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 or Sorry 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 hands to mailman Mailman takes to post office Mail system uses many tools to transport Mailman picks up at post office Mailman delivers to assistant Assistant hands to boss

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 Boss gives to assistant Assistant hands to boss Boss tells to ship Assistant hands to boss Boss says wrong part Boss told wrong 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.

Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Network Data Link Physical Machine A Machine B Intermediate Nodes Effective Client-Server Interaction

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 addressMessage typeInformation 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

Middleware More overhead -> Easy transition! Application AApplication B Database Library ODBC Microsoft Access Database SQL Oracle Application A ODBC Microsoft Access Database

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 Application Presentation Session Transport Network Data Link Physical First Second Third Etc. Start at the top because that is the place where you have familiarity