Survivable IP over WDM Networks.
Current issue: How to map the physical topology and the logical topology to makethe network survivable.


Physical Topology
The Physical Topology is the description of how the network is physically connected. The physical layout of devices on a network. Every LAN has a topology, or the way that the devices on a network are arranged and how they communicate with each other. The way that the workstations are connected to the network through the actual cables that transmit data -- the physical structure of the network -- is called the physical topology.


Logical Topology
The logical topology, in contrast, is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices.

Sometimes even when physical links exist, if there is no logical Topology, the connection will not be allowed. To think of this, just consider the router, if it does not forward the packet to the next hop, then the connection could not be setup.