What is OSI Model?
The Open Systems Interconnection model (OSI model) is a conceptual model that
'provides a common basis for the coordination of [ISO] standards development for the
purpose of systems interconnection'.[2] In the OSI reference model, the
communications between a computing system are split into seven different
abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation,
and Application.

What is OSI Model?
The model partitions the flow of data in a communication system into
seven abstraction layers to describe networked communication from the physical
implementation of transmitting bits across a communications medium to the highest-
level representation of data of a distributed application. Each intermediate layer
serves a class of functionality to the layer above it and is served by the layer below it.
Classes of functionality are realized in all software development through all and any
standardized communication protocols.

•Layer 1 defines devices’ electrical and physical aspects, for example, cables,
connectors, electrical signals, pinouts, etc. It provides the physical interface between a
device and the transmission media.
•Layer 2 – The data link layer provides the transmission protocol that controls the
data flow between network devices, for example, a network switch. Data packets are
encoded and decoded into bits, flow control, and error checks in layer 1. This layer
has two sub-layers, Media Access Control (MAC) and the Logical Link Control (LLC)
layer.
•communication are functions of the application layer.

•Layer 3 provides routing and switching technologies that route variable length data
packets from the source to the destination network. It creates logical routes (Virtual
Circuits) which transmit the data from one network to another. Layer 3 includes
packets, IP, ICMP, IPSec, and IGMP.
•Layer 4 ensures complete data transfer is error-free, in sequence, and with no losses
or duplications, for example, TCP and UDP. The transport layer controls a link’s
reliability through flow control, segmentation and desegmentation, and error control.
Providing the transfer of data between end-users.

•Layer 5 of the OSI model handles the management, establishment, and termination
of connections between two end-users of a network. Communication sessions consist
of requests and responses that occur between applications.
•Layer 6 translates data for the application layer for the network. Encryption and
authentication, for example, SSL and TLS, and data compression are processed in the
presentation layer.
•Layer 7 is the layer that handles the end-user direct interaction with the application,
for example, SSH, HTTP, and DNS. Resource evaluation, synchronization, and
communication are functions of the application layer.

OSI Model Explained
Each layer in the OSI model has its own well-defined functions, and the functions of
each layer communicate and interact with the layers immediately above and below it,
unless the layer does not have layers below or above.
The Internet protocol suite has a separate model, the layers of which are mentioned
in RFC 1122 and RFC 1123. That model combines the physical and data link layers of
the OSI model into a single link layer, and has a single application layer for all
protocols above the transport layer, as opposed to the separate application,
presentation and session layers of the OSI model.

Advantage of OSI Model
•It helps you to standardize router, switch, motherboard, and other hardware
•Reduces complexity and standardizes interfaces
•Facilitates modular engineering
•Helps you to ensure interoperable technology
•Helps you to accelerate the evolution
•Protocols can be replaced by new protocols when technology changes.
•Provide support for connection-oriented services as well as connectionless
service.
•It is a standard model in computer networking.
•Supports connectionless and connection-oriented services.
•Offers flexibility to adapt to various types of protocols

OSI Layers
•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.
•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.

OSI Model TCP/IP model
OSI model provides a clear distinction between
interfaces, services, and protocols.
TCP/IP doesn’t offer any clear distinguishing
points between services, interfaces, and
protocols.
OSI uses the network layer to define routing
standards and protocols.
TCP/IP uses only the Internet layer.
OSI model use two separate layers physical
and data link to define the functionality of the
bottom layers
TCP/IP uses only one layer (link).
OSI model, the transport layer is only
connection-oriented.
A layer of the TCP/IP model is both connection-
oriented and connectionless.
In OSI model, data link layer and physical are
separate layers.
In TCP data link layer and physical layer are
combined as a single host-to-network layer.
The minimum size of the OSI header is 5 bytes. Minimum header size is 20 bytes.

Layer Name Protocols
Layer 7 Application
SMTP, HTTP, FTP, POP3,
SNMP
Layer 6 Presentation MPEG, ASCH, SSL, TLS
Layer 5 Session NetBIOS, SAP
Layer 4 Transport TCP, UDP
Layer 3 Network
IPV5, IPV6, ICMP, IPSEC, ARP,
MPLS.
Layer 2 Data Link
RAPA, PPP, Frame Relay, ATM,
Fiber Cable, etc.
Layer 1 Physical RS232, 100BaseTX, ISDN, 11.
Protocols supported at various levels

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