OSI REFERENCE MODEL
Objectives
The objective of this blog
is to familiarize with : -
i)
Concept of data encapsulation
ii)
Characteristics of the OSI Layers
iii)
OSI Model and Communication Between Systems
iv)
OSI layers
1.1 INTRODUCTION
The International Organization introduced the OSI layer for
Standardization (ISO) in 1984 in order to provide a reference model to make
sure products of different vendors would interoperate in networks. OSI is short
for Open System Interconnection.
The OSI layer shows WHAT needs to be done to send data from
an application on one computer, trough a network, to an application on another
computer, not HOW it should be done. A
layer in the OSI model communicates with three other layers: the layer above
it, the layer below it, and the same layer at its communication partner. Data
transmitted between software programs passes all 7 OSI layers. The Application,
Presentation and Session layers are also known as the Upper Layers.
The Data Link
and Physical layers are often implemented together to define LAN and WAN
specifications.
- Data Encapsulation
- Application Layer
- Presentation Layer
- Session Layer
- Transport Layer
- Network Layer
- Data Link Layer
- Physical Layer
1.2 Data
Encapsulation
Data Encapsulation is the process of adding a header to
wrap the data that flows down the OSI model.
Each OSI layer may add it's own header to the data received from above.
(from the layer above or from the software program 'above' the Application
layer.)
There are five steps of Data Encapsulation : -
- The Application, Presentation and Session layers create DATA from
users' input.
- The Transport layer converts the DATA to SEGMENTS
- The Network layer converts the SEGMENTS to PACKETS (or datagrams)
- The Data Link layer converts the PACKETS to FRAMES
- The Physical layer converts the FRAMES to BITS.
At the sending computer the information goes from top to
bottom while each layers divides the information received from upper layers in
to smaller pieces and adds a header. At the receiving computer the information
flows up the model discarding the corresponding header at each layer and
putting the pieces back together.
The Figure shows layered
model of two directly interconnected end systems. The transmission media is not
included in the seven layers and, therefore, it can be regarded as layer number
zero. Functions and services of various layers are described
1.3 Characteristics
of the OSI Layers
The
seven layers of the OSI reference model can be divided into two categories:
upper layers and lower layers.
The
upper layers
of the OSI model deal with application issues and generally are implemented
only in software. The highest layer, the application layer, is closest to the
end user. Both users and application layer processes interact with software
applications that contain a communications component. The term upper layer is
sometimes used to refer to any layer above another layer in the OSI model.
The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the network cabling, for example) and is responsible for actually placing information on the medium.
PROTOCOLS
The
OSI model provides a conceptual framework for communication between computers,
but the model itself is not a method of communication. Actual communication is
made possible by using communication protocols. In the context of data
networking, a protocol is a formal set of rules and conventions that
governs how computers exchange information over a network medium. A protocol
implements the functions of one or more of the OSI layers. A wide variety of
communication protocols exist. Some of these include:
LAN protocols operate at the physical and data
link layers of the OSI model and define communication over the various LAN
media.
WAN protocols operate at the lowest three
layers of the OSI model and define communication over the various wide-area
media.
Routing protocols are network layer protocols that
are responsible for exchanging information between routers so that the routers
can select the proper path for network traffic.
Network protocols are the various upper-layer
protocols that exist in a given protocol suite. Many protocols rely on others
for operation.
For
example, many routing protocols use network protocols to exchange information
between routers. This concept of building upon the layers already in existence
is the foundation of the OSI model.
1.4 OSI
Model & Communication Between Systems
Information
being transferred from a software application in one computer system to a
software application in another must pass through the OSI layers. For example,
if a software application in System A has information to transmit to a software
application in System B. The application program in System A will pass its
information to the application layer (Layer 7) of System A.
The
application layer then passes the information to the presentation layer (Layer
6), which relays the data to the session layer (Layer 5), and so on down to the
physical layer (Layer 1). At the physical layer, the information is placed on
the physical network medium and is sent across the medium to System B. The
physical layer of System B removes the information from the physical medium,
and then its physical layer passes the information up to the data link layer
(Layer 2), which passes it to the network layer (Layer 3), and so on, until it
reaches the application layer (Layer 7) of System B. Finally, the application
layer of System B passes the information to the recipient application program
to complete the communication process.
1.5 Interaction
between OSI model layers
A
given layer in the OSI model generally communicates with three other OSI
layers: the layer directly above it, the layer directly below it, and its peer
layer in other networked computer systems. The data link layer in System A, for
example, communicates with the network layer of System A, the physical layer of
System A, and the data link layer in System B
OSI LAYER SERVICES
One
OSI layer communicates with another layer to make use of the services provided
by the second layer. The services provided by adjacent layers help a given OSI
layer communicate with its peer layer in other computer systems. Three basic
elements are involved in layer services: the service user, the service
provider, and the service access point (SAP).
In
this context, the service user is the OSI layer that requests services
from an adjacent OSI layer. The service provider is the OSI layer that
provides services to service users. OSI layers can provide services to multiple
service users. The SAP is a conceptual location at which one OSI layer can
request the services of another OSI layer.
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