OSI MODEL

OSI MODEL
• OSI stands for Open Systems Interconnection. It has been developed by ISO –
‘International Organization of Standardization‘, in the year 1984.

• It 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.

• OSI consists of seven layers, and each layer performs a particular network function.

• Each layer is self-contained, so that task assigned to each layer can be performed
independently.
LAYERS OF OSI MODEL
 LAYER 1- PHYSICAL LAYER
• Physical layer includes the physical equipment involved in the data transfer, such as the
cables, hubs and switches.

• It transmits the individual bits (string of 1s and 0s) from one node to another node.

• It establishes, maintains and deactivates the physical connection.

• It specifies the mechanical, electrical and procedural network interface specifications.

 FUNCTIONS OF PHYSICAL LAYER

• Line Configuration: It defines the way how two or more devices can be
connected physically.

• Data Transmission: It defines the transmission mode whether it is simplex, half-

duplex or full-duplex mode between the two devices on the network.

• Topology: It defines the way how network devices are arranged.

• Signals: It determines the type of the signal used for transmitting the
information.
 LAYER 2- DATA LINK LAYER
• It takes packets from the network layer and breaks them into smaller pieces called
frames. Responsible for the error-free transfer of data frames.

• It defines the format of the data on the network.

• It provides a reliable and efficient communication between two or more devices.

• Hop-to-hop or Node-to-node delivery (not source to destination).

• It is mainly responsible for the unique identification of each device that resides on a local
network.

• Data link layer is also responsible for flow control and error control in intra-network
communication
 Functions of Data Link Layer
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 packets. The header
which is added to the frame contains the hardware destination and source
address.

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.
 Functions of Data Link Layer
Flow Control
• 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.
• Stop and Wait, Go Back N, Selective Repeat

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 occur, then the receiver sends the acknowledgment for the
retransmission of the corrupted frames.

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.
DATA LINK LAYER…
• It contains two sub-layers:
• Logical Link Control Layer
• It is responsible for transferring the packets to the Network layer of the receiver
that is receiving.
• It identifies the address of the network layer protocol from the header.
• It also provides flow control.

• Media Access Control Layer

• A Media access control layer is a link between the Logical Link Control layer and
the network's physical layer.
• It is used for transferring the packets over the network.
• MAC sublayer manages the device’s interaction, responsible for addressing
frames, and also controls physical media access.
 LAYER 3- NETWORK LAYER
• Responsible for facilitating data transfer between two different networks, Host-to-host
communication, Source-to-destination delivery using IP addressing.

• The network layer breaks up segments from the transport layer into smaller units, called
packets, on the sender’s device, and reassembling these packets on the receiving
device.

• The network layer also finds the best physical path for the data to reach its destination
based on the network conditions, the priority of service, and other factors; this is known
as Routing.

• 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
• Internetworking: An internetworking is the main responsibility of the network layer.
It provides a logical connection between different devices.

• 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.

• 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.

• Packetizing: A Network Layer receives the segments from the upper layer and
converts them into packets. This process is known as Packetizing. It is achieved by
internet protocol (IP).
 LAYER 4- TRANSPORT LAYER
• 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.

• It ensures that messages are transmitted in the order in which they are sent and
there is no duplication of data.

• This includes taking data from the session layer and breaking it up into chunks
called segments before sending it to layer 3 i.e. network layer.
 Functions of Transport Layer
• 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.

• 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.

• 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.
Flow control: The transport layer also responsible for flow control but it
is performed end-to-end rather than across a single link.
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.
Protocols used in TRANSPORT layer
Transmission Control Protocol
• It is a standard protocol that allows the systems to communicate over the internet.
• It establishes and maintains a connection between hosts.
• 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.

User Datagram Protocol

• User Datagram Protocol is a transport layer protocol.
• 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.
Difference between TCP and UDP:
Basis for TCP UDP
comparison
Full form Transmission Control Protocol User Datagram Protocol or Universal Datagram
Protocol
Connection Connection-oriented protocol. Connection less protocol.
Function TCP reads data as streams of bytes, and the UDP messages contain packets that were sent
message is transmitted to segment boundaries. one by one. It also checks for integrity at the
arrival time.

Usage TCP is suited for applications that require high UDP is suitable for applications that need fast,
reliability, and transmission time is relatively less efficient transmission, such as games. UDP's
critical. stateless nature is also useful for servers that
answer small queries from huge numbers of
clients.

Header size 20 bytes 8 bytes.

 LAYER 5- SESSION LAYER
• The Session layer is used to establish, maintain and synchronizes the interaction between
communicating devices.

Functions of Session layer:

• 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 which
can be either half-duplex or full-duplex.

• 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.
 LAYER 6- PRESENTATION LAYER
• Mainly concerned with the syntax and semantics of the information exchanged between
the two systems.

• It acts as a data translator for a network.

• Purpose: Preparing data so that it can be used by the application layer; in other words,
layer 6 makes the data presentable for applications to consume.

• This layer is a part of the operating system that converts the data from one presentation
format to another format.

• The Presentation layer is also known as the syntax layer.

 Functions of Presentation Layer

• 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.

• 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.

• 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.
 LAYER 7- APPLICATION LAYER
• This layer provides the network services to the end-users.

• It handles issues such as network transparency, resource allocation, etc.

• Software applications like web browsers and email clients rely on the application
layer to initiate communications.

• Application layer protocols include HTTP as well as SMTP (Simple Mail Transfer
Protocol is one of the protocols that enables email communications).
 Functions of Application Layer

• File transfer, access, and management (FTAM): An application layer allows a user
to access the files in a remote computer, to retrieve the files from a computer
and to manage the files in a remote computer.

• Mail services: An application layer provides the facility for email forwarding and
storage.

• Directory services: An application provides the distributed database sources and

is used to provide that global information about various objects.
Advantages of OSI Model
• It divides network communication into 7 layers which makes it easier to
understand and troubleshoot.

• It standardizes network communications, as each layer has fixed functions and

protocols.

• Diagnosing network problems is easier with the OSI model.

• It is easier to improve with advancements as each layer can get updates

separately.
Disadvantages of OSI Model
• Complexity: The OSI Model has seven layers, which can be complicated and hard to
understand for beginners.

• Not Practical: In real-life networking, most systems use a simpler model called the
Internet protocol suite (TCP/IP), so the OSI Model isn’t always directly applicable.

• Slow Adoption: When it was introduced, the OSI Model was not quickly adopted by the
industry, which preferred the simpler and already-established TCP/IP model.

• Overhead: Each layer in the OSI Model adds its own set of rules and operations, which
can make the process more time-consuming and less efficient.

• Theoretical: The OSI Model is more of a theoretical framework, meaning it’s great for
understanding concepts but not always practical for implementation.