Figure 5-12. The Protocol Stack
Between two protocol stacks, members of the same layer are known as peers and communicate by well- known (open and published) protocols. Within a protocol stack, adjacent layers communicate by an internal interface. This interface is usually not publicly documented and is frequently proprietary. It has some of the same characteristics of a protocol and two stacks from the same software vendor may communicate in the same way. Two stacks from different software vendors (or different products from the same vendor) may communicate in completely different ways. As long as peers can communicate and interoperate, this has no impact on the functioning of the network.
The communication between layers within a given protocol stack can be both different from a second stack and proprietary, but communication between peers on the same OSI layer is open and consistent.
A brief description of the most commonly used functional layers is helpful to understand the scope of how protocol layering works.
Layer 1
This is referred to as the physical layer. It handles the electrical connections and signaling required to make a physical link from one point in the network to another. It is on this layer that the unique Media Access Control (MAC) address is defined.
Layer 2
This layer, commonly called the switching layer, allows end station addressing and the establishment of connections between them.
Layer 2 switching forwards packets based on the unique MAC address of each end station and offers
Layer 2 does not ordinarily extend beyond the intranet. To connect to the Internet usually requires a router and a modem or other device to connect to an Internet Service Provider’s WAN. These are Layer 3 functions.
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