Why both VPI *and* VCI?

[A7med Baraka]

The Virtual Path concept originated with concerns over the cost of
controlling BISDN networks. The idea was to group connections
sharing common paths through the network into identifiable units (the Paths).
Network management actions would then be applied to the smaller number of
groups of connections (paths) instead of a larger number of individual
connections (VCI). Management here including call setup, routing, failure
management, bandwidth al******** etc. For example, use of Virtual Paths in
an ATM network reduces the load on the control mechanisms because the function
needed to set up a path through the network are performed only once for all
subsequent Virtual Channels using that path. Changing the trunk mapping
of a single Virtual Path can effect a route change for every Virtual Channel
using that path.

Now the basic operation of an ATM switch will be the same, no matter if it is
handling a virtual path or virtual circuit. The switch must identify on
the basis of the incomming cell's VPI, VCI, or both, which output port to
forward a cell received on a given input port. It must also determine what
the new values the VPI/VCI are on this output link, substituting these
new values in the cell.