching. Optical transducers usually, or always, do a bottom level framing activity to achieve frame sync. The ATM people and now the IP people utilize this low level sonet function and then say that they are running over sonet. The sonet hardware to do this is so cheap and transparent that new designs to support ATM or IP make no economic sense. Sonet has its own simple switching protocol, unused by ATM or IP, that assumes that successive 8 bit octets within the 24*8+1 bit frame are from 24 different voice streams. Time slot interchange is used to switch these sonet streams.
Nick McKeown: Stanford CS, Switching, gave a Sonet switching tutorial in Europe.
I would call this: switching of IP packets that are carried in a sonet framing form.