Internetworking

An isolated LAN is almost as useless as an isolated computer. Different LANs must be interconnected to each other and must be connected to MANs and WANs. Different LANs may use different protocols, such as CSMA/CD, Token Ring, FDDI, etc. How these protocols interact and co-oporate is a very challenging issue. Type network components that acomplish the internetworking functions are:

• Repeater: it basically copies whatever being sent in one LAN to another LAN. requires very little processing, mostly a physically layer issue.
• Bridge: a bridge interconnects two or more LAN (similar or dissimilar) at the MAC level (can be treated as part of the Data link Layer). The following diagram shows how a bridge functions between two LANs:

  

  

  The main function of a bridge is

  • Forwarding: filtering through the received frame to see if it needs to be forwarded to another LAN or ignore it. Other functions include learning and updating source, destination (physical) addresses and save them in its address database. This is necessary to keep up with the ever changing configurations of the LANs.
  • Conversions of frame formats. Since frames from an Ethernet or Token Ring are not the same, they have to be converted when the frame crosses the bridge.
  • Decide on the proper frame size: different MAC protocols have different default frame size (1500bytes for Ethernet, much more for FDDI, etc).
  • Implement priority: forward high priority frames first (but not for Ethernet frames since it does not have priority built-in.).
• Router: its main function is to do network routing, which we will discuss later. From the diagrams we can see that in order to use a bridge or router, the two LANs must have the same set of transport (and upper) layer protocols. From the transport layer's point of view, the two LANs are operating in the same environment. In order to cross a router, the network layer of one LAN needs to specify the network address of the packet, which will be used by the router. Some of the tough routing problems, which we will discuss later, have to be delt with in routers.

  

  

• Gateway: takes care of upper layer issues. One of the main function of gateways is to fragment and reassemble packets as they go through the gateway. This is because the packet from one LAN may be too long for transmission over another LAN. Gateway is also responsible to put the fragmented packets back to one piece upon arrival to the destination.

Routers and gateways play a major role in the internetworking between LANs, MANs and WANs. Companies who research and develop commercial router and gateway product represents the fastest sector of high tech industry.

The networking industry is an extremely fast changing one. Networking research is also different from many other research topics in that many of the issues are not clearly defined before it's put into production and use. Because of the sheer magnitude of the issues, what we can cover are only some of the representative issues. In fact, you can write a thick book on any one of the topics. Some of the books on standards deal with only one small part of a particular layer, still takes thousands of pages.