Introduction to OSI Design Issues in OSI OSI Standard OSI Protocol OSI Layer Function & Data Control

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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