Page 84 - Handout Computer Network.

Chapter (3) Networking Media and Data Link Layers Protocols

Using the OSI model as an example, the goal is to build the network’s functionality in
independent modules. The desire is to allow a diversity of LAN technologies at Layers 1 and 2.
You want to allow a diversity of applications functioning at Layers 5, 6, and 7.

However, you want a system that hides the details of the lower and upper layers,
allowing intermediate networking devices to relay traffic without having to bother with the
details of the LAN (best administered locally, and the network envisioned will be global) or the
applications generating network traffic. This leads to the concept of internetworking—building
networks of networks. A network of networks is called an internet (with a lowercase i). (An
uppercase I is used to refer to the networks that grew out of the DoD on which the WWW runs,
and to refer to the Internet.)
Internetworking must have the following characteristics:

• It must be scalable in the number of networks and computers attached.

• It must be able to handle the transport of data across vast distances, including
entire-earth and near-earth space.

• It must be flexible to account for constant technological innovations.

• It must adjust to dynamic conditions on the network.

• It must be cost-effective.

• It must be a system that permits anytime, anywhere data communications to
anyone.

Figure 4-1 illustrated the connection of one physical network to another through a
specialpurpose computer called a router. This diagram is not unlike the problem that led to the
beginning of Cisco Systems at Stanford University in 1984 and the invention of the router. These
networks are described as “directly connected” to the router.

The router here is useful for handling any “translations” required for the two networks
to communicate. However, because users seek anytime and anywhere connections to anyone,
this scheme for connecting just two networks quickly becomes inadequate. Figure 4-2 shows
two routers connecting three physical networks. Now the routers must make more-complex
decisions. Because all users on all networks want to communicate with each other, even without
being directly connected to one another, the router must have some way of dealing with this.

66