Protocol Data Unit: A Suitcase for Electrons

The data, conversations, or pictures of your cute pet you send or view across this web of connections all have to be packed up for travel to the various addresses linked through the strands of the web. Where IP addresses are the street addresses and cables of copper, strands of glass, or the air waves themselves are the roadways you still have to get those ideas of yours you so diligently produced on that machine we call a computer into a format ready for travel.

To help understand the process of packing up those electrons Network Admins and Gurus have developed two models illustrating what happens to the data we see on the screen to the point of the 1’s and 0’s being transmitted back and forth across the web or for that matter any network. The two models are called the OSI (Open System Interconnection) model and the TCP/IP model which we touched on in the article Internet Protocol: Street Address of the Internet.

4 Layer Model of TCP/IP
4 Layer Model of TCP/IP -Image by D²

As web developers the 4 layer TCP/IP model fits better for visualizing the process of packaging your information you are gathering or disseminating for the web content focused professional.

The OSI model offers a more detailed 7 layer model which some layers are combined in layers of the TCP/IP model for a clearer picture without muddying concepts left better for the Network Admin to worry with.

7 Layer OSI Model
7 Layer OSI Model – Image by D²

It should be noted though that can be beneficial for developers to compare the two models side by side to get a clearer picture of what happens to the data at each layer but is not necessary for the web content professional. The thing to take away from the models regardless of which you use is understanding the process of giving your electrons we so carefully sculpt a suitcase we call packets.

Layer 1: The Application Layer

At the most top level of the models you are interfacing with the data through applications. This layer you can consider the items raw and not ready for transport across the etheric strands of the web. You are writing your blog, editing graphics to be uploaded to a web site, etc. You get the picture, this is you and the machine working in harmony to create!

Layer 2: The Transport Layer

Then when you click send, upload, or type a URL to view in a browser the system at the other end has to prepare that data for Transport which is a layer unto itself in the TCP/IP model. This is where the system begins to divide the data up into different suitcases (packets) for transport over the Internet. This is our TCP layer of the model.

Cisco defines this layer as:

Transport: Defines data segments and numbers them at the source, transfers the data, and reassembles the data at the destination.

– Network Fundamentals, CCNA Exploration Companion © 2008 Cisco Systems Inc.

Think of this layer as the luggage drop off or pick up point at an airport. Your luggage is allocated and numbered for the order to be loaded and unloaded.

Layer 3: The Internet Layer

After the Transport Layer your luggage/packet then is tagged with the destination, this is the Internet layer for the packets. The IP address is applied to the packet so that it will find the destination for pickup at your destination airport pickup station. Just as luggage can be lost in transit so can packets travailing the various paths of the Internet which may cause issue in the reassembly of the data at the Transport layer. This layer is also where the best path or”airplane” to load the data packet on to reach the destination address is decided. It is through this path that we loose our luggage/packets at the various equipment the packet will encounter on its way to the destination address.

Layer 4: The Network Interface Layer

At the Network Interface Layer the packets are being loaded onto the Internet though the actual hardware we call the Network Interface Controller or “NIC.” The NIC negotiates with the gateway and the end destination hardware to say “Hey I’m me and I have something for you” where the NIC at the other end says “Hey, I am me, send what you have for me” This is what Admins call the Physical Layer where the packets are traveling across actual media such as the copper or fiber lines.

In a Blink of an Eye

The amazing part of this is the speed that all this happens. Several negotiations or handshakes occur at the Network Interface layer that you don’t see unless you are at that layer watching the packets flow much like the luggage handlers at airports to insure that the most reliable and fastest path is chosen for delivery of your data. Other considerations are also considered at this layer such as security and rerouting of packets if a route seems to have gone down much like when a flight is delayed and the ticket counter offers a sooner flight which might not be as direct but will still get you there.

What Goes Down Must Come Up

Just as the data travels down the layers it must travel back up the layers to the Application Layer for human consumption and understanding. This means when you request a web page that page on a server is packaged as it travels down the layers to travel across the web to reach your computer NIC then starts the process of being unpacked while traveling up the layers form, Network Interface Layer to the Internet Layer where the IP addresses are verified, then to the Transport Layer where the packets are put back together in order to the final layer the Application layer for your consumption and enjoyment. Much like reaching your final destination and unpacking your suitcases so you can enjoy that vacation you so much deserve.

Image of palmtrees and resort.
Carribean Dominican Republic Vacation Photo
-Courtesy of All Free Download
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