The OSI, TCP/IP, and NetWare Protocol Architectures
79
·
A better environment for interoperability is created.
·
Reduced complexity allows easier program changes and faster product evolution.
·
Each layer can define headers and trailers around the user data. Anyone examining these
headers or trailers for troubleshooting can find the header or trailer for Layer
X
and know
what type of information should be found.
·
One layer uses the services of the layer immediately below it. Therefore, remembering
what each layer does is easier. (For example, the network layer needs to deliver data from
end to end. To do this, it uses data links to forward data to the next successive device along
that end-to-end path.)
Interaction Between OSI Layers
CCNAs frequently deal with the concepts of layer interaction and encapsulation, particularly
because routers build new data link headers and trailers to encapsulate the packets they route.
The process of how layers interact on the same computer, as well as how the same layer
processes on different computers communicate with each other, is all interrelated. The software
or hardware products implementing the logic of some of the OSI protocol layers provide two
general functions:
·
Each layer provides a service to the layer above it in the protocol specification.
·
Each layer communicates some information with the same layer's software or hardware
on other computers. In some cases, the other computer is connected to the same media; in
other cases, the other computer is on the other end of the network.
In the coming pages, you will learn more about each of these two functions.
Interactions Between Adjacent Layers on the Same Computer
To provide services to the next higher layer, a layer must know about the standard interfaces
defined between layers. These interfaces include definitions of what Layer
N
+1 must provide
to Layer
N
to get services, as well as what information Layer
N
must provide back to Layer
N
+1.
Figure 3-2 presents a graphical representation of two computers and provides an excellent
backdrop for a discussion of interactions between layers on the same computer.
ch03.fm Page 79 Monday, March 20, 2000 4:58 PM