OSI (Open Systems Interconnection) is a reference model for how
applications communicate over a network. This model focuses on
presenting a visual representation of how each communications layer—from
the physical cabling to the application that is attempting to communicate
with other devices on a network—is built on top of the other.
The OSI reference model's objectives are to create a clear framework that
explains the functions of an operational networking or telecommunications
system and to provide direction to technology vendors and developers so
that the digital communications products and software programs they
develop can interoperate.
Most vendors involved in telecommunications try to describe their
products and services in relation to the OSI model. This helps them
differentiate among the various transport protocols, addressing schemes
and communications packaging methods. And, although it's useful for
guiding discussion and evaluation, the OSI model is theoretical in nature
and should be used only as a general guide. That's because few network
products or standard tools keep related functions together in well-defined
layers, as is the case in the OSI model. The Transmission Control
Protocol/Internet Protocol (TCP/IP) suite, for example, is the most widely
used network protocol, but even it doesn't map cleanly to the OSI model
History of the OSI model
Technology researchers started looking into the best ways for computer
systems to communicate with one another in the 1970s. Several competing
models were developed and released to the public over the following few
years. However, it wasn't until 1984 that the International Organization for
Standardization (ISO) proposed OSI as a way to finally create a framework
that technology companies all over the world could use as the foundation
of their networking technologies. OSI was created by taking the best parts
of competing networking reference models.
From ISO's perspective, the easiest way to create a conceptual model was
to organize the models into different abstraction layers required to organize
and send data between computing systems. Looking inside each abstracted
layer to see the details shows one part of this network communication
process. Each layer can be thought of as a separate communication module
or piece of the puzzle. But, to actually accomplish the goal of sending data
from one device to another, each module must work together.
How the OSI model works
Information technology (IT) networking professionals use OSI to model or
conceptualize how data is sent or received over a network. Understanding
this is a foundational part of most IT networking certifications, including the
Cisco Certified Network Associate (CCNA) and CompTIA Network+
certification programs. As mentioned, the model is designed to break down
data transmission standards, processes and protocols over a series of
seven layers, each of which is responsible for performing specific tasks
concerning sending and receiving data.
The main concept of OSI is that the process of communication between two
endpoints in a network can be divided into seven distinct groups of related
functions, or layers. Each communicating user or program is on a device
that can provide those seven layers of function.
In this architecture, each layer serves the layer above it and, in turn, is
served by the layer below it. So, in a given message between users, there
will be a flow of data down through the layers in the source computer,
across the network and then up through the layers in the receiving
computer. Only the application layer at the top of the stack doesn't provide
services to a higher-level layer.
The seven layers of function are provided by a combination of applications,
operating systems (OSes), network card device drivers, networking
hardware and protocols that enable a system to transmit a signal over a
network through various physical mediums, including twisted-pair copper,
fiber optics, Wi-Fi