Also, the frame sizes on each network must be the same. Bridges are useful because they can be used to interconnect networks that have related but independent applications. Bridges separate the data traffic in each network from the other so that the token rotation time and the amount of data traffic on each network are kept at lower levels. The stations that need to communicate across the bridge do so transparently, without having to know that a bridge exists. In situations where two different networks are to be interconnected, a router is used.
Routers overcome the limitations of the bridge.
Routers can also take care of the different frame sizes used on the two networks by segmenting large frames into smaller ones and reassembling small frames into larger ones. There is a price to pay for these features, however. Routers and the stations that use them have to use another protocol called the ISO Network protocol. This protocol is identified by the LSAP discussed in Section 5. Frames from a station on one network that need to be sent to a station on another network have to be explicitly addressed and sent to the router with directions on how to forward the frame to its ultimate destination.