The OSI Model

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The OSI Model

• A laymans view of the internet

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OSI Structure
Each layer has a specific function. It relies
on the layer below to provide a service.
Application
Presentation
Please do this for me
Session
Transport
Network
Data Link
OK Its Done or Sorry I couldnt
Physical
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A Network
Machine A
Machine B
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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An everyday example

• Consider a situation with which you are very
  familiar sending a package/mail.
• Concentrate on the division of responsibility
• How does each person/group rely on the other to
  do a job.
• Consider real vertical communication
• Consider effective horizontal communication

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Boss gives to assistant
Assistant hands to boss
Assistant hands to mailman
Mailman delivers to assistant
Mailman takes to post office
Mailman picks up at post office
Mail system uses many tools to transport
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Can you see the OSI model?
Each relies on the person/group below to provide
a service
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Effective horizontal communications
Order repair parts
Delivery of parts.
No direct horizontal communications!
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Real vertical communications
Boss gives to assistant
Assistant hands to boss
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Something goes wrong
(1)
(5)
Boss gives to assistant
(3)
Boss tells to ship
Boss says wrong part
Assistant hands to boss
Assistant hands to boss
Boss told wrong part
(4)
(2)
(6)
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Effective horizontal communications
Order repair parts
Delivery of parts.
Return parts
Resend parts.
No direct horizontal communications!
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OSI communications

• This effective horizontal communications is
  between like levels of the model at each end
• Similar horizontal communications takes place
  with intermediary nodes
• Real direct communications is vertical with the
• ONE EXCEPTION (Bottom) Physical Layer

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Order repair parts
Delivery of parts.
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Effective Client-Server Interaction
Machine A
Machine B
Application
Application
Presentation
Presentation
Intermediate Nodes
Session
Session
Transport
Transport
Network
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
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What do these layers do?

• Each layer has a limited responsibility
• Typical vertical interactions are simple
• request service
• receive confirmation or rejection
• The total system solves the problem
• Each layer solves some aspect of the problem

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Physical layer

• Proper formatting of the message

address
Message type
Information

• Timing transmission of the message
• everyone cant send at the same time

• Reception of the message

• Detecting and reporting errors in the message
• NOT fixing them

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Data Link Layer

• Effective Point to Point communications
• Retransmitting messages when necessary
• Timing to avoid sending too much or too little
  (flow control)
• Think about Lucy and the Chocolate Assembly Line
  show

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Network Layer

• How to properly route messages
• Is the message for me
• Do I forward to someone else

• Universal Naming of addresses
• All networks dont name the same
• IP addresses are one universal approach
• must have someway to translate
• like higher level languages and machine language

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Transport Layer

• Having Point-to-Point delivery does not assure
  end-to-end
• Message can go halfway across the world and die
• Transport provides guaranteed delivery across
  networks (if desired)
• Provides tuning the transmission process

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Session Layer

• Provides for process of
• connection
• interaction
• disconnection
• In TCP/IP actually done in transport layer (if
  desired)

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Presentation Layer

• Formatting requirements
• internal representation differences
• ascii vs ebcdic
• byte order .. Little/big endian
• Encryption

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Application Layer(s)

• Applications themselves may be layered into
  multiple layers for the same reasons that many
  computer systems use layers
• Common applications forming the basis for other
  applications include
• mail
• ftp

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Why use a layered design?

• Providing a well-defined interface allows for
  substituting other modules while applications
  stay the same
• In networking, allows for
• different physical layers (hardware)
• different protocols (SNA, Decnet, Appletalk, ..)
• In compilers, reuse program on another machine
  (type of processor)
• Networks do a better job than compilers

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Recall the basic layers of a computer
User
Application
Compiler
Operating System
Machine Language
Micro Code
Processors
Gates and Switches
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MiddlewareMore overhead -gt Easy transition!
Application A
Application B
Application A
Database Library
ODBC
Microsoft Access Database
ODBC
SQL
Microsoft Access Database
Oracle
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Layered Design is EVERYWHERE!
You can NOT avoid it! Learn it. Use it.
Use the delivery problem to get the idea of the
OSI model The basic goal of the course is to
understand the principles in the context of the
OSI model.
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Start at the topthenBottom up
First
Application
Presentation
Start at the top because that is the place where
you have familiarity
Session
Transport
Etc.
Network
Third
Data Link
Second
Physical