OSI Network Model
The Open Systems Interconnection (OSI) model
began as a reference model, but has since been
It was created by the International Organization
for Standardization (ISO) to provide a logical
framework for how data communication
processes should interact across networks.
There are 7 layers in the OSI model. Each layer is
responsible for a particular aspect of data
For example, one layer may be responsible for
establishing connections between devices, while another
layer may be responsible for error checking during
The layers of the OSI model are divided into two
groups: the upper layer and lower layer .
The upper layers focus on user applications and how
files are represented on the computers prior to transport.
For the most part, network engineers are more
concerned with the lower layers.
It's the lower layers that concentrate on how the
communication across a network actually occurs.
The Application layer provides network
services directly to the user's application
such as a web browser, email software
and Windows Explorer. This layer is said
to be "closest to the user".
o Protocols that operate on this layer
include: TELNET, HTTP, FTP, TFTP,
SMTP, NTP, SNMP, EDI.
This layer 'represents' the data in a
particular format to the Application layer. It
defines encryption, compression,
conversion and other coding functions.
o Specifications defined at this layer include:
GIF, TIFF, JPEG, MPEG, MIME, and
The session layer is responsible for
dialog control and synchronization
layer requests for a logical
connection to be established on an enduser’s request.
o Any necessary log-on or password
validation is also handled by this layer.
o Session layer is also responsible for
terminating the connection.
This layer provides services like dialogue
discipline which can be full duplex or half
o Session layer can also provide checkpointing mechanism such that if a failure
of some sort occurs between checkpoints,
all data can be retransmitted from the last
Purpose of this layer is to provide a reliable
mechanism for the exchange of data between
two processes in different computers.
Ensures that the data units are delivered error
Ensures that there is no loss or duplication of
Provides connectionless or connection oriented
Provides for the connection management.
multiple connection over a single
(packets) through the network.
o Defines the most optimum path the
packet should take from the source
to the destination
o Defines logical addressing so that
any endpoint can be identified.
Handles congestion in the network.
o Facilitates interconnection between
o The network layer also defines how
to fragment a packet into smaller
packets to accommodate different
Data Link Layer
Data link layer attempts to provide reliable
communication over the physical layer interface.
Breaks the outgoing data into frames and
reassemble the received frames.
Create and detect frame boundaries.
acknowledgement and retransmission scheme.
Implement flow control.
o Supports points-to-point as well as
o Supports simplex, half-duplex or fullduplex communication.
Provides physical interface for transmission of
Defines rules by which bits are passed from one
system to another on a physical communication
Covers all - mechanical, electrical, functional
Such characteristics as voltage levels, timing of
voltage changes, physical data rates, maximum
transmission distances, physical connectors,
and other similar attributes are defined by
physical layer specifications.