2. THE OSI MODEL
Although, when speaking of the Internet, everyone talks
about the TCP/IP protocol suite, this suite is not the only
suite of protocols defined. Established in 1947, the
International Organization for Standardization (ISO) is
a multinational body dedicated to worldwide agreement on
international standards. Almost three-fourths of the
countries in the world are represented in the ISO. An ISO
standard that covers all aspects of network
communications is the Open Systems Interconnection
(OSI) model. It was first introduced in the late 1970s.
3. An open system is a set of protocols that allows any two
different systems to communicate regardless of their
underlying architecture. The purpose of the OSI model is to
show how to facilitate communication between different
systems without requiring changes to the logic of the
underlying hardware and software. The OSI model is not a
protocol; it is a model for understanding and designing a
network architecture that is flexible, robust, and
interoperable. The OSI model was intended to be the basis
for the creation of the protocols in the OSI stack. The OSI
model is a layered framework for the design of network
systems that allows communication between all types of
computer systems. It consists of seven separate but related
layers, each of which defines a part of the process of
moving information across a network
29. OSI versus TCP/IP
When we compare the two models, we find that two
layers, session and presentation, are missing from
the TCP/IP protocol suite. These two layers were not
added to the TCP/IP protocol suite after the
publication of the OSI model. The application layer in
the suite is usually considered to be the combination
of three layers in the OSI model.
31. Two reasons were mentioned for this decision.
First, TCP/IP has more than one transport-layer
protocol. Some of the functionalities of the session
layer are available in some of the transport-layer
protocols. Second, the application layer is not only
one piece of software. Many applications can be
developed at this layer. If some of the
functionalities mentioned in the session and
presentation layers are needed for a particular
application, they can be included in the
development of that piece of software.