7.2: OSI and TCP/IP Models

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Open Systems Interconnect Model

The Open System Interconnection (OSI) reference model was proposed by the International Organization for Standardization (ISO), a nonprofit organization that develops and publishes standards including information technology. It is headquartered in Geneva, Switzerland and comprised of 162-member countries. The OSI model for networking explains how networks function in an orderly and structured, seven-layered approach. The OSI model is a theoretical concept that is used for teaching and learning in the field of computer networking; and, serves as a general conceptual framework on how networked systems should operate to be able to work together.

Screen Shot 2022-10-14 at 9.35.43 AM.png — Figure \(\PageIndex{1}\): OSI Model. ("Information Security" by Umar Khokhar Binh Tran is licensed under CC BY 4.0)

Let us look at this example of why the reference model is needed and how the layered approach is so valuable using this non-networking example: Mary who lives in Atlanta, Georgia would like to send some information to Bob who lives in Los Angeles, California via the United States Postal Service or snail mail like many IT folks like to call it. Here is the breakdown of what has to happen:

Mary, who lives in Atlanta has some information she would like to send to Bob, who lives in Los Angeles. She writes a letter and puts the information with the letter to get to Bob and decides to use the postal service.
Mary folds the letter with the information and puts it in an envelope which is the standard “protocol” for sending letters in the United States ready to be sent but now has to address the envelope.
Mary addresses the envelope with Bob’s name, street address, city and state information in the middle and put her return address on the top left which is the standard “protocol” for sending letters.
Mary must then purchase and put a stamp on the top right of the envelope per the stamp location standard “protocol”.
Mary then goes to the post office to drop it off or she can enable the flag at her home mailbox both of which are standard “protocols”.
The mail carrier obtains the envelope and brings it to the main sorting center in Atlanta and then it is sorted for the destination zip code which is in Los Angeles. The envelope is then placed on a plane to travel to Los Angeles main sorting center.
Upon arrival the letter then needs to be sorted according to Bob’s zip code and location for local delivery by the mail carrier.
The mail carrier then uses the street address to be sure to deliver it to Bob’s mailbox.
Bob then receives the envelope, open it up, discard the envelope and retrieves the letter and information that Mary wanted to send to him.

As you can see there are a number of structured, orderly tasks that has to happen at each “layer” and one must be completed before the next layer take over. This layered approach may seem complex and in reality it is but it allows for each task to be handled separately thus allowing for more simplified troubleshooting. This “divide and conquer” strategy allows for solving big problems by breaking them down into smaller components and a change to one of the steps would not affect the entire process very much if at all.

Adapted from:
"Information Security" by Umar Khokhar Binh Tran, OpenALG is licensed under CC BY 4.0