PNNI signalling is used between ATM switches. PNNI is an ATM Forum standard protocol applicable to private ATM switching networks that may also be used in public 1.1.1 1.1.2 Figure 12.14 The ATM peer group hierarchy structure Understanding Changing Telecommunications - Building a Successful Telecom Business networks. The PNNI signalling system consists of two protocol categories: routing and signalling. This protocol is further explained below. B-ICI is the interface between different ATM providers. The AINI protocol is based on PNNI signalling and is used between ATM networks. A permanent virtual circuit (PVC) is a logical connection between ATM endpoints. Once the connection is set-up it remains up, until it is manually torn down. PVC is suitable for connections that are frequently used. A switched virtual circuit SVC is a temporary logical connection established on demand. In each ATM node the information will be analysed in order to find the next point to be connected to. When the connection is not needed any more it is disconnected. PNNI Routing Protocol The routing part of the PNNI protocol is responsible for distributing topology information between switches and clusters of switches. The PNNI routing protocol computes the routing of switched connections in the con- nectivity network based on the QoS requirements specified in the call set-up request. Depending on the size of the network there can be several levels in the routing hierarchy, as seen in Figure 12.14. PNNI Signalling Protocol The signalling part of PNNI handles the signalling between ATM switches in the network. It establishes point-to-point or point-to-multipoint connections across the ATM network. The PNNI signalling protocol is used to establish and release connections in the connec- tivity network. A signalling control channel (SCC) is used to transport the PNNI messages between ATM switching nodes in the connectivity plane. 12.8 IP ROUTING Routing is probably the most important function of IP. It is the act of moving information across the