This is the next packet switched WAN protocol. People are moving from frame-relay and going to multiple protocol label switching, or MPLS. The reason for this is the need for speed. In this type of communication, only the edge routers are the ones that do routing lookup of all other routers. The core routers do label switching with a mechanism called POP HOP and SWAP. Routers will have their own local label and forward a new label to their neighbor router, which in turn will swap for their own label until it reaches the customer edge router and then it POPs the label and just does a naked IP. The labels carry all the layer 3 destination addresses.
MPLS works in combination with CEF, Cisco Express Forwarding, which will make looking for routes much faster. There is no need for serial connections anymore because MPLS supports Ethernet technology.