OSI Structure
Application
Presentation
Session
Transport
Network
Data Link
Physical
Each layer has a specific function.
It relies on the layer below to provide a service.
Please do this for me
OK It’s Done orSorry I couldn’t
A Network
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
Machine A Machine B
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 handsto mailman
Mailman takesto post office
Mail system uses many tools to transport
Mailman picksup at post office
Mailman deliversto assistant
Assistant handsto boss
Effective horizontal communications
Order repair parts
Delivery of parts.
No direct horizontal communications!
Something goes wrong
Boss gives to assistant
Assistant handsto boss
Boss tellsto ship
Assistant handsto boss
Boss sayswrong part
Boss toldwrong part
(1)
(2)
(3)
(4)
(5)
(6)
Effective horizontal communications
Order repair parts
Delivery of parts.
No direct horizontal communications!
Return parts
Resend parts.
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.Order part 12345
Order part 1234
There is no such part
OK Order adjusted
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network
Data Link
Physical
Machine A Machine B
IntermediateNodes
Effective Client-Server Interaction
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
N
D
P
Machine A Machine B
IntermediateNodes
Intermediate Nodes are Routers
N
D
P
N
D
P
…..
Another View of Routers
workstation
workstation
LAN/MAN
routerLAN
LAN
router
LAN/MAN
routerIntermediate Nodes
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
Application
Compiler
Operating System
Machine Language
Micro Code
Processors
Gates and Switches
User
MiddlewareMore overhead -> Easy transition!
Application A Application B
Database Library
ODBC
Microsoft AccessDatabase
SQL
Oracle
Application A
ODBC
Microsoft AccessDatabase
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.