network equipment vendors that focus on developing
devices that operate at Layer 2 of the OSI model, called
switches, and devices that operate at Layer 3 of the
model, called routers, and so on. But probably the
biggest advantage of layering is the level of innovation
that this model allows at each layer. As long as the
functionality provided by each layer stays consistent with
the expectations of the layer above, protocols, hardware,
and software can be developed and improved. Take, for
example, the way the Ethernet protocol and
specifications have evolved over the years from transfer
rates of 10 Mbps in the early days of the protocol to 400
Gbps today. This improvement of several orders of
magnitude has been possible because of the layering
philosophy. As long as the data transmission layer
provides a set of primitives to the layer above, it can
freely innovate and improve.
In the layered approach, each layer provides a set of
functionality and features to the layer above it and
consumes the features of the layer below it. The OSI
model defines seven layers for exchanging data between
networked devices (see Figure 16-1). Two or more
devices that would exchange data with each other using
the OSI model need to implement all seven layers.