The document explains the 7-layer OSI model for network communication. The OSI model decomposes communication into 7 layers, with each layer responsible for specific functions. The layers are: Physical (transmits raw bits), Data Link (frames bits into packets), Network (routes packets between nodes), Transport (delivers data reliably), Session (manages connections between applications), Presentation (translates between application and network formats), and Application (supports end-user network applications and services). The document describes the functions of each layer in detail.

1. Explain OSI 7 Layer
Architecture
BY S. ANANTHA KUMAR
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2. OSI Layers
Arrangement
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Fig: OSI Layers in Arranged in a Hierarchy
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3. Introduction of OSI 7 Layer
Open System Interconnection Model (OSI) was developed by ISO (International
Standard Organization) in 1983, for sending and receiving of data, between two
computers. It deals with connecting open system i.e., system that follow a
standards are open for communicating with other systems, irrespective of a
manufactures.
OSI represents a concept of intel process communication so that any open
system may be able to communicate with another open system.
The OSI architecture decomposes the communication process into functional
layers. Each layer is responsible for performing special functions. Therefore, OSI
architecture is reference model to all open system inter connections.
OSI reference model consists of 7 layers, as shown in fig.
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4. Functions of each layer
Source Destination
Application Layer
Presentation Layer
Transport Layer
Session Layer
Data Link Layer
Network Layer
Physical Layer
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer101010011
Transmission Medium
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5. Functions of each layer
 Application Layer:
The application Layer enables the user, either humun or software access the network.
It provides end user for processing of data and supports for services such as e-mail,
file transfer, shared data base management, network software services and other
types of distributed information service.
This layer acts as an interface end user and network. This layer mainly allows access to
network resources.
 Presentation Layer:
Presentation layer is concerned with syntax and semantics of the information
exchanged between two systems.
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6. Functions of each layer
It translates the application into network format and vice versa. It provides format and
encrypt/ decrypt data to be send across a network.
 Session Layer:
It allows to establish, maintains and disconnect between communicating systems.
It allows the communication between two devices either in simplex or half duplex
mode of transmission.
It allows a process to add checkpoints into a stream of data.
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7. Functions of each layer
 Transport Layer:
The transport layer is responsible for source to destination delivery of the entire
message.
This layer concerts data into smaller “segments” for sending and at the receiving end
the segments are converted into original data. This layer is also responsible for error
control and flow control.
 Network Layer:
This layer converts data segments into packets and at the receiving end, the packets
are converted into data segments.
This determines paths for transmitting data from source to destination.
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8. Functions of each layer
 Data Link Layer:
This layer converts data packets from network Layer into frames at the receiving end,
the packets are converted into data segments.
The data link layer divides the stream of bits received from the network layer into
manageable data units called frames. If frames are to be distributed to different
system on the network. The data link layer adds a header to the frame to define the
physical address of the sender and receiver of the frame.
Data link layer receives the data to be sent from the network layer, adds header and
trailer to it which is now known as a frame. This frame is then transmitted to the
physical layer.
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9. Functions of each layer
Data
(Packets) Trailer Data Header
T Data
DestinationSource
Fig: Role of Data Link Layer
Data link layer
Frame
Network layer
To physical layer
Source address : AA Destination address : ZZTrailer Header
Trailer conains extra bits needed for
error detection Source address
Destination address
Fig: Frame format of a Datalink layer
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10. Functions of each layer
The data link layer can be devided into two sub-layers. They are :
1. LLC (Logicsal Link Control)
2. MAC (Medium Access Control)
Data link
layer
Logical link control (LLC)
Medium access control
(MAC)
It is used for error control & flow control
In LAN, it decides who
can send data, when
and how muc
Fig: Sub-layers in Datalink layer
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11. Functions of each
 Physical Layer:
The physical layer is responsible for transmitting raw bits over a communication channel.
It converts frames from data link layer into bits and at the receiving end, bits from the
physical layer is given to the data link layer.
For sending raw bits from source to destination, to do this, the source and destination
nodes have to agree on a number of factors such as what voltage constitute a bit value ‘0’
what what voltage constitute a bit value’1’, what is the bit interval, whether the
communication is in only one direction or both the directions simultaneously i.e., simplex,
half duplex or full duplex and so on. It also deals with the electrical specifications of the
cables, connectors and interfaces such as RS 232.
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12. Functions of each layer
Data in frames Data in frames
101010
(bits)
101010
(bits)
Communication channel
Sender Receiver
Data link
layer
Physical
Layer
Fig: Relation of Physical Layer with Communication channel
Data link
layer
Physical
Layer
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13. Functions of Physical Layer
 Functions of Physical Layer:
1. Signal encoding
2. Medium
3. Bit Synchronization
4. Transmission Byte
5. Transmission Mode
6. Multiplexing
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14. Short Notes
7. Application Supports application and end – user processes.
Provides application service for the file transfers, e-mail
and other network software services
6. Presentation Translates from application to network format and vice
versa. Format and encrypts/decrypts data to be send
across a network.
5. Session Establishes, manages and terminates connections
between applications. Data with session and connection
coordination.
4. Transport Responsible for end-to-end error recovery and flow
control. Ensures complete data transfer.
3. Network Creates logical paths for transmitting data from node
to node. Includes switching, routing and forwarding.
2. Data Link Packets are encoded and decoded into bits. Teo sub
layers. Media Access Control (MCA) Logival Link Control
(LLC)
1. Physical Conveys the bit stream – electrical light or radio signal
through the network at the electrical and mechanical
level.
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15. OSI Model – 7 Layer
Fig: OSI Model – 7 Layer
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16. THANK YOU
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By S. Anantha Kumar