Foundations of Networking

Many advanced features are being supported by the physical hardware through the application of Moore's law. Those of us responsible for networking these many devices follow a theoretical framework that allows the required functionality to be deployed within our networks. This framework is more commonly known as the OSI reference model.

OSI stands for open system interconnection, where open system refers to the specifications surrounding the model's structure as well as its nonproprietary public availability. Anyone can build the software and hardware needed to communicate within the OSI structure. If you know someone that has written a script to access information in a router, at some level, he is following the OSI reference model.

Why Was the OSI Reference Model Needed?

Before the development of the OSI reference model, the rapid growth of applications and hardware resulted in a multitude of vendor-specific models. In other words, one person's solution would not work with anyone else's because there was no agreed-upon method, style, process, or way for different devices to interoperate. In terms of future network growth and design, this rapid growth caused a great deal of concern among network engineers and designers because they had to ensure that the systems under their control could interact with every standard. This concern encouraged the International Organization of Standardization (ISO) to initiate the development of the OSI reference model.

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Foundations of Networking 7

The work on the OSI reference model was initiated in the late 1970s and came to maturity in the late 1980s and early 1990s. The ISO was the primary architect of the model that is in place today.

Figure 1-2 demonstrates how the layers are spanned by a routing protocol. You might also want to contact Network Associates, as its protocol chart shows how almost every protocol spans the seven layers of the OSI reference model. Figure 1-2 provides a good illustration of how the seven layers are grouped in the model. For a better picture of how protocols are positioned in the OSI reference model, visit to the following websites and request a copy of the applicable posters:

Acterna (aka W&G) offers free OSI, ATM, ISDN, and fiberoptics posters at

www.acterna.com/shared/forms/poster_form.html.

Network Associates offers its Guide to Communications Protocols at

www.sniffer.com/dm/protocolposter.asp.

Characteristics of the OSI Layers

8 Chapter 1: Networking and Routing Fundamentals

Figure 1-2 How a Routing Protocol Spans the OSIModel

Application

Figure 1-2 How a Routing Protocol Spans the OSIModel

Application

Understanding the Seven Layers of the OSI Reference Model 9

Table 1-1 outlines an effective mnemonic tool to help you remember the seven OSI layers and their order, working either from Layer 7 down or from Layer 1 up.

Table 1-1 Mnemonics Used to Remember OSI Layers

OSI Layer (Upper to Lower)

Mnemonic

OSI Layer (Lower to Upper)

Mnemonic

Application (Layer 7)

All

Physical (Layer 1)

Please

Presentation (Layer 6)

people

Data Link (Layer 2)

do

Session (Layer 5)

seem

Network (Layer 3)

not

Transport (Layer 4)

to

Transport (Layer 4)

take

Network (Layer 3)

need

Session (Layer 5)

sales

Data Link (Layer 2)

data

Presentation (Layer 6)

peoples

Physical (Layer 1)

processing

Application (Layer 7)

advice

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