l plane. As a matter of fact, it should be remembered that ASON is not a complete protocol suite, but more a reference architecture. Thus it is well possible that different vendors use differ- ent specific sets of protocols, all ASON-compatible. In this case, naturally every different protocol suite will be confined to a different admin- istrative domain and suitable interfaces will guarantee the cross-compatibility. The first standard interface that is the optical user network interface (O-UNI). This is the interface connecting into the data plane the client equipment with a network equipment through an optical signal. Somehow the O-UNI is a limited functionality interface, since it has also to shield the net- work from any improper client activity that could either interfere with the network working or try to detect information about the network that the carrier wants to hide to the customer. The activity of the O-UNI is thus limited to resource discovery, signaling and, naturally, data exchange. In particular, the following protocols flow through the O-UNI: Connection request and creation Connection parameters change Connection delete Client identity change Data transit Just to do an example, it is through the O-UNI that the Ethernet switch belonging to a cor- porate network asks to the public network the creation of a virtual connection for a data center back-up operation. In particular, the ASON-enabled Ethernet switch sends through the O-UNI a suitable signal to the multilayer node to which it is connected for specifying the request of a mis- sion-critical data connection between two points of the network. The desired format is Gbit Ethernet and the network back-up service foresees a specific service level agreement. The ASN uses all the control planes to provision the connection and, when the con- nection is correctly provisioned, a return message is returned through the O-UNI to the switch that starts the data transfer. At the end of the back-up, the private network manager decides not to destroy the conn