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OSI Model

Posted on 2004-08-27
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Hi there,

What is the real problem OSI model is trying to solve?
How does it impact, say a network student in today's context, does not know about OSI?

Things seems to be working so fine now a day, and it makes me wonder to importance of OSI for a student to know practically.

Any comments?

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Question by:kwseow
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by:Pete Long
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>>What is the real problem OSI model is trying to solve?

When the OSI was concieved, every major vendor had their own propriatart systems, IBM wouldnt connect to Digital etc, firms had to but a system and then HAD TO get support from the people they bought it from - who could charge what they wanted.

getting dissimilar systems to talk to each other cost companies a lot, so the OSI was designed for the sole reason of getting dissimilar systems to talk to each other using accepted standards.

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by:Pete Long
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>>How does it impact, say a network student in today's context, does not know about OSI?

In many ways it doesnt - but its like a child learning its times tables, to begin with everything is disjointed and nothing makes sense, the more bits it learns the easier the resst of it becomes.

learning at what level of the OSI things are happening makes it easier to understand switching, routing and network technologies, if your implementing VLANS how do you know the difference between a layer 3 switch and a switch if you dont understand OSI?

it easier to dicern different types of protocol - how can you understand the difference between a ROUTING protocol and a ROUTED protocol without OSI (or the ting and ted story :)
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kwseow,
The OSI Model isn’t trying to solve anything. It is a Model built by the ISO to REPRESENT the flow of data between nodes on a network. This model is absolutely essential in understanding the communication process. Here are some examples and analogies for you and Im sure you will see that the relevance of this model is still if not more important today as the day in was created.

https://secure.linuxports.com/howto/intro_to_networking/c4412.htm
http://www.pe.net/~rlewis/Resources/james.html
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

Good luck
Shinds57
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by:srpoage
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It can help to troubleshoot issues without having to know the details about every program or system.  When you troubleshoot a network connectivity problem, you know that layer 1 issues would be hardware and cabling.  Is the link light on the card green?  Ok, that's ok, so next check layer 2 and 3 with a ping.  Inside the segment is ok, but outside the segment isn't?  Sounds like a layer 3 problem (router).  Or, we can ping the ip of the node we need to get to, but the program won't connect.  Well, a packet sniffer will show you if there is a problem in session establishment, or maybe the application level is having issues if all else is working.  That sort of thing is how the OSI can be handy.  I started out sceptical about the OSI model, but over time, I've come to see it's usefullness.  Hope this helps.
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by:pseudocyber
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When I memorized the 7 layers, I did so to pass the exams.  

Now that I'm a network engineer analyzing issues with Internet flowing through our 2 Cisco Routers (BGP & VRRP) on the edge, through 2 Nortel Switches, through VRRP interfaces to Checkpoint Firewalls, to more Nortel switches, to a users desktop and they want to tell me "the Internet is down" when they can't open a web page or access email or printing is slow ...

I use the OSI model intuitively every day in the real world.  I typically troubleshoot from the bottom up - check the cabling (layer 1), does the switch see the mac address? (layer 2), can you PING this that everything & check the ARP table (layer 3), is the port open & listening (layer 4), and so on.

If you as a student don't learn the basics - such as the OSI - you are doing a huge disservice to yourself.  Everything is built on it.
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by:PennGwyn
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You don't have to be a certified mechanic to drive a car, but if you're serious about driving professionally, you'd better have some idea what those guages and indicator lights are telling you -- and than means at least theoretical familiarity with what's under the hood.  You don't need to know the OSI model to surf the web, but if thirty other people are looking at you to find out why htey can't, it's kind of useful to break it down into a structure and determine what poieces are working and what are broken.

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by:SithLoaded
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"The OSI Model isn’t trying to solve anything. It is a Model built by the ISO to REPRESENT the flow of data between nodes on a network."

I would beg to disagree.  PeteLong had it right when he was explaining it was built out of a need to get disimilar systems to exchange information.  They developed the OSI model of connectivity and then created the OSI protocol suite that maps directly to the OSI model layer for layer.  The TCP/IP protocol suite doesn't map directly to the OSI model.  There are not 7 layers to the TCP/IP model.

"The Open System Interconnection (OSI) protocol suite is comprised of numerous standard protocols that are based on the OSI reference model. These protocols are part of an international program to develop data-networking protocols and other standards that facilitate multivendor equipment interoperability. The OSI program grew out of a need for international networking standards and is designed to facilitate communication between hardware and software systems despite differences in underlying architectures.

The OSI specifications were conceived and implemented by two international standards organizations: the International Organization for Standardization (ISO) and the International Telecommunication Union-Telecommunications Standards Sector (ITU-T). This chapter provides a summary of the OSI protocol suite and illustrates its mapping to the general OSI reference model."

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/osi_prot.htm




The Internet protocol suite is the set of protocols that implement the protocol stack on which the Internet runs. It is sometimes called the TCP/IP protocol suite, after the two most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were also the first two defined.

The internet protocol suite can be described by ANALOGY with the OSI model, which describes the layers of a protocol stack, not all of which correspond well with internet practice. In a protocol stack, each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the higher layers. Higher layers are logically closer to the user and deal with more abstract data, relying on lower layers to translate data into forms that can eventually be physically manipulated.

The internet model was produced as the solution to a practical engineering problem. The OSI model, on the other hand, was a more theoretical approach, and was also produced at an earlier stage in the evolution of networks. Therefore, the OSI model is easier to understand, but the TCP/IP model is the one in ACTUAL USE. It is helpful to have an understanding of the OSI model before learning TCP/IP, as the same principles apply, but are easier to understand in the OSI model.
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by:SithLoaded
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The OSI model was not built to be a learning tool.  It was built to create a set of universal protocols for communication between disimilar systems.  It HAS BECOME a learning tool, because the TCP/IP suite of protocols has taken the majority of the market.  It is easier to use the OSI model to learn, because of the modularized approach.
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by:kwseow
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Hi all, I'm thinking of splitting points equally to all who comment.  Any objections?
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ThanQ
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