Following the success of MPLS in the provision, configuration and management of virtual circuits for IP networks, some thought was given as to whether MPLS might be used to handle other sorts of virtual circuits, not as a transport mechanism, but as a signaling and control plane for:
§  Add a note hereLayer-2 virtual circuits for Frame Relay,
§  Add a note hereTDM virtual paths for SONET and SDH,
§  Add a note hereWavelengths in an optical transport network (OTN),
§  Add a note hereFiber segments linked via spatial physical port switches.
Add a note hereThus was born Generalized MPLS (GMPLS), which applies the MPLS control plane (with extensions) to these other layers—the focus is typically on SONET/ SDH and optical (wavelength) networks. Cross-connects and Add-Drop Multiplexers in these networks need to exchange GMPLS protocol messages. This is not necessarily strange—all these devices today run element managers or SNMP agents that communicate via a management IP layer. In the TDM world, MPLS label allocation/de-allocation is identified with time-slot allocation; in the optical world it is equivalent to wavelength allocation.
Add a note hereIn fact, GMPLS has had a mixed reception in the world’s carriers. Optical and transmission engineers don’t necessarily believe that the IP guys know best when it comes to controlling their networks. There is a history of virtual path management in SONET/SDH networks and optical channel management for OTNs being organized through the network management systems. With increasing element intelligence and more powerful management tools, it is widely felt that the management plane is adequate, and that replicating its functions in a new signaling plane is not required. Of course, opinions differ.