The OSI reference model has 7 layers - physical, data link, network, transport, session, presentation and application layer. Each layer has a specific function like the physical layer deals with transmission of raw bits over a communication channel, the data link layer handles error checking and frame delimiting, the network layer handles logical addressing and routing, the transport layer handles flow control and error checking, the session layer establishes and manages communication sessions, the presentation layer handles syntax and semantics of the information and the application layer supports applications and end-user processes. Data flows from the application layer downwards to the physical layer for transmission.

1. H. Vignesh Ramamoorthy
M.Sc (Softw Engg), MCA, Assistant
Professor
UG – Computer Science
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2. OSI Reference Model
• OSI: Open System Interconnection .
• Developed by the International
Organization for Standardization in
1974.
• There are seven layers in OSI
reference Model .
• process or role
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3. OSI Model Layers
Layer - 7 Application
Application
Upper Layer
Layer - 6 Presentation
Presentation or
Software Layer
Layer - 5 Session
Session
Layer - 4 Transport
Transport Heart of OSI
Layer - 3 Network
Network
Lower Layer
Layer - 2 Data Link
Data Link or
Hardware Layer
Layer - 1 Physical
Physical
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4. Application Layer
Application Layer is
Application
Application
Application
Application responsible for providing
Networking Services to the
Presentation
Presentation user. It is also known as
Desktop Layer. Identification of
Session
Session Services is done using Port
Numbers.
Transport
Transport
Ports are Entry and Exit Points
Network
Network to the Layer
Data Link
Data Link Total No. Ports 0 – 65535
Physical Reserved Ports 0 – 1023
Physical
Open Client Ports 1024 – 65535
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5. Examples of Networking Services
Service Port No.
HTTP 80
FTP 21
SMTP 25
TELNET 23
TFTP 69
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6. Data flow from Application Layer
Application
Application Data
80 21 25 53 67 69
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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7. Presentation Layer
Presentation Layer is
Application
Application responsible for converting data
into standard format.
Presentation
Presentation
Presentation
Presentation
Examples : ASCII, EBCDIC,
Session
Session JPEG, MPEG, BMP, MIDI, WAV,
MP3
Transport
Transport
Network Following tasks are perform at
Network
Presentation layer :
Data Link
Data Link
Encoding – Decoding
Physical
Physical Encryption – Decryption
Compression – Decompression
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8. Data flow from Presentation Layer
Application
Application Data
Presentation
Presentation Data
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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9. Session Layer
Application
Application
Session Layer is responsible for
Presentation
Presentation establishing, maintaining and
terminating session.
Session
Session
Session Session ID works at Session
Layer.
Transport
Transport
Network
Network Examples :
Data Link
Data Link
RPC  Remote Procedure Call
Physical SQL  Structured Query Language
Physical
NFS  Network File System
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10. Data flow from Session Layer
Application
Application Data
Presentation
Presentation Data
Session
Session Data
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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11. Transport Layer
Transport Layer is
Application responsible for end-to-end
Application
connectivity. It is also known as
Presentation the heart of OSI Layers.
Presentation
Following tasks are performed at
Session the Transport Layer : -
Session
Transport
Transport
Transport
Transport • Identifying Service
• Multiplexing & De-multiplexing
Network
Network
• Segmentation
Data Link
Data Link • Sequencing & Reassembling
• Error Correction
Physical
Physical
• Flow Control
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12. Identifying Service
TCP UDP
• Transmission Control • User Datagram
Protocol Protocol
• Connection Oriented • Connection Less
• Acknowledgement • No Acknowledgement
• Reliable • Unreliable
• Slower • Faster
• Port No. 6 • Port No. 17
• e.g. HTTP, FTP, SMTP • e.g. DNS, DHCP, TFTP
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13. Multiplexing & De-multiplexing
Application
Application
Presentation
Presentation
Session
Session
80 21 25 53 67 69
Transport
Transport
TCP - 6 UDP - 17
Network
Network
Data Link
Data Link
Physical
Physical
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14. Data flow from Transport Layer
Application
Application Data
Presentation
Presentation Data
Session
Session Data
Transport TH
Segment
Data
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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15. Network Layer
Network Layer is
Application
Application responsible for providing best
path for data to reach the
Presentation
Presentation destination. Logical Addressing
works on this layer. Router is a
Session
Session Network Layer device.
Transport
Transport It is divided into two parts
Network
Network • Routed Protocols
Network
Network
e.g. IP, IPX, Apple Talk.
Data Link
Data Link • Routing Protocols
Physical e.g. RIP, IGRP, OSPF, EIGRP
Physical
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16. Data flow from Network Layer
Application
Application Data
Presentation
Presentation Data
Session
Session Data
e.g. Router Transport
Transport Segment
Network
Network Packet
NH Segment
Data Link
Data Link
Physical
Physical
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17. Datalink Layer
Datalink Layer is
Application
Application divided into two Sub Layers :
Presentation
Presentation
• LLC – Logical Link Control
Session It talks about Wan protocols e.g.
Session
PPP, HDLC, Frame-relay
Transport
Transport
• MAC – Media Access Control
Network
Network It talks about Physical Address.
It is a 48 bit address i.e. 12 digit
Data Link
Data Link
Data Link
Data Link Hexadecimal Number.
It is also responsible for Error
Physical
Physical Detection
Devices working on Data Link
Layer are Switch, Bridge, NIC.
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18. Data flow from Data Link Layer
Application
Application Data
Presentation
Presentation Data
Session
Session Data
Transport
Transport Segment
e.g. Switch Network
Network Packet
Data Link
Data Link DH Frame DT
Packet
Physical
Physical
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19. Physical Layer
Physical Layer is
Application
Application responsible for electrical,
mechanical and procedural checks.
Presentation
Presentation Data will be converted into Binary
(i.e.) 0’s & 1’s. Data will be in the
Session
Session form of electrical pulses if it is
Coaxial or Twisted Pair cable and in
Transport
Transport the form of Light if it is Fiber Optic
Cable.
Network
Network
Devices working at Physical Layer
Data Link
Data Link are Hubs, Repeaters, Cables,
Modems etc.
Physical
Physical
Physical
Physical
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20. Data flow from Physical Layer
Application
Application Data
Presentation
Presentation Data
Session
Session Data
Transport
Transport Segment
Network
Network Packet
e.g. Hub Data Link
Data Link Frame
Physical
Physical Bits
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21. Data Encapsulation & De-capsulation
A B
Application Data Data Application
Application Application
Presentation
Presentation Data Data Presentation
Presentation
Session
Session Data Data Session
Session
Transport TH TransportTH
Transport Data
Segment TH
Segment
Data Transport
Network
Network NH Segment
Packet NH Segment
Packet Network
Network NH
Data Link
Data Link DHFrameDT
Packet DTFrameDH
Packet
Packet DH
Data LinkDT
Data Link
Physical
Physical Bits Bits Physical
Physical
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22. Comparing OSI with TCP/IP Layers
OSI Layers TCP/IP Layers
Application
Application
Presentation
Presentation Application
Application
Session
Session
Transport
Transport Transport
Transport
Network
Network Internet
Internet
Data Link
Data Link Network
Network
Access
Access
Physical
Physical
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23. Thank You
hvigneshram@yahoo.com
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