Understanding how data moves across an internetwork is a very important component of being a network engineer.You need a comprehensive grasp of the technologies and the standards they support, and you also need to know how those technologies and standards relate to the actual network.The OSI system model bridges that gap for you. Knowing the details of the network as well as the way end-user applications interact with the network is a powerful troubleshooting tool.

One of the easiest analogies used to understand the OSI system model is that of sending a letter through the mail. A number of items must be completed for your letter to be delivered to the appropriate recipient.We walk a letter through the postal system and illustrate the parallel connections to the OSI system model.

The first thing that you need to do to send a letter is to write it.You sit down at your desk and write a letter to your friend that lives on the other side of the country. After you finish writing the letter, you get an envelope and address it to your friend.You then walk to your mailbox and place the letter inside.These actions correlate to the OSI system model layers nicely.Writing the letter corresponds roughly to the Application layer. If you used a word processor to write the letter, then print it out to place in the envelope, the act of printing the letter would be similar to what happens at the Application layer.The fact that you printed the letter means that you relinquished control of the letter to the network, the postal system in this case.Your actual words on the paper correspond to the Presentation layer in that you needed to ensure that the recipient, your friend, can read the letter.You presented your thoughts in a format your friend can read and comprehend. Addressing the letter can correspond to the Session, Transport, and Network layers. In networking terms, the steps of sealing the letter in the envelope and addressing it relate to the actions of UDP in a TCP/IP network. The data, your letter, was encapsulated in the envelope and passed down through the OSI model to the Network layer where it was addressed.Without the address, your letter cannot be delivered and the same principle applies to networking. Data cannot be delivered without an address. Placing the envelope in the mailbox is comparable to what happens at the Data-link and Physical layers of the OSI system model.The envelope was placed or encapsulated in the correct format for delivery on the network where it will be transmitted to the recipient. The mailbox maps to the Data-link layer and the postal carrier that picks up the envelope would be the Physical layer, responsible for ensuring that the envelope is delivered

Now that the envelope is in the network, the postal system, it may pass through many different offices. If you view these offices as nodes on a network, they would correspond to routers.The envelope reaches your local post office, or default gateway in a TCP/IP network, and is scanned by a computer to determine if the envelope requires routing for proper delivery. In this example, your friend lives across the country, so the envelope does need to be routed.The computers in the post office review the destination address and determine the best path for the envelope to take to reach its final destination.The next office, or hop, on the path the envelope takes may be a regional office or some other central location with routes to the next hop.Your envelope is transported by mail truck, plane, or other form of transportation.The actual path and transmission medium are unimportant to you as you relinquished control of your letter when you placed it in your mailbox.You are trusting that the postal service will ensure that your letter arrives.

Your envelope finally reaches the local post office for your friend.The envelope is delivered to your friend and is opened.Your friend opens the envelope, pulls out the letter, and reads it.These last steps correlate to the OSI system model working in reverse.The data, your letter, is de-encapsulated when the envelope is opened.The contents are then delivered to the recipient when your friend reads the letter, a mapping to the Presentation layer, and comprehends through the Application layer.

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