This section discusses an incorporation of Virtual Network Configuration (VNC) and the Network Control Protocol (NCP) as described in the previous sections into the Private Network-Network Interface (PNNI)  to facilitate seamless ATM hand-off. An attempt is made to minimize the changes to the evolving PNNI standard. Figure 9 shows a high level view of the PNNI Architecture. Terminology used in the PNNI Specification. In this version of mobile PNNI, the standard PNNI route determination, topology database, and topology exchange would reside within the NCP. The NCP stack with VNC is shown in Figure 10.
Figure 9: PNNI Architecture Reference Model.
Figure 10: Virtual Network Configuration Stack.
The enabling mechanism is the fact that VNC will cause the NCP to create a topology which will exist after a hand-off occurs at a time prior to the hand-off. This will cause PNNI to perform its standard action of updating its topology information immediately before the hand-off occurs. Note that this is localized within a single Peer Group (PG).
The second enabling mechanism is a change to the PNNI signaling protocol. In mobile PNNI, standard PNNI signaling is allowed to dynamically modify logical links when triggered by a topology change. This is similar to a CALL ABORT message except that the ensuing RELEASE messages will be contained within the scope of the Peer Group (PG). This new message will be called a SCOPED CALL ABORT message.
When the topology changes due to an end system hand-off, a check is made to determine which end system (RN) has changed logical nodes (LN). An attempt is made to establish the same incoming VCs at the new LN as were at the original LN and connections are established from the new LN to the original border LNs of the Peer Group. This allows the RN to continue transmitting with the same VCI as the hand-off occurs. The connections from the original LNs to the border LNs are released after the hand-off occurs. If the new LN is already using a VCI that was used at the original LN, the HANDOFF packet will contain the replacement VCIs to be used by the end system (RN). There are now two branches of a logical link tree established with the border LN as the root. After the hand-off takes place the old branch is removed by the new SCOPED CALL ABORT message.
Note that link changes are localized to a single Peer Group. The fact that changes can be localized to a Peer Group greatly reduces the impact on the network and implies that the mobile network should have many levels in its PNNI hierarchy. In order to maintain cell order the new path within the Peer Group is chosen so as to be equal to or longer than the original path based on implementation dependent metrics.
Consider the network shown in Figure 11. Peer Groups are enclosed in circles and the blackened nodes represent the lowest level Peer Group Leader for each Peer Group. End system A.1.2.X is about to hand-off from A.1.1 to A.2.2. The smallest scope which encompasses the old and new LN is the LN A.
Figure 11: Mobile PNNI Example.
A.3.1 is the outgoing border node for LN A. A CALL SETUP uses normal PNNI operations to setup a logical link from A.3.1 to A.2.2. After A.1.2.X hands off, a SCOPED CALL ABORT message releases the logical link from A.3.1 to A.1.1.