wn. The adoption of the virtual circuit concept through the VLAN technology opens the possibility to empower the carrier class Ethernet with a set of monitoring and QoS capa- bilities that justify the name "Carrier Class." Networks Fundamentals and Present Architectures As a matter of fact, the concept of virtual circuit binds the packet belonging to the same communication to travel through the same path in the network. If we consider that the reception of each Ethernet frame is acknowledged there are all the elements to make it possible to control frames delay, loss, and duplication, while eliminating frames out of order arrival. Once virtual circuits are used, survivability can be implemented via fast techniques, operating on the virtual circuits themselves. The fast protection mechanism introduced in the standards is the 1:1 VLAN protection. This mechanism is based on the fact that, when it is needed to provision a VLAN, two of them are provisioned by the control plane. The two VLANs have the same endpoints but no other network element in common. The traffic can be either divided among the two routes (implementing the so-called traf- fic balancing) or sent over one of them (the traditional protection). Moreover, the protection capacity can be also used to transmit best effort frames. When a failure happens on a traffic-loaded path, the protocol managing protection starts. First of all, a control frame is sent by the terminal detecting the failure to the other end terminal of the involved VLAN. If no acknowledgment is detected, the terminal sets itself in the bidirectional failure mode, switches on the protection route after stopping low-priority traffic, and looks for the contact with the other end terminal via a control frame. If the other end terminal has done the same thing, both the extremes of the affected VLAN are now on the protection route, they can verify the good performance of the route and their readiness to switch with a handshake, and after that, they deviate the high- priority traffic on the pro