A transport-service bridge is essentially a router that copies PDUs, as opposed to translating them. Like gateways, transport-service bridges introduce a single point of failure. For example, OSI includes different transport protocols, such as TP0 for wide area networks and TP4 for local area networks. Transport-service bridges are viable candidates for linking those different OSI transports. Network tunnels are one level down from the transport-service bridge approach.
Network tunnels operate at the network layer instead of at the transport layer.
They encapsulate OSI CLNP packets in IP packets and run them over IP networks. Network tunnels are essentially CLNP routers.