 a specific public Ethernet envelope containing the B-VLAN ID and the other backbone related fields. The carrier class Ethernet network is thus divided into virtual networks where the rout- ing of Ethernet frames is carried out through the spanning tree algorithm. Since there are up to 4096 B-VLAN identifier, also a carrier having 5 million users has an average VLAN constituted by about 1220 users SO that the spanning tree can manage it. Routing in the backbone Ethernet network is carried out by carrier class Ethernet switches. 7.9.1 Generalities on Carrier Class Ethernet Switches The carrier class Ethernet switch procedure when receiving a packet from a line interface can be simplified as follows: The Ethernet frame is obtained by terminating level 1 envelops. The frame is stored in the input queue, located in the line card. The B-VLAN tag is accessed and the frame is assigned to the correct backbone VLAN. The correct B-VLAN routing table is accessed using the incoming frame media access control (MAC) to find the address of the output interface where the frame has to be forwarded. This operation requires, in standard switches, the accessi- bility to the network spanning tree to determine allowed and forbidden routes through the network. The forwarding engine performs output interface lookup to detect its availability, QoS criteria are used to manage conflicts. Telecommunication Networks When the output interface is available, the frame is forwarded to the correct out- put interface via the switch fabric. The new layer 1 envelop is created while the frame is stored in the output interface. The QoS is managed taking into account the QoS class of the frame and, when the output line is available, the frame is transmitted through the transmission system. Besides Ethernet frame switching, a switch also manages the spanning tree protocol as detailed in Appendix B. The steps listed assume that the frame belongs to a unicast service. Multicast is sup- ported by carrier class Ethernet and several multicast services can be