istributed but coordinated fashion, or on the switch fabric card itself as a powerful and centralized unit. Special provisions enable the correct handling of multicast packets. Unicast packets are switched from an input port to an output port and represent the bulk of the switch fabric's typical workload. Different architectures handle multicast packets in different ways. Some switch fabrics will switch the original packet only once over the crossconnect. They will then replicate it to the corresponding output port buffers without adversely affecting the crossconnect switching bandwidth. However, other architectural approaches copy the input packet and then switch it over the crossconnect in multiple instances in order to feed the appropriate output port buffers with the output of a single thread through the crossconnect. However, the switch architect has to deal with a series of trade-offs regarding the overall efficiency, switching bandwidth usage, line-card logic complexity translating into development cost and time, and SO on. We will not be expanding on them here. For more information, consult the recommended reading at the end of the chapter. As some processing overhead needs to be expended in order for the fabric to create, affix, and remove internal special tags (facilitating the internal routing inside the fabric) to and from these inter- nally created cells (after disassembly of the incoming packet), to enable the assembly and disassem- bly (internal to the switch fabric) of outgoing and incoming packets, and to properly handle and manage multicast traffic packets over the crossconnect, some extra switching bandwidth is required. A fabric's overspeed is the ratio of the actual data throughput over the aggregate user-port bandwidth. In this definition, the backplane encoding is not taken under consideration. For example, if a switch fabric is using a backplane and crossconnect structure that switches data with a speed of 20 Gbps and if traffic is received from a port that operates at a wire speed of 10 G