Download The OSI Model

The OSI Model
A layman’s view of the internet
OSI Structure
Application
Presentation
Each layer has a specific
function.
It relies on the layer below
to provide a service.
Please do this for me
Session
Transport
Network
Data Link
Physical
OK It’s Done
or
Sorry I couldn’t
A Network
Machine A
Machine B
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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
Boss gives
to assistant
Assistant hands
to boss
Assistant hands
to mailman
Mailman delivers
to assistant
Mailman takes
to post office
Mail system uses many tools to transport
Mailman picks
up at post office
Can you
see the
OSI model?
Each relies on the
person/group below
to provide a service
Effective horizontal
communications
Order repair parts
Delivery of parts.
No direct horizontal communications!
Real vertical communications
Boss gives
to assistant
Assistant hands
to boss
Something goes wrong
(1)
(5)
Boss gives
to assistant
Boss says
wrong part
Assistant hands
to boss
(4)
(3)
Boss tells
to ship
Assistant hands
to boss
(2)
Boss told
wrong part
(6)
Effective horizontal
communications
Order repair parts
Delivery of parts.
Return parts
Resend parts.
No direct horizontal communications!
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
Order repair parts
Delivery of parts.
Effective Client-Server Interaction
Machine A
Machine B
Application
Application
Presentation
Presentation
Session
Intermediate
Nodes
Transport
Session
Transport
Network
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
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
Physical layer
•Proper formatting of the message
address
Message type
Information
•Timing transmission of the message
•everyone can’t send at the same time
•Reception of the message
•Detecting and reporting errors in the message
•NOT fixing them
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
Network Layer
• How
to properly route
messages
•Is the message for me
•Do I forward to someone else
• Universal Naming of addresses
•All networks don’t name the same
•IP addresses are one “universal” approach
•must have someway to translate
•like higher level languages and machine language
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
Session Layer
• Provides for process of
– connection
– interaction
– disconnection
• In TCP/IP actually done in transport layer
(if desired)
Presentation Layer
• Formatting requirements
– internal representation differences
• ascii vs ebcdic
• byte order .. Little/big endian
• Encryption
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
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
Recall the basic layers of a
computer
User
Application
Compiler
Operating System
Machine Language
Micro Code
Processors
Gates and Switches
Middleware
More overhead -> Easy transition!
Application A
ODBC
Microsoft Access
Database
Application A
Application B
Database Library
ODBC
Microsoft Access
Database
SQL
Oracle
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.
Start at the top
then
Bottom up
First
Application
Presentation
Session
Transport
Etc.
Network
Third
Data Link
Second
Physical
Start at the top
because that is
the place
where you have
familiarity