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OSI Model: The 7 Layers of
Network Architecture
The Open Systems Interconnection (OSI) Reference Model is a
conceptual framework that describes functions of the
networking or telecommunication system independently from the
underlying technology infrastructure. It divides data
communication into seven abstraction layers and standardizes
protocols into appropriate groups of networking functionality
to ensure interoperability within the communication system
regardless of the technology type, vendor, and model.
The OSI model was originally developed to facilitate

interoperability between vendors and to define clear standards
for network communication. However, the older TCP/IP model
remains the ubiquitous reference framework for Internet
communications today.
The 7 layers of the OSI model
This image illustrates the seven layers of the OSI model.
Below, we’ll briefly describe each layer, from bottom to top.
1. Physical
The lowest layer of the OSI model is concerned with data
communication in the form of electrical, optic, or
electromagnetic signals physically transmitting information
between networking devices and infrastructure. The physical
layer is responsible for the communication of unstructured raw
data streams over a physical medium. It defines a range of
aspects, including:
Electrical, mechanical, and physical systems and

networking devices that include specifications such as
cable size, signal frequency, voltages, etc.
Topologies such as Bus, Star, Ring, and Mesh
Communication modes such as Simplex, Half Duplex, and
Full Duplex
Data transmission performance, such as Bit Rate and Bit
Synchronization
Modulation, switching, and interfacing with the physical
transmission medium
Common protocols including Wi-Fi, Ethernet, and others
Hardware including networking devices, antennas, cables,
modem, and intermediate devices such as repeaters and
hubs
2. Data Link
The second layer of the OSI model concerns data transmission
between the nodes within a network and manages the connections
between physically connected devices such as switches. The raw
data received from the physical layer is synchronized and
packaged into data frames that contain the necessary protocols
to route information between appropriate nodes. The data link
layer is further divided into two sublayers:
The Logical Link Control (LLC) sublayer is responsible

for flow controls and error controls that ensure error-
free and accurate data transmission between the network
nodes.
The Media Access Control (MAC) sublayer is responsible
for managing access and permissions to transmit data
between the network nodes. The data is transmitted
sequentially and the layer expects acknowledgement for
the encapsulated raw data sent between the nodes.
3. Network
The third layer of the OSI model organizes and transmits data
between multiple networks.
The network layer is responsible for routing the data via the
best physical path based on a range of factors including
network characteristics, best available path, traffic
controls, congestion of data packets, and priority of service,
among others. The network layer implements logical addressing
for data packets to distinguish between the source and
destination networks.
Other functions include encapsulation and fragmentation,

congestion controls, and error handling. The outgoing data is
divided into packets and incoming data is reassembled into
information that is consumable at a higher application level.
Network layer hardware includes routes, bridge routers, 3-
layer switches, and protocols such as Internet (IPv4) Protocol
version 4 and Internet Protocol version 6 (IPv6).
4. Transport
The fourth layer of the OSI model ensures complete and
reliable delivery of data packets.
The transport layer provides mechanisms such as error

control, flow control, and congestion control to keep
track of the data packets, check for errors and
duplication, and resend the information that fails
delivery. It involves the service-point addressing
function to ensure that the packet is sent in response
to a specific process (via a port address).
Packet Segmentation and reassembly ensure that the data
is divided and sequentially sent to the destination
where it is rechecked for integrity and accuracy based
on the receiving sequence.
Common protocols include the Transmission Control Protocol

(TCP) for connection-oriented data transmission and User
Datagram Protocol (UDP) for connectionless data transmission.
5. Session
As the first of three layers that deal with the software
level, the session layer manages sessions between servers to
coordinate communication. Session refers to any interactive
data exchange between two entities within a network. Common
examples include HTTPS sessions that allow Internet users to
visit and browse websites for a specific time period. The
Session Layer is responsible for a range of functions
including opening, closing, and re-establishing session
activities, authentication and authorization of communication
between specific apps and servers, identifying full-duplex or
half-duplex operations, and synchronizing data streams.
Common Session Layer protocols include:
Remote procedure call protocol (RPC)

Point-to-Point Tunneling Protocol (PPTP)
Session Control Protocol (SCP)
Session Description Protocol (SDP), as described here
6. Presentation
The sixth layer of the OSI model converts data formats between
applications and the networks. Responsibilities of the
presentation layer include:
Data conversion
Character code translation
Data compression
Encryption and decryption
The presentation layer, also called the syntax layer, maps the
semantics and syntax of the data such that the received
information is consumable for every distinct network entity.
For example, the data we transfer from our encryption-based
communication app is formatted and encrypted at this layer
before it is sent across the network.
At the receiving end, the data is decrypted and formatted into

text or media information as originally intended. The
presentation layer also serializes complex information into
transportable formats. The data streams are then deserialized
and reassembled into original object format at the
destination.
7. Application
The application layer concerns the networking processes at the
application level. This layer interacts directly with end-
users to provide support for email, network data sharing, file
transfers, and directory services, among other distributed
information services. The upper most layer of the OSI model
identifies networking entities to facilitate networking
requests by end-user requests, determines resource
availability, synchronizes communication, and manages
application-specific networking requirements. The application
layer also identifies constraints at the application level
such as those associated with authentication, privacy, quality
of service, networking devices, and data syntax.
Common application layer protocols include:
File Transfer Protocol (FTP)

Simple Mail Transfer Protocol (SMTP)
Domain Name System (DNS)
The internet doesn’t welcome OSI
The OSI model is widely criticized for an inherent
implementation complexity that renders networking operations
inefficient and slow. The academic approach to developing the
OSI protocol suite relied on replacing existing protocols
across all communication layers with better alternatives.
This approach failed to gain traction in the industry; vendors

had already invested significant resources in TCP/IP products
and had to manage interoperability with the vast choices of
protocols and specifications offered by the OSI model.
Additionally, academia itself considered the OSI model as an
invention politically inspired by the European
telecommunication and U.S. government authorities.
The older TCP/IP architecture model had already itself in

real-world network environments. It served as a solid
foundation for the Internet–including all of the security,
privacy, and performance-related challenges. Continued
research and development, investments, and industry-wide
adoption of the OSI model could have made today’s cyber world
a different (and perhaps better) place, but the pragmatism of
the TCP/IP model gave us the internet that prevails today.
Additional resources
Learn more with these resources:
BMC IT Operations Blog

IT Infrastructure Management: An Introduction
IT Infrastructure Planning: Getting Started
Network Engineer vs Network Administrator: Roles,
Responsibilities, and Job Descriptions
What Is a Virtual Network?
Osi model 7 Layers from Siddique Ibrahim
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OSI Model: The 7 Layers of

The OSI model was originally developed to facilitate

Electrical, mechanical, and physical systems and

The Logical Link Control (LLC) sublayer is responsible

Other functions include encapsulation and fragmentation,

The transport layer provides mechanisms such as error

Common protocols include the Transmission Control Protocol

Common Session Layer protocols include:

Remote procedure call protocol (RPC)

At the receiving end, the data is decrypted and formatted into

Common application layer protocols include:

File Transfer Protocol (FTP)

This approach failed to gain traction in the industry; vendors

The older TCP/IP architecture model had already itself in

BMC IT Operations Blog

Original reference image:

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