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There is another critical difference between the top layer in Figure 1.2 and the lower three
layers. The application layer is concerned with the details of the application and not with the
movement of data across the network. The lower three layers know nothing about the application
but handle all the communication details.
We show four protocols in Figure 1.2, each at a different layer. FTP is an application layer
protocol, TCP is a transport layer protocol, IP is a network layer protocol, and the Ethernet
protocols operate at the link layer. The TCP/IP protocol suite is a combination of many
protocols. Although the commonly used name for the entire protocol suite is TCP/IP, TCP and
IP are only two of the protocols. (An alternative name is the Internet Protocol Suite.)
The purpose of the network interface layer and the application layer are obvious-the former
handles the details of the communication media (Ethernet, token ring, etc.) while the latter
handles one specific user application (FTP, Telnet, etc.). But on first glance the difference
between the network layer and the transport layer is somewhat hazy. Why is there a distinction
between the two? To understand the reason, we have to expand our perspective from a single
network to a collection of networks.
One of the reasons for the phenomenal growth in networking during the 1980s was the
realization that an island consisting of a stand-alone computer made little sense. A few standalone systems were collected together into a network. While this was progress, during the 1990s
we have come to realize that this new, bigger island consisting of a single network doesn't make
sense either. People are combining multiple networks together into an internetwork, or an
internet. An internet is a collection of networks that all use the same protocol suite.
The easiest way to build an internet is to connect two or more networks with a router. This is
often a special-purpose hardware box for connecting networks. The nice th...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12