Posted on 2004-11-07
Last Modified: 2013-11-29
Hi all,

   Please can anyone give me the nice description of what the lower 4 layers (Physical,Data link, Network , Transport) actually do when one system wants to send the packet to another system.

Please don`t give me any reference. Explain in your own words.

thanks in advance,
Question by:dineshb_2001
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      Don't know if we are allowed to answer this based on
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    donkey's dont have layers..  - sorry just had to..

    Ok.. well when a packet comes from the upper layers (application, presentation, session) it is still in data form... but as it goes through each layer, it adds a header onto the data that comes in....  eg, through the transport layer, it adds the TCP header to the data, (now called a frame), at the network layer, it adds for instance an IP header with IP address, (and now called a packet), at datalink layer, it adds a header and a trailer... has data like the MAC address... (now called a segment) and finally goes down to the physical layer, where it is translated into bits to go onto the wire...

    On the other end, as it goes up the layers, it strips the header, and reads the information...
    LVL 11

    Accepted Solution

    I'll do my best ... I'll try at least
    so ... it's important to understand first some basics. When information moves from upper layers to lower layers a process called "encapsulation" occurs. This is ... imagine a plates stack. Each plate is different colors. Now each plate is a OSI layer.

    If there are 7 people eating at your table, then after they finished you just have to clean the table? right? :) Put each plate over the other and you should have your plates "encapsulation". Now, in netwroking this happen like this (starting with the transport layer - in your case. Actually, the process starts at the App Layer):

    1. Transport Layer: PDU (packet data unit) are Segments
    2. Network Layer: Segments are converted to Packets
    3. Data Link Layer: Packets are converted to Frames
    4. Physical Layer: Frames are converted to Bits.
    5. Bits goes into your physical media.

    Actually we can say that there's always a virtual communication between the same layer on both ends. For example, the transport layer at one end, exchange information with the transport layer on the other end. Is this clear so far? I hope it is.

    As you might have already read, each packet contains specific information and has its own function. If you want to know more detail one this, in my own words, just let me know.

    It might be usefull to you to know that Routers works at the first three (or more - some routers) layers; switches and bridges function at the first 2 layers; Hubs function at the physical layer. This is important in understanding the difference between these networking devices. If you know what each layer does, then you know what each device does.

    I know u haven't asked for online articles but you might took a look at this link:


    Author Comment

    Hi rafael,

    Anyway the points will be given to you but before giving I want to clear one thing.
     As u said the   "Actually we can say that there's always a virtual communication between the same layer on both ends. For example, the transport layer at one end, exchange information with the transport layer on the other end."

    U mean it is the virtual communication and not the real communication. here virtual means the two corresponding end layers communicate with the help of their lower layers or they send packet to each other.

    please clear to me this point.

    Thanks in advance,
    LVL 11

    Expert Comment

    Yep. This is very true!!!
    Very happy for being of some help to you. I'm not here for the points itself. It's just something I like to do.

    Take care man
    LVL 11

    Expert Comment

    "with the help of their lower layers ..."

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