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8-port Gigabit Ethernet Switch User’s Guide
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Protocol Layering
The task of connecting users to networks, and then networks to networks, is made somewhat easier by dividing up the
overall job into simpler, but related, tasks. Each task is structured to be resilient to failures in the connecting hardware,
software, data loss, data corruption, and data received out of order. Taken together, these tasks are referred to as a
protocol suite.
Each task, or protocol, must communicate with other protocols. To manage this communication, the concept of layering was
introduced as a way of structuring the overall network. The idea of protocol layering is to start with the most basic layer,
the physical (or hardware) layer, and to define data formats and functions for that layer. The physical layer passes data to
next higher layer, the data link layer, and so on until one user is connected to another.
Protocol layering then provides clearly defined breaks in the process of communicating over a network. Each break in the
process has a clearly defined data format so that the layer below can perform its task in any way that is suitable, so long
as the data it generates is in the format expected by the next layer. The advantage of this approach is that the exact
method and tools (or software and hardware) used to accomplish the task at each layer is not critically important.
Hardware and software designers are free to improve the performance or to reduce the cost of accomplishing the task of
each protocol layer, so long as the data format between layers conforms to the defined formats (and of course, the layer’s
task is accomplished).
The protocol layer concept currently used by the Internet, the OSI seven-layer model, was developed from earlier, simpler
layered models. Much of the current layer model owes its origin to the Xerox Network Systems (XNS) model.
The OSI (Open Systems Interconnection) model actually refers to a system of protocols proposed by ISO (the International
Standards Organization) that are themselves not widely used or supported. The layered model itself is, however, taken as
the framework for nearly all modern networking.
A diagram of the OSI model is shown below (note that this is not a complete listing of the protocols contained within each
layer of the model):
Figure 5-8. OSI Seven Layer Network Model
Each layer has a distinct set of tasks to accomplish and clearly defined formats in which to receive and forward data and
messages. A distinct set of programs, executing a distinct set of protocols, is required to accomplish the task set by each
layer.
Although the layers are separated from other layers in the model, they must all communicate and interoperate. For this to
work, there must be very well-defined and well-known methods for transferring messages and data. Within a device
connected to a network, this inter-layer communication is managed by the device’s protocol stack.