D-Link DGS-3308FG User Manual page 46

8-port Gigabit Ethernet Switch User's Guide
Using the protocol layering model to visualize the organization of the network software, Layer 2 represents switching and
Layer 3 represents routing. In fact, the protocol layering model gives only guidelines for writing programs to accomplish
certain tasks and functions. How the layers communicate within a protocol stack (for example, within a network device or a
computer) is determined by the operating system programmers. So long as the communication between devices on the
network follows the well-defined and well-known methods and data formats, the protocol stack can accomplish its tasks in
any way suitable.
Figure 5-9. The Protocol Stack
Elements on the same layer of a protocol stack are known as peers. They communicate with other peers, in other protocol
stacks (on other network devices) using the well-defined and well-known methods and formats. Messages and data are
transferred via published (and therefore well-known) protocols.
Elements within the same stack communicate using an internal interface. This interface is part of the operating system
and is usually not published (and therefore not well-known). In addition, internal protocol stack interfaces are generally
proprietary. This means that communication within the protocol stack has the same characteristics as a protocol in that
two protocol stacks from the same operating system vendor will communicate (within the stack) in the same way. The
difference from a protocol is that stacks from different operating system vendors (or two different operating system
products from the same vendor) may communicate within the stack in completely different ways.
The result is that communication between layers in within a protocol stack (and within a given network device) are often
proprietary and different from communication within a second protocol stack.
Communication between peers (between two protocol stacks, but at the same layer) is accomplished by well-known and
published protocols. So, peers communicate in an open and consistent way, and peers from completely different systems
from different vendors can communicate easily. This principle has allowed the rapid growth of layered networking.
A brief description of the most commonly used layers of the OSI model is helpful to understand the scope of how protocol
layering works.
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