These end stations may be located in physically separate LANs, but because they are both connected to the ATM network, they are part of the same logical subnetwork, or virtual subnetwork. If an end station is using PVCs, the ATM service provider or network administrator manually maps the IP addresses of reachable destination end stations to ATM virtual circuits. The destination end station then converts the AAL 5 cells back into IP packets. To send packets to a destination IP host in another virtual subnetwork, a sending IP host must send the packets through a router. The sending IP host uses a virtual circuit to connect to the router, and the router uses another virtual circuit to connect to the destination IP host.
Routers create a bottleneck because they are slower than switches.
Furthermore, Classical IP Over ATM does not solve delay or congestion problems because Classical IP Over ATM cannot take advantage of ATM's Qualities of Service. Also, because end stations can be part of the same virtual subnetwork even if they are on physically separate LANs, Classical IP Over ATM gives you a lot of flexibility in configuring your network. You can enable users who are hundreds of miles apart to share the same resources over a high-speed ATM backbone. LANE Developed by the ATM Forum, the LANE standard defines a way to bridge LANs over a high-speed ATM backbone. LANE also enables you to connect workstations running traditional protocols directly to an ATM network, creating a high-speed workgroup.