The OSI Model

A layman’s view of the internet

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

Can you see the

OSI model?

Each relies on the person/group belowto provide a service

Effective horizontal communications

Order repair parts

Delivery of parts.

No direct horizontal communications!

Real vertical communications

Boss gives to assistant

Assistant handsto boss

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 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.

Start at the topthen

Bottom upApplication

Presentation

Session

Transport

Network

Data Link

Physical

First

Second

Third

Etc.

Start at the top because that is

the place where you have

familiarity