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OSI Model

Discussion in 'Network+' started by obsolete, Nov 14, 2008.

  1. obsolete

    obsolete Bit Poster

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    I'm having trouble understanding the OSI model. When I read the book, I try to make a visualization but it doesn't fall in to place. I checked many books, and a few try to explain it like the post office. The problem is that I'm having trouble relating them. This is how I interpreted when I read. When the books mentioned peer-to-peer layering, I think of making requests. Similar to the post office where you talk to someone over the phone to get the information on location or to double check on delivery. Whereas, with adjacent layering, that's when the actual data, like sending files being processed. I'm not sure, the problem is my view of data. They use the word data often in the books. When two machines communicating for info, I see that as data. I also see sending a file as data as well. If I start from the top, I visualize my machine making a request to download a file from Computer B. In order to make a request, my computer has to send a message (communicating) to Computer B as data during the Application Layer phase through browser. But how does Computer A and other network devices like routers and switches gather info on where other machines are located? The problem is that I think maybe I'm trying too hard to visualize what's happening at the data at each layer and on each device. I know it may sound farfetched, but maybe I'm being too concrete and detailed. Similar to ports (not the physical ones), those are hard to visualize as well. Any help will greatly be appreciated. An explanation in a very detailed step by step with vivd imagery will help.
     
  2. dmarsh

    dmarsh Terabyte Poster

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    Ok, I'll try my best.

    Say I want to send a letter. So first thing I do is write down my message on some piece of paper, I probably include some details like the message, my name, contact details, date etc for the recipient. I've typically encoded the message in a form suitable for the recipient, say modern english. Now I want to send the letter, in order to have the letter routed by the postal system I have to put it in an envelope, on the envelope I put the routing information, the address and the postal code. The postal system routes the letter based on this routing information, it goes through various sorting offices, depots, sorting machines, until eventually it gets received and the recipient removes the envelope and reads the message contained in the letter.

    Now things could have been different if the recipient lived in a different country, maybe I would have to translate the message or I would send it to a translation service and they would translate and re-transmit.

    Maybe the recipient would live in a country with different addressing schemes.

    Thus there are many different aspects to communication systems, latency, fault tolerance, information loss, contention, encoding, addressing, routing, translation etc. These exist if its a roman messenger running with a scroll, a carrier pigeon, modern airmail, or saterlite communications or even a space probe.

    Its not just 'data' its information, the data is formated and used in very specific ways. Its not so hard to understand this, look at the frames/packets, read about the protocols.

    The famous Two Generals problem is an interesting puzzle regarding information loss and unreliable communications that demonstrate some of these aspects.

    Ok taking a browser as an example the browser is an application in the application layer, it uses an application layer protocol called HTTP. The data is encoded using a MIME encoding and packaged up into a HTTP request. The HTTP request is then passed to the operating system using the TCP/IP WinSock library. This library deals with the details of TCP/IP, the operating systems TCP/IP stack and interfaces with the NIC driver, the NIC driver interfaces with the NIC. The OS then deals with creating a valid TCP/IP request. First the Transport layer is entered, here TCP headers and footers are added, then Internet layer, here IP headers and footers are added. Finally the Link layer is entered, here the ARP cache is read or ARP requests are made to determine the next routable address. The frame is routed though network layer (Ethernet), devices in this layer may use details from higher up layers like the Link or Internet layers to help transmit the frame. Finally the frame gets to the web server which is the destination host, here the process is reversed going through the same layers in the reverse order, the headers and footers are ignored until finally the web server in the application layer on the server gets the HTTP request and gets to interpet it and form a HTTP response. This HTTP response is then transmittted using a similar process.

    Browsers are multi threaded and HTTP is stateless, plus websites can have many different types of dynamic content so one page can result in anything from 1 to 200+ HTTP requests and responses. You can use Firebug, Fiddler, IEWatch, Charles, ngrep, tcpdump, Wireshark, Network Monitor etc to observe this.

    See here :-

    http://en.wikipedia.org/wiki/TCP/IP

    You are far better off investing in some good materials than trying to ask such in depth questions on a forum, buy some books, get a CBT, join a local college etc.
     
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  3. BosonMichael
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    BosonMichael Yottabyte Poster

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    Yep... that was my problem in responding to this thread when I first read it, because entire books have been written on this subject.

    Ob, my advice to you would be, if you don't understand the OSI model, skip it and learn everything else - how does networking and addressing and connectivity work - THEN come back to it, and it will likely be a lot clearer.
     
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  4. Obinna Osobalu

    Obinna Osobalu Banned

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    The OSI Reference Model
    The OSI reference model was created in the late 1970s, and the main reason the International
    Organization for Standardization (ISO) released the OSI model was so different vendor networks
    could work (communicate) with each other. One of the greatest functions of the OSI specifications
    is that they assist in data transfer between disparate hosts; this means, for example, that you can
    use them to transfer data between a Unix host and a PC or a Mac.
    The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application developers
    can use to create and implement applications that run on a network. It also provides a
    framework for creating and implementing networking standards, devices, and internetworking
    schemes.
    The OSI has seven different layers, which are divided into two groups. The top three layers
    define how the applications within the end stations communicate with each other and with users.
    The bottom four layers define how data is transmitted end-to-end.


    The above is just an intro culled from Sybex CCNA FastPass E-Book by Todd Lamle. The OSI reference model has been explined in some materials but i find this particular material more concise and abridged.. Avail yourself of this material.
     
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  5. BosonMichael
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    BosonMichael Yottabyte Poster

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    By "culled", you actually mean, "copied from a copyrighted text". :dry
     
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  6. Luddym

    Luddym Megabyte Poster

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    That's the least of it BM. I'm pretty sure the poster of that won't bat an eyelid. :blink
     
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  7. Obinna Osobalu

    Obinna Osobalu Banned

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    Wouldn't use the word "copied" because i just tried to explain the intro of what I have prievously read from that material:rolleyes:
     
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  8. Obinna Osobalu

    Obinna Osobalu Banned

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    Yeah Yeah...:(
     
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  9. BosonMichael
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    BosonMichael Yottabyte Poster

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    You're probably right. And to top it off, he'll likely make some snarky comment to me just as he has Zeb, Harry, and other respected members of this forum in the past few days.
     
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  10. BosonMichael
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    BosonMichael Yottabyte Poster

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    Please... you didn't explain ANYTHING... you copied what is IN HIS BOOK, word-for-word, and punctuation-for-punctuation! :blink

    Don't take us for naive fools, Obinna, because we're not. I may have been born at night... but I wasn't born LAST night. :dry
     
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  11. Obinna Osobalu

    Obinna Osobalu Banned

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    This is actually the only one thing that you have said correctly in the past few days.... actually didnt want to prove you on that but then, what the heck.....



    And yet you still ask if you already know.... now what does that make you; "a naive fool" nah, something worse:eek:
     
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  12. dmarsh

    dmarsh Terabyte Poster

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    I agree, the OSI model itself IS abstract, personally I'd stick it right at the end of any networking communications course and not near the beginning like the Network+. Ignore it as much as you can and learn other stuff, then review it every now and then, if it still does not make sense, go and learn some more, then come back later. The general concepts of layering, encapsulation and abstraction possibly can be picked up earlier. If you need a more concrete example I'd try and stick to TCP/IP and its 4 layer model. Bear in mind that the layers in both models are in reality no way fixed in stone, they are design concepts for people trying to logically break down a system, depending on what they look at there could be more or less, they might define sublayers for instance. These layers may be obvious or not in the finished system depending on how it was built. For example some people built TCP/IP stacks rigourously to the OSI model others realised this was not necessary and wrote TCP/IP stacks with less layers, these tended to be smaller and more efficient if not as 'architectually pure'.

    http://www.tcpipguide.com/free/t_TCPIPProtocols.htm

    This is the trouble with many of the certifications, many of them aren't really designed for beginners or pros, they pick and choose some quite arbitary stuff to test on. This makes it tough for both parties, beginners find they need to master too much stuff too fast with lots of gaps and random ordering. Seasoned pros find they need to dumb down or give the 'approved' answer.

    Thats why many say only take certifications after some experience, but hopefully still while you are learning so you get some benefit out of the required study.
     
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