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Thread: OSI Model difficulty CCNA semester 1 student here

  1. #1

    OSI Model difficulty CCNA semester 1 student here

    Hi everyone, first off thanks for viewing.

    I'm trying very hard to grasp the concept of the 7 layer OSI model. I have a few questions and would greatly appreciate any help. Firstly, my current view of the application layer is that it can be any common platform that is dependent on the network for use and connects the client to a server. Examples would be an e-mail server, AIM, etc. The presentation layer then is responsible to code (or decode) data sent to the node. Next, the session layer sets up a connection for the information to be sent across. Next, the transport layer breaks the data up into smaller segments and sets up the rate and speed at which the connection will be made. The network layer and below seems to be where i'm having trouble. I know that routers are a part of the network layer, but how can this be when there has not been any physical transmission of data up to this point? I was thinking routers made the decision on which way to send the packets once the actual electrical signals had been received by them. I'm assuming firstly that the network layer assembles these segments into packets and establishes the origination IP and destination IP addresses. So at this layer the routers must also predetermine the best path for the bits to be sent (sense they haven't already been sent yet). The packets are then transformed into frames at the data link layer and then are transformed into electrical signals with meaning to be sent over the physical mediums. Up until this point, non actual data has been transferred. Next at the physical layer, the electrical signals are sent over the physical cables and wires that actually make up the physical network and the process is reversed from that point.

    I know alot of this may be off but i'm just beginning the class and i'm scared i'm not grasping the concepts right. I really want to succeed in this class (and the IT field in general) and will put as much work in as necessary.

  2. #2
    fem the retarded rabbit mccoffee's Avatar
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    Like you I went though the cisco program the osi model is theroalitcal model. The router is a layer three since it deals with ip adressing. Some tips to help ya if you understand most of this below your on the right track. To me from what you said you are getinng it ,and your not that far off
    Layer 1 is cabling/wireless singals hubs
    Layer 2 is switches and anything that deals with mac addressing primarlly
    Layer 3 is the network card , routers , layer three switching which you cover more off in cisco 3 semsester.
    Layer 4 is the transporting such as tcp udp igrp eigrp ospf
    Kayer 5 session layer tunelling ,and i want to say vpn protocols
    Layer 6 is pop3 dns http sockets
    Layer 7 is the application it self for pop3 would be outlook http will be ineternet xplorer.

    layver 1 bits
    layer 2 frames
    layer 3 packet
    layer 4 and up data

    Sending end Receving end
    Phys ---> app
    data--->press
    netowork-->session
    trans--->trans
    session-->network
    pres--->dat
    app-- phys

    Now as far as routers making a decision they use many factors you mettioned path detmenation by electrical signals which is true depending on what routing protocol is used if one link is down ,and there is a another reduntant link on the router to that the next hop it will use that reduntant link.

    Routing itself occurs on layer 4 read up more on egirp ,igrp,ospf, rip ,and rip v2 each one of thease protocls use differnet criteria to route the traffic. It's more about the protocol than the router making the decision hopefully that makes sense if you have any promblems ,or more question feel free to ask.
    Last edited by mccoffee; 08-27-08 at 09:12 AM.
    Comptia a+ n+

  3. #3
    fem the retarded rabbit mccoffee's Avatar
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    Sorry to double but had more things to add this a good site for cisco help it's an eaiser read since it has some humor in it. http://www.routergod.com/?p=44

    NOthing to fear with the cisco stuff it does take time I took the tranning twice once six years ago ,and once more with in the past three. The first time some of the concepts didn't stick due to lack of expirence with networking. Another suggestion is try also looking at some comptia network plus stuff the network+ is a very simmilar test to the ccna part 1 ,and 2. Plus comptia's take on networking is more concept based for me it helped taking that with the cisco stuff that's just me.

    Here is a free test site for comptia's ,and cisco ,and some other certs a good way to practice alot of it is memorization. http://www.free-tests.com/comptia.shtml
    Comptia a+ n+

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