Internetworking: Devices

Internetworking, i. e. the connection of networks ( LANs or WANs), has become more and more important during the last years. The goal is always to make two networks logically look like one. There are two main reasons for this: different applications and environments need different networks, which nevertheless should be able to exchange information. Secondly, needs for worldwide communications via computer networks. To accomplish the task of interconnecting networks special interconnection devices (hubs) are needed. There are five important types, which can be distinguished by their starting point in the OSI layer model:

1. Repeaters: they just forward bits from one network to another. This is often done to extend the maximum cable length. Repeaters do not process the data in any way, they simply copy bits. Thus, repeaters work on the physical layer of the OSI model.
2. Bridges and Switches: are used to connect two networks at the level of the data link layer. This might be useful, when the networks differ at data link layer but not at network layer (e. g. CSMA/CD and token bus). They can be programmed to make necessary changes to the transmitted data. Thus bridges always store a complete frame of data before they transmit it.
3. Routers: are needed when two networks have the same transport layer but different network layers. Just like their name suggests, they are responsible for routing, i. e. the process which determines how data can flow from source to destination (in the most efficient way). If two networks use different addresses, the routers have to adjust them.
4. Brouters: are a combination of a bridge and a router combined for performance reasons. Whenever possible a brouter bridges data (more efficient), if not, the data is routed.
5. Gateways: are used to connect two networks that use completely different network architectures, for example one that does not use the OSI model at all. Their work has to be done in the application layer in most cases. They are specific to the two protocols of the two networks they connect.

Sometimes interconnection devices have other tasks than to connect two networks.