The document outlines the OSI (Open Systems Interconnection) model, which serves as a framework for understanding computer networking through its seven-layer architecture. It discusses the challenges and standards involved in computer communication, the functionality of each layer, and the importance of structured protocols for effective data transfer. Additionally, it compares the OSI model to the TCP/IP model to highlight differences in functionality and implementation.

1. MTN 2013
Computer Network Technology
Chapter 3
OSI Model

2. Outline
1. Introduction to OSI Design
2. Issues in OSI
3. OSI Standard
4. OSI Protocol
5. OSI Layer Function & Data Control
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3. 1. Introduction to OSI Design
• Open Systems Interconnected Model
– was created in the 70’s by the ISO (International
Organization for Standardization)
• Several different people got together and considered all functions
of communications
– The concept of a 7 layer model was provided by the work
of Charles Bachman, Honeywell Information Services.
– Various aspects of OSI design evolved from experiences
with the ARPANET
– Integrated Services Digital Network (ISDN) was a big
factor
– 7 layers = 4 upper and 3 lower
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4. OSI Model
• OSI Model
– Model for understanding and developing computer-to-
computer communication
– Divides network architecture into seven layers
– Modular approach
– Ensures error free delivery
– Defines functionality NOT services or protocols
• Network Architect
– Designs networks
• Network Administrators
– Need to understand
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5. OSI Reference Model in Practice
• The OSI reference model is used in many ways:
– To provide assistance when troubleshooting network
problems
– To provide a common terminology and framework for
networking technology developers
– To facilitate the development of connectivity standards
needed for flexible open architectures
– To enable the development of protocol stacks that allow
network nodes to communicate with one another
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6. Seven Layers of Network Architecture
ALL
PEOPLE
SEEM
TO
NEED
DATA
PROCESSING
FIGURE : OSI Model
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7. 2. Issues in OSI
Sr. No. TCP/IP Reference Model OSI Reference Model
Defined after the advent of
1 Defined before advent of internet
Internet
Service interface and protocols
Service interface and protocols are clearly
2 were not clearly distinguished
distinguished
before
3 TCP/IP supports Internet working Internet working not supported
4 Loosely layered Strict layering
5 Protocol Dependent standard Protocol independent standard
6 More Credible Less Credible
TCP reliably delivers packets, IP
7 All packets are reliably delivered
does not reliably deliver packets
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8. 3. OSI Standard
• Was created because people realized that our
computers needed to talk to each other(and
there was no one dominant computer system)
• Consist of different technologies – hardware and
software
• i.e each computer vendor, had a different
hardware design, OS and a proprietary protocol
to enable communication between two
computers of the same make.
• Need one standard protocol to define the
functionality
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9. 4. OSI Protocol
• It is a way of sub-dividing a communications
system into smaller parts called layers.
• Similar communication functions are grouped
into logical layers.
• A layer provides services to its upper layer
while receiving services from the layer below.
• On each layer, an instance provides service to
the instances at the layer above and requests
service from the layer below
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10. 5. OSI Layer Function & Data Control
• 7 layers
1. Physical
2. Data link
3. Network
4. Transport
5. Session
6. Presentation
7. Application
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11. OSI Reference Model: Layer 1
Transmits and receives raw data
to communication medium.
Does not care about contents.
voltage levels, speed,
connectors
1 Physical
Examples: RS-232, 10BaseT
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12. OSI Reference Model: Layer 2
Detects and corrects errors.
Organizes data into packets before
passing it down. Sequences packets (if
necessary).
Accepts acknowledgements from
receiver.
2 Data Link
1 Physical
Examples: Ethernet MAC, PPP
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13. OSI Reference Model: Layer 3
Relay and route information to
destination.
Manage journey of packets and
figure out intermediate hops (if
needed).
3 Network
2 Data Link
1 Physical
Examples: IP, X.25
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14. OSI Reference Model: Layer 4
Provides a consistent interface
for end-to-end (application-to-
application) communication.
Manages flow control.
Network interface is similar to a
4 Transport mailbox.
3 Network
2 Data Link
1 Physical
Examples: TCP, UDP
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15. OSI Reference Model: Layer 5
Services to coordinate dialogue
and manage data exchange.
Software implemented switch.
5 Session
Manage multiple logical
4 Transport
connections.
Keep track of who is talking:
3 Network establish & end
communications.
2 Data Link
1 Physical Examples: HTTP 1.1, SSL,
NetBIOS
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16. OSI Reference Model: Layer 6
Data representation
6 Presentation
Concerned with the
5 Session
meaning of data bits
4 Transport
Convert between machine
representations
3 Network
2 Data Link
1 Physical Examples: XDR, ASN.1,
MIME, MIDI
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17. OSI Reference Model: Layer 7
7 Application
Collection of application-specific
protocols
6 Presentation Provides interface to the
software enabling programs to
5 Session use network devices
4 Transport
3 Network
Examples:
2 Data Link email (SMTP, POP, IMAP)
file transfer (FTP)
1 Physical directory services (LDAP),
browser,
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18. END
Q&A
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