OSI Model
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OSI stands
for Open System Interconnection is a reference model that
describes how information from a software application in one computer moves
through a physical medium to the software application in another computer.
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OSI consists of
seven layers, and each layer performs a particular network function.
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OSI model was developed
by the International Organization for Standardization (ISO) in 1984, and it is
now considered as an architectural model for the inter-computer communications.
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OSI model divides
the whole task into seven smaller and manageable tasks. Each layer is assigned
a particular task.
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Each layer is
self-contained, so that task assigned to each layer can be performed
independently.
Characteristics of OSI Model:
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The OSI model is
divided into two layers: upper layers and lower layers.
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The upper layer of
the OSI model mainly deals with the application related issues, and they are
implemented only in the software. The application layer is closest to the end
user. Both the end user and the application layer interact with the software
applications. An upper layer refers to the layer just above another layer.
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The lower layer of
the OSI model deals with the data transport issues. The data link layer and the
physical layer are implemented in hardware and software. The physical layer is
the lowest layer of the OSI model and is closest to the physical medium. The
physical layer is mainly responsible for placing the information on the
physical medium.
Functions of the OSI Layers
Application Layer:
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An application
layer serves as a window for users and application processes to access network
service. Like chrome, fire fox etc.
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It handles issues
such as network transparency, resource allocation, etc.
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An application
layer is not an application, but it performs the application layer functions.
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This layer
provides the network services to the end-users.
Presentation Layer:
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A Presentation
layer is mainly concerned with the syntax and semantics of the information
exchanged between the two systems.
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It acts as a data
translator for a network.
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This layer is a
part of the operating system that converts the data from one presentation
format to another format.
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The Presentation
layer is also known as the syntax layer.
Functions of Presentation layer:
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Translation: The processes in two systems exchange
the information in the form of character strings, numbers and so on. Different
computers use different encoding methods, the presentation layer handles the
interoperability between the different encoding methods. It converts the data
from sender-dependent format into a common format and changes the common format
into receiver-dependent format at the receiving end.
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Encryption: Encryption is needed to maintain
privacy. Encryption is a process of converting the sender-transmitted
information into another form and sends the resulting message over the network.
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Compression: Data compression is a process of
compressing the data, i.e., it reduces the number of bits to be transmitted.
Data compression is very important in multimedia such as text, audio, video.
Session Layer:
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It is a layer 3 in
the OSI model.
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The Session layer
is used to establish, maintain and synchronizes the interaction between communicating
devices.
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Session layer
operates in 3 phases.
1.
Session management:
Creates a separate session for request.
2.
Authentication: Verification
of user name and password.
3.
Authorization:
Verification of access control.
Functions of Session layer:
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Dialog control: Session layer acts as a dialog
controller that creates a dialog between two processes or we can say that it
allows the communication between two processes.
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Synchronization: Session layer adds some checkpoints
when transmitting the data in a sequence. If some error occurs in the middle of
the transmission of data, then the transmission will take place again from the
checkpoint. This process is known as Synchronization and recovery.
Transport Layer:
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The Transport
layer is a Layer 4 ensures that messages are transmitted in the order in which
they are sent and there is no duplication of data.
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The main
responsibility of the transport layer is to transfer the data completely.
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It receives the
data from the upper layer and converts them into smaller units known as
segments.
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This layer can be
termed as an end-to-end layer as it provides a point-to-point connection
between source and destination to deliver the data reliably.
The two protocols
used in this layer are:
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Transmission
Control Protocol
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It is a standard
protocol that allows the systems to communicate over the internet.
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It establishes and
maintains a connection between hosts.
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When data is sent
over the TCP connection, then the TCP protocol divides the data into smaller
units known as segments. Each segment travels over the internet using multiple
routes, and they arrive in different orders at the destination. The
transmission control protocol reorders the packets in the correct order at the
receiving end.
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User Datagram
Protocol
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User Datagram
Protocol is a transport layer protocol.
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It is an
unreliable transport protocol as in this case receiver does not send any
acknowledgment when the packet is received, the sender does not wait for any
acknowledgment. Therefore, this makes a protocol unreliable.
Functions of Transport Layer:
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Service-point
addressing: Computers
run several programs simultaneously due to this reason, the transmission of
data from source to the destination not only from one computer to another
computer but also from one process to another process. The transport layer adds
the header that contains the address known as a service-point address or port
address. The responsibility of the network layer is to transmit the data from
one computer to another computer and the responsibility of the transport layer
is to transmit the message to the correct process.
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Segmentation and
reassembly: When the
transport layer receives the message from the upper layer, it divides the
message into multiple segments, and each segment is assigned with a sequence
number that uniquely identifies each segment. When the message has arrived at
the destination, then the transport layer reassembles the message based on
their sequence numbers.
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Connection
control: Transport
layer provides two services Connection-oriented service and connectionless
service. A connectionless service treats each segment as an individual packet,
and they all travel in different routes to reach the destination. A
connection-oriented service makes a connection with the transport layer at the
destination machine before delivering the packets. In connection-oriented
service, all the packets travel in the single route.
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Flow control: The transport layer also responsible
for flow control but it is performed end-to-end rather than across a single
link.
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Error control: The transport layer is also responsible
for Error control. Error control is performed end-to-end rather than across the
single link. The sender transport layer ensures that message reach at the
destination without any error.
Network Layer:
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It is a layer 3
that manages device addressing, tracks the location of devices on the network.
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It determines the
best path to move data from source to the destination based on the network
conditions, the priority of service, and other factors.
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The Data link
layer is responsible for routing and forwarding the packets.
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Routers are the
layer 3 devices, they are specified in this layer and used to provide the
routing services within an internetwork.
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The protocols used
to route the network traffic are known as Network layer protocols. Examples of
protocols are IP and Ipv6.
Functions of Network Layer:
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Addressing: A Network layer adds the source and
destination address to the header of the frame. Addressing is used to identify
the device on the internet.
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Routing: Routing is the major component of the
network layer, and it determines the best optimal path out of the multiple
paths from source to the destination.
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Path Determination:
The physical determination of path from source to destination.
Data-Link Layer:
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This layer is
responsible for the error-free transfer of data frames.
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It defines the
format of the data on the network.
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It provides a
reliable and efficient communication between two or more devices.
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It is mainly
responsible for the unique identification of each device that resides on a
local network.
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It contains two
sub-layers:
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Logical Link Control
Layer
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It is responsible
for transferring the packets to the Network layer of the receiver that is
receiving.
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It identifies the
address of the network layer protocol from the header.
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It also provides
flow control.
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Media Access
Control Layer
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A Media access
control layer is a link between the Logical Link Control layer and the
network's physical layer.
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It is used for
transferring the packets over the network.
Functions of the Data-link layer
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Framing: The data link layer translates the
physical's raw bit stream into packets known as Frames. The Data link layer
adds the header and trailer to the frame. The header which is added to the
frame contains the hardware destination and source address.
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Physical
Addressing: The Data
link layer adds a header to the frame that contains a destination address. The
frame is transmitted to the destination address mentioned in the header.
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Flow Control: Flow control is the main functionality
of the Data-link layer. It is the technique through which the constant data rate
is maintained on both the sides so that no data get corrupted. It ensures that
the transmitting station such as a server with higher processing speed does not
exceed the receiving station, with lower processing speed.
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Error Control: Error control is achieved by adding a
calculated value CRC (Cyclic Redundancy Check) that is placed to the Data link
layer's trailer which is added to the message frame before it is sent to the
physical layer. If any error seems to occurr, then the receiver sends the
acknowledgment for the retransmission of the corrupted frames.
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Access Control: When two or more devices are connected
to the same communication channel, then the data link layer protocols are used
to determine which device has control over the link at a given time.
Physical layer
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The main
functionality of the physical layer is to transmit the individual bits from one
node to another node.
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It is the lowest
layer of the OSI model.
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It establishes,
maintains and deactivates the physical connection.
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It specifies the
mechanical, electrical and procedural network interface specifications.
Functions of a Physical layer:
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Line
Configuration: It defines
the way how two or more devices can be connected physically.
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Data Transmission: It defines the transmission mode
whether it is simplex, half-duplex or full-duplex mode between the two devices
on the network.
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Topology: It defines the way how network devices
are arranged.
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Signals: It determines the type of the signal
used for transmitting the information.
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