1. Unit03:
The OSI Model

2. Overview
• Open Systems Interconnection
• Encapsulation
• Physical
• Data-link
• Network
• Transport
• Session
• Presentation
• Application

3. Open Systems Interconnection
• Proposed by the International Organization for
Standardization (IOS) and released in 1984
• Decidedly structured set of seven protocol layers
interconnecting as a stack
• Developed to maintain the ability to connect diverse
types of computers
• Valuable as a teaching and development tool

4. OSI

5. Encapsulation
The sending
station converts
data into
increasingly
smaller
capsules until
they are mere
bits
Puts those bits
back together
into increasingly
larger capsules
at the receiving
station

7. Physical Layer
• Transmission of raw data in the form of a stream of bits
across physical media
• Physical Layer conveys the bit stream data through
electrical impulses, light, or radio signals
• The connections that are supported at the physical layer
are either multipoint or point-to-point

8. Physical Layer

9. Data Link Layer
• It is the only layer with two sublayers: Media Access
Control (MAC) and Logical Link Control (LLC).
• The data-link layer supplies hardware addresses,
identifies errors, and manages flow control.
• Bit stream received from the physical layer is assembled
into larger pieces, called frames
• Able to insert control information into a frame header
• Source and destination of the frame on that network
segment

10. Data Link Layer
• Media Access Control (MAC)
– This sublayer defines how devices are able to gain
access to the media
– maintains the hardware address for the device
• The three types of MACs are:
– Contention
– Polling
– Token passing

11. Data Link Layer
• MAC Address
– also called the hardware address
– MAC address is unique to each interface

12. Data Link Layer
• Logical Link Control
– second data-link sublayer
– Responsible for logical link functions of single or
multiple connections
– LLC provides services to the higher-level network
layer
– The LLC primarily provides flow control and frame
sequencing services

13. Network Layer
• Providing an address for each network segment
• Providing a logical address for each device that is
separate from the MAC address
• Routing and forwarding data
• Selecting a route if there are multiple routes to the same
network segment
• Discovering the routes to other network segments
• Error handling, congestion control, and packet
sequencing

14. Network Layer

15. Network Layer
• RIP
– Distance Vector routing protocols periodically
broadcast the entire routing table to neighboring
routers
• OSPF
– These advanced protocols use a hello process to
announce a new router on the network
– the time it takes for the new routes to be added to the
network is greatly reduced

16. Transport Layer
• Provides for a transparent transfer of data
between the sender and receiver nodes
• Control information provides end-to-end
recovery and flow control
• Ensures that the data is completely
transferred

17. Transport Layer
• Connection Orientation
– Connection-oriented
• TCP
• Reliable Transfer
• use of acknowledgments (ACK)
– Connectionless
• UDP
• Does not use ACK
• Smaller headers and less time needed

18. Session Layer
• Establishes a connection between applications and
manages that connection until it terminates.
• Also terminate a session in an orderly manner
• Remote procedure calls (RPCs) function at the session
layer
• Example : Telnet and Citrix MetaFrame ICA sessions
• Data is transmitted in a session in three ways
– Simplex.
– Half-duplex..
– Full-duplex.

19. Presentation Layer
• Provides the way for data to be presented to the
application
• Encryption/decryption and data compression/expansion
are handled at this layer
• provide formatting data
• Bit order
• Byte order
• Character code
• File syntax.

20. Application Layer
• Supports end-user processes
• Specific to the application being used.
• Application-layer protocols connect across the
internetwork in three ways
– Collaborative computing
– Operating system call interception
– Remote operation