The OSI Model in Operation

There are four general rules that control the operation of the OSI model:

1. Data created at one layer must move down all layers below it before it can be transmitted to a receiving station. If that data was created at the Application layer, then it would move down through the six layers below it prior to transmission. If the data was created in the form of a network address at Layer 3, then it would move down through Layers 2 and 1 prior to transmission.

2. When data arrives at the receiving station, it must move up to the layer (peer layer) where it was created at the sending station. If the sending station created the data at the Application layer, then the data would have to move down through six layers of the sending station, be transmitted to the receiving station, and then move up through six layers at the receiving station until it arrived at the Application layer. If the data was generated at Layer 3, then it would only move up to the peer layer (Layer 3) at the receiving station.

3. A layer at the sending station can only communicate with its peer layer at the receiving station and vice versa. In other words, a packet of data that is assembled at the Network layer of the sending station can only be disassembled by the Network layer of the receiving station.

4. Each layer provides services to the layer directly above it in the OSI stack.

The OSI stack was presented previously in reverse order because it seems to be a little easier to understand that way. When you see the OSI stack in the real world, it will always be ordered from Layer 7 down to Layer 1. If you have trouble remembering the order, or the layers themselves , try saying; " A ll P eople S eem T o N eed D ata P rocessing." This should remind you of the layers and their order from Layer 7 (Application) to Layer 1 (Physical).

Moving Through the OSI Model from the Application Layer

Let's follow a stream of data as it moves from the Application layer at a sending station to the Application layer at a receiving station. Remember that for a user application to be considered an application at Layer 7, it must require communications to another node or station. Email is an excellent example of a user application requiring communications, and as such is a Layer 7 application.

So, starting with an email message at Layer 7, the message is first passed to Layer 6 (Presentation). Layer 6 encrypts and/or compresses the message as required, and passes it to Layer 5 (Session).

Now the plot thickens. Starting with Layer 7, the message will move through five steps of encapsulation. Encapsulation is the process of putting something inside of something else, much like placing a letter in an envelope. Each layer from this point on will add information to the message. The additional information includes processing instructions for the same layer (Layer 5) at the receiving station. We say the message is encapsulated because the additional information is added as a header to the front of the message and often includes a trailer at the end of the message. So in our case, the now encrypted and/or compressed message is encapsulated at Layer 5 with information indicating that the message is part of a single session. When information is encapsulated, it becomes a protocol data unit ( PDU ) . Each layer has its own PDU and the name of the PDU for Layer 5 is "data" .

Occasionally, the name of a PDU will be the only clue you have as to the correct answer for a question. You need to know protocol data units and the function of each layer of the OSI model backwards and forwards. Sounds simple right? Well, if the plot thickened in the previous paragraph, this is where it coagulates into an ugly goo! People in the industry (including the fine folks at Cisco) cannot agree on how to properly describe encapsulation in the OSI model. Older Cisco instructors, and a majority of industry documentation, describe five steps of encapsulation as we are doing here with the PDU for Layers 7 and 6 called an "information unit." We have seen test questions approach encapsulation both ways. Of course, current Cisco instructional material, at least the material we have seen, clarifies the matter by ignoring the issue completely. So, what should you do on the test? Well, everybody agrees that encapsulation occurs at Layers 5, 4, 3, 2, and 1. They also agree that the PDUs for these layers are respectively "data", "segment", "packet", "frame" , and "bits" (we will discuss this in detail later). The only areas of uncertainty are whether Layers 7 and 6 are encapsulating levels, and should the protocol data units for these layers be called "data" or "information unit" . So here are our recommendations:

Read the question very carefully . It will probably provide a clue.

• There are two ways of looking at encapsulation. Think before answering any question dealing with encapsulation or PDUs.

• If there are equally correct responses for both views of encapsulation, GUESS. A guess may not be right, but no answer is definitely wrong.

Figure 2.2 shows five levels of encapsulation with associated protocol data units and OSI layers. There is still uncertainty as to whether Layers 6 and 7 should be viewed as encapsulating with a protocol data unit of " data" or as not encapsulating with a protocol data unit of " information unit" .

Figure 2.2. Protocol data units and encapsulation.

When a layer formats information from the layer above and adds additional information, usually in the form of a header, the resulting assembly becomes a "Protocol Data Unit" (see Figure 2.2). In effect, the data from the higher layer (the letter) is encapsulated in data from the lower layer (the envelope). Layers 5 through 1 have PDUs with information specific to that layer. PDUs that are specific to a given layer are why only peer layers (layers at the same level) at the sending and receiving station can communicate. If all of this seems a bit murky, read on and it should become clearer.

The Transport layer takes the Data PDU from the Session layer, adds additional information as a header (encapsulating), and passes the resulting unit, now called a Segment PDU , to the Network layer. The Network layer in turn encapsulates the Segment into a Packet PDU with a logical source and destination address, and then passes the Packet to the Data Link layer, which encapsulates the Packet into a Frame with a physical source and destination address. The Frame is then passed to the Physical layer where it is turned into a Bits PDU and finally placed on the network medium.

If we have done everything right we will have used five steps of encapsulation, while passing through seven layers, and yes you will need to keep all of this straight for the test. Maybe Figure 2.3 will help.

Figure 2.3. Data flow between network stations .

Figure 2.3 shows the flow of data between two network stations. Five levels of encapsulation imply the PDUs for Layer 6 and Layer 7 would be an "information unit". Seven levels of encapsulation would use "Data" as the PDU for Layers 5, 6, and 7 and identify the upper two layers as encapsulating.

Packet Handling at the Receiving Station (Reversing the Process)

Now that we have gone through seven layers and five steps of encapsulation, what do you think happens at the receiving station? Right, everything happens in reverse as the information flows up the OSI stack. The Physical layer of the receiving station senses voltage on the medium, converts it into Bits , and strips off any other signals placed there by the Physical layer of the sending station. The remaining Frame moves up to the Data Link layer where the physical source and destination address are removed and the resulting Packet is passed to the Network layer. There the logical source and destination address are removed and the remaining Segment is passed to the Session layer. Well, you get the idea. The process of moving up the OSI stack at the destination node is called de-encapsulation and when we finally get the information unit to the Application layer of the receiving station, we have successfully passed data from one node to another as described by the OSI model.