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History of the OSI Model
The OSI model wasn’t the first or only attempt to streamline networking protocols or estab-
lish a common communications standard. In fact, the most widely used protocol today, 
TCP/IP (which is based on the 
DARPA model
, also known now as the 
TCP/IP model
), was 
developed in the early 1970s. The OSI model was not developed until the late 1970s.
The OSI protocol was developed to establish a common communication structure or 
standard for all computer systems. The actual OSI protocol was never widely adopted, but 
the theory behind the OSI protocol, the OSI model, was readily accepted. The OSI model 
serves as an abstract framework, or theoretical model, for how protocols should function 
in an ideal world on ideal hardware. Thus, the OSI model has become a common reference 
point against which all protocols can be compared and contrasted.
OSI Functionality
The OSI model divides networking tasks into seven distinct layers. Each layer is respon-
sible for performing specific tasks or operations for the ultimate goal of supporting data 
exchange (in other words, network communication) between two computers. The layers are 
always numbered from bottom to top (see Figure 11.1). They are referred to by either their 
name or their layer number. For example, layer 3 is also known as the Network layer. The 
layers are ordered specifically to indicate how information flows through the various levels 
of communication. Each layer communicates directly with the layer above it as well as the 
layer below it, plus the peer layer on a communication partner system.
F I g u r e 11.1
Representation of the OSI model
Application
7
Presentation
6
Session
5
Transport
4
Network
3
Data Link
2
Physical
1
The OSI model is an open network architecture guide for network product vendors. This 
standard, or guide, provides a common foundation for the development of new protocols, 
networking services, and even hardware devices. By working from the OSI model, vendors 
are able to ensure that their products will integrate with products from other companies 
and be supported by a wide range of operating systems. If all vendors developed their own 
networking framework, interoperability between products from different vendors would be 
next to impossible.

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Chapter 11 
■
Secure Network Architecture and Securing Network Components
The real benefit of the OSI model is its expression of how networking actually functions. 
In the most tangible sense, network communications occur over a physical connection 
(whether that physical connection is electrons over copper, photons over fiber, or radio sig-
nals through the air). Physical devices establish channels through which electronic signals 
can pass from one computer to another. These physical device channels are only one type 
of the seven logical communication types defined by the OSI model. Each layer of the OSI 
model communicates via a logical channel with its peer layer on another computer. This 
enables protocols based on the OSI model to support a type of authentication by being 
able to identify the remote communication entity as well as authenticate the source of the 
received data.