Page 85 - M. Abrahim Thesis

Computer Network 2021

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.

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