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  • #3938004

    sending data (OSI/TCP) – steps involved

    by nickjarvis1 ·

    I understand roughly the OSI model, and I also understand the TCP/IP model. What I am struggling to grasp is the actual process and how many times it might happen during the ONE request. If I open up a browser and goto a website, then using the theory of the OSI model, EVERY place I read up on this implies that everything happens in one go, going from layer 7 to layer 1, before reaching the destination, where it will go from 1 to 7. Surely, an ARP request will happen, or a DNS lookup and many many other things will happen. Surely each of these process use this model seperately. So, if I enter a web site name, it cannot be a simple case of going through this theoretrical model once, from 7 to 1 and then when reaches its destination 7 to 1. It must do this many times. Every website I goto implies it is done just ONCE??

    When trying to learn something, I cannot just accept things. I have to understand why and how. Mayber wireshark would list all the processes?

    Also, if the physical layer is responsible for sending the bits of data (1s and 0s), but I am using my wireless laptop, does this mean that data is transferred from my laptop using RF and NOT digital bits??

    No idea whether this question(s) makes sense?
    Thanks
    Nick

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    • #3939383

      Re: OSI

      by kees_b ·

      In reply to sending data (OSI/TCP) – steps involved

      You’re right. For example, transferring a file using FTP (in the application layer) involves a lot of messages if it’s a big file. Nobody will ever say that it’s one message going through the layers once after logging in and checking, say, if the input file and output folder exist.

      The physical layer is used to transfer one defined message from point A to point B. It has a defined interface: both input and output are defined, the physical medium is inside and can be everything. For WiFi, the input and output are bits, the medium is an electromagnetic wave.
      For a dial-up modem (did you ever use it?), the input and output are bits (in, for example, a serial interface or USB), the medium is audible cracking beeps. Which, inside the AT&T network, can be digital for part of the way (transmitted by copper or air or glass fiber or qbits), but not identical to the bits sent and received by the modem. But for every step (A to B, B to C, …, T to U) there’s a protocol (defining an input and an output) and a medium, and in the end what goes in into A comes out of T which U can hear or read.

      It’s like a radio broadcast. The sound from a microphone goes in, the sound from the speaker comes out. In between it’s electromagnetic: analog (AM, FM) or digital (DAB via the air).

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