to the forwarding state, albeit after a delay of about 30s while it goes through the listen- ing and learning states. The time spent in the listening and learning states is determined by a value known as the forward delay (the default is 15s; in every situation the value of this time interval is set by the root bridge). If, instead, another switch is connected, the port may remain in blocking mode if it is determined that it would cause a loop in the network. B.3.2 Topology Change Notification TCN BDUs are used to inform other switches of port changes. TCNs are injected into the network by a non-root switch and propagated to the root. Upon receipt of the TCN, the root switch will set a topology change flag in its normal BPDUs. This flag is propagated to all other switches to instruct them to change their topology data base and, as a conse- quence, to update the forwarding table entries. B.4 Protocol Extensions The STP is SO diffused and is applied in SO huge a number of different situations that sev- eral updates and variations of the protocol have been done. The first important variation is the so-called RSTP (rapid STP). The scope of this STP variation is to speed the incorporation of new entities into the network. In the standard STP it can take up to 30s, while in RSTP it generally takes 6s if the administrator has left default values for the process characteristic time. Another important variation of the STP is the incorporation of the virtual local area net- work (VLAN) concept into STP, allowing different instances of the protocol to coexist, each one representing a different VLAN built on the same physical network structure. Appendix C: Inter-Symbol Interference Indexes Summation Rule C.1 Cascade of Linear Filters In this appendix, we will demonstrate the rule used in the design of transmission systems that, under opportune hypothesis, the ISI indexes relative to independent phenomena can be added. We will consider regular pulses, that is, either finite duration pulses or pulses with expo- nentially decre