2. OSI Reference Model
• OSI was developed by the International Organization
for Standardization (ISO) and introduced in 1984.
• It is a layered architecture (consists of seven layers).
• Each layer defines a set of functions in data
3. OSI Model Layers
Layer - 7
Layer - 6
Layer - 5
Layer - 4
Layer - 3
Layer - 2
Layer - 1
Core layer of
4. Application Layer
Application Layer is responsible for providing an interface
for the users to interact with application services or
Networking Services .
Ex: Web browser etc.
Identification of Services is done using Port Numbers.
Port is a logical communication Channel
Port number is a 16 bit identifier.
Total No. Ports
0 – 65535
1 - 1023
1024 – 65535
5. Examples of Networking Services
6. Presentation Layer
defines a specific format for the date to be sent over
The major functions described at this layer are..
Ex: ASCII, EBCDIC (Text)
Encryption – Decryption
Ex: DES, 3-DES, AES
Compression – Decompression
7. Session Layer
• It deals with sessions or Interactions between the
• It is responsible for establishing, maintaining and
terminating the sessions.
• Session layer organizes communication through
simplex, half-duplex or full-duplex
8. Transport Layer
It is responsible for end-to-end transportation of data
between the applications.
The major functions described at the Transport Layer are..
• Identifying Service
• Multiplexing & De-multiplexing
• Sequencing & Reassembling
• Error Correction
• Flow Control
9. Identifying a Service
Services are identified at this layer with
the help of Port No’s.
The major protocols which takes care of Data Transportation
at Transport layer are…TCP,UDP
• Transmission Control
• User Datagram
• Connection Oriented
• Connection Less
• Unreliable communication
communication( with Ack’s )
( no Ack’s )
• Slower data Transportation
• Faster data Transportation
• Protocol No is 6
• Protocol No is 17
Eg: HTTP, FTP, SMTP
Eg: DNS, DHCP, TFTP
10. Network Layer
• It is responsible for end-to end Transportation of data across
• Logical addressing & Path determination (Routing) are described
at this layer.
The protocols works at Network layer are
IP,IPX, AppleTalk.. Etc
Routed protocols acts as data carriers and defines logical
RIP, OSPF.. Etc
Routing protocols performs Path determination (Routing).
Routers, Multilayer Switches operate at this layer
11. Datalink Layer
It is responsible for end-to-end delivery of data between the devices on
a Network segment.
Data link layer comprises of two sub-layers.
• MAC (Media Access Control)
It deals with hardware addresses (MAC addresses).
MAC addresses are 12 digit Hexa-decimal identifiers used to
identify the devices uniquely on the network segment.
It also provides ERROR DETECTION using CRC (Cyclic Redundancy
Check) and FRAMING (Encapsulation).
Ex: Ethernet, Token ring…etc
• Logical Link Control (LLC)
LLC used to support multiple network protocols with
single NIC card.
• Switches, Bridges Work in this layer
12. Physical Layer
• It transfers the data in bits format i.e. 01010101010
(zeros and ones).
• Conversion of bits over the media depends on type of the
• Copper media : Electrical signals of different voltages
• Fiber media
: Light pulses of different wavelengths
• Wireless media: Radio frequency waves
• Hubs,repeaters, modems operate at this layer
13. Data format at different Layers
14. Comparing OSI with TCP/IP Layers