Why OSI

Rearden

The other day I was having a conversation with my boss about the new switches we were deploying. I asked him if we would be having them use any of the layer three functions that they were capable of. The thing is, him and I both knew we were talking about OSI. But, nobody uses a fuly implemented OSI model, we all use TCP/IP, or some proprietary model. So why is it that everything we talk about is in reference to OSI?

keatron

Because the OSI model is general theoretical frame of reference that those "proprietary" models are supposed to be based. While you say no one uses the entire model, that is an area of dispute and opinion. When you execute a ping or trace route from a switch or router, which layers come into play?

Webmaster

But, nobody uses a fuly implemented OSI model

Every complete network does, but there's really no such thing as a 'fully implemented OSI model" nor partly implemented OSI model.

we all use TCP/IP, or some proprietary model.

No, the OSI model encapsulates (;)) pretty much all the network systems we use. TCP/IP is a protocol suite, not a model. And even though TCP/IP has it's own model, the DoD model, TCP/IP protocols map to the OSI model as well.

Your question seems to be based on some misunderstandings on what the OSI model is, and once you cleared those up, the answer will become obvious. Actually, it's in your question: "So why is it that everything [regarding networking] we talk about is in reference to OSI?"

In short: because it's a reference 'model'. It's not system, nor a technology or a protocol. ( well actually it can be, if you're referring to OSI's own protocol suite, but that's not the OSI 'model' we, and you and your boss refer to.

From my TechNotes:

The OSI (Open System Interconnection) model is developed by ISO in 1984 to provide a reference model for the complex aspects related to network communication. It divides the different functions and services provided by network hardware and software in 7 layers. This facilitates modular engineering, simplifies teaching and learning network technologies, helps to isolate problems and allows vendors to focus on just the layer(s) in which their hardware or software is implemented and be able to create products that are compatible, standardized and interoperable.

www.techexams.net/technotes/ccna/osimodel.shtml

Network technologies that are developed today still map to the OSI model. Keep in mind that the OSI model is just that, a model of something real, so not every network technology has a 1-on-1 relation with the layers of the OSI model.

sprkymrk

Email starts at layer 7, application. So does surfing the web - Internet Explorer or Firefox, etc. Arp/MAC is at layer 2. TCP/IP would have no way to get from point A to point B without the physical devces at layer 1 (physical). A firewall that breaks down a packet, inspects it, runs an AV scan on it, or whatever is considered an "Application" level firewall. Many more examples could be used, but I hope this helped a little.

Rearden

keatron wrote:

Because the OSI model is general theoretical frame of reference that those "proprietary" models are supposed to be based. While you say no one uses the entire model, that is an area of dispute and opinion. When you execute a ping or trace route from a switch or router, which layers come into play?

That's how I had thought of it as well until I read a couple things in the cisco book. . .

First, where it explained the 4 'layers' of TCP/IP as Network Interface, Internetwork, Transport, and Application. . .

But then, I read the part about "The difficulty these days when discussing the OSI protocol specifications is that you have no point of reference - you simply cannot typically walk down the hall and use a computer whose main, or even optional networking protocols conform to OSI."

Until I read that part about the TCP/IP layers, I had known OSI to be the standard thing that we say. . . .'so and os protocol in TCP/IP relates to Layer X in OSI'. Is this the 'right' way to look at it?

Webmaster

Rearden wrote:

First, where it explained the 4 'layers' of TCP/IP as Network Interface, Internetwork, Transport, and Application.

That's the DoD model I mentioned in my previous reply, if you follow the link to my TechNotes I posted you can see how it relates to the OSI model.

Rearden wrote:

Until I read that part about the TCP/IP layers, I had known OSI to be the standard thing that we say. . . .'so and os protocol in TCP/IP relates to Layer X in OSI'. Is this the 'right' way to look at it?

You must have missed my last line: "Keep in mind that the OSI model is just that, a model of something real, so not every network technology has a 1-on-1 relation with the layers of the OSI model."

"The difficulty these days when discussing the OSI protocol specifications is that you have no point of reference - you simply cannot typically walk down the hall and use a computer whose main, or even optional networking protocols conform to OSI."

The OSI model is 'the' reference for almost anything you will learn about networking. Even though the model is never an exact representation of the real world, and the protocols don't entirely conform to the model.

Rearden

Thanks. Sorry for the seemingly ignorant reply. I was just a little confused.

Webmaster

Don't be hard on yourself, the OSI model 'is' confusing, but eventually, when you see through the mysterical clouds that surround it, it will actually make networking technologies less confusing.

HELLZxPHADER

Osi model is useful for pinpointing network related problems, to cancel out certain layers. This will save you time in the troubleshoot and configuration process in both real life and ccna test.