The TCP/IP Model

As we learned in the previous section, the TCP/IP model has four layers. You can view a diagram of the model below.

Keep in mind that we divide the stack into four separate layers because they each perform a certain role or task. As data is being sent from one computer, it will pass from the top layer to the bottom. On the receiving end, the data will then be rebuilt from the bottom layer to the top. You can view an example of this process below.

Each layer a packet of information travels through adds what is called a header. Think of it in terms of a Russian doll. You’re probably familiar with them: each doll has another smaller doll inside of it. Just like the dolls, each layer a sending packet passes through gains another header (or doll). When the packet is being rebuilt on the receiving end, each header is unpackaged the same way. You can see an example of a sending packet gaining header information below.

Note that at the receiving end, we would have the reverse process (Headers would be taken away at each layer, until the receiving packet is by itself.)

Since each layer of the TCP/IP model does a unique task separate of the other layers, we refer to the data package at each layer with different names. For instance, the data package at the Application Layer is called a message, while the same data package at the Internet Layer is called a datagram. Review the diagram below for the complete list of names.

Notice that the Transport Layer may have one of two names- a segment or a datagram. If the TCP protocol is being used, it is called a segment. If the UDP protocol is being used, it is called a Datagram.

The data then passes through the Internet Layer onto the Network Access Layer, where a frame is created. Once the data packet leaves this level it is converted into a bitstream of electrical pulses, commonly referred to as 1’s and 0’s.

Finally, you should note that Cisco demands CCNA students to know specific information on the Data Link Layer and encapsulation. As you can see, we haven’t used the OSI model, but the TCP/IP model (so we use the Network Access Layer as opposed to the Data Link and Physical Layer). Specifically, Cisco demands that students know that packets are packaged into frames at the Data Link Layer. And, like other layers, a header and trailer are added to the information at the Data Link Layer. You can see the encapsulation process with the OSI model below.

We know what you’re thinking- where’s a good pneumonic when you need it? The easiest one we could find was “Dirty Sick People Feel Bad,” whereas each letter of each word corresponds to Data, Segments, Packets, Frames, and Bits.

Putting it All Together

Now that we have the basics down, we can finally review the entire process of data encapsulation. Refer to the below list to see a real-life example of encapsulation. If needed, you can view the above diagrams if you get lost.

The Data Encapsulation Process

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• 1. One computer requests to send data to another over a network.
• 2. The data message flows through the Application Layer by using a TCP or UDP port to pass onto the internet layer.
• 3. The data segment obtains logical addressing at the Internet Layer via the IP protocol, and the data is then encapsulated into a datagram.
• 4. The datagram enters the Network Access Layer, where software will interface with the physical network. A data frame encapsulates the datagram for entry onto the physical network. At the end of the process, the frame is converted to a stream of bits that is then transmitted to the receiving computer.
• 5. The receiving computer removes the frame, and passes the packet onto the Internet Layer. The Internet Layer will then remove the header information and send the data to the Transport layer. Likewise, the Transport layer removes header information and passes data to the final layer. At this final layer the data is whole again, and can be read by the receiving computer if no errors are present.

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And there you have it: encapsulation at its finest. We realize that encapsulation isn’t the easiest concept to grasp, but steady review of this and previous sections will ensure you will grasp the concept soon enough.

Closing Comments

Data encapsulation allows devices to communicate, and therefore, it is completely necessary to learn. It is present on networking exams such as the CCNA, so be sure to review these concepts before exam day.

In the following sections we’ll be looking at each layer in more specifics. The next section we will review the Network Access Layer in particular. Physical addresses, frames and LAN technology- clearly we will have our hands full!

The TCP/IP Model and Modular Design

TCP/IP is responsible for a wide range of activity: it must interface with hardware, route data to appropriate places, provide error control, and much more. If you are starting to think the TCP/IP suite can get confusing, you wouldn’t be the first.

The developers of TCP/IP thankfully designed what we call a modular design- meaning that the TCP/IP system can be divided into separate components. You may call these layers or modules. But why use a modular design? Not only does it aid in the education process, but it also lets manufacturers easily adapt to specific hardware and operating system needs.

For example- if we had a token ring network and an extended star network, we surely wouldn’t want to create entirely different network software builds for each one. Instead, we can just edit the network layer, called the Network Access Layer, to allow compatibility. Not only does this benefit manufacturers, but it greatly aids networking students in education. We can dissect the TCP/IP suite into different layers, and then learn about each layer’s specifics one at a time. Below you’ll see the TCP/IP model divided into four layers.

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• Network Access Layer – The Network Access Layer is fairly self explanatory- it interfaces with the physical network. It formats data and addresses data for subnets, based on physical hardware addresses. More importantly, it provides error control for data delivered on the physical network.
• Internet Layer – The Internet Layer provides logical addressing. More specifically, the internet layer relates physical addresses from the network access layer to logical addresses. This can be an IP address, for instance. This is vital for passing along information to subnets that aren’t on the same network as other parts of the network. This layer also provides routing that may reduce traffic, and supports delivery across an internetwork. (An internetwork is simply a greater network of LANs, perhaps a large company or organization.)
• Transport Layer – The Transport Layer provides flow control, error control, and serves as an interface for network applications. An example of the transport layer would be TCP- a protocol suite that is connection-oriented. We may also use UDP- a connectionless means of transporting data.
• Application Layer – Lastly, we have the Application Layer. We use this layer for troubleshooting, file transfer, internet activities, and a slew of other activities. This layer interacts with many types of applications, such as a database manager, email program, or Telnet.

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The above layers are more complex than the general descriptions provided, but rest assured, we will get into the specifics in later sections. For now we have another model to learn- the OSI model.

The Open System Interconnection Model

The Open System Interconnection Model, more commonly known as simply OSI, is another model that can help break the TCP/IP suite into modules. Technically speaking, it is exactly the same as the TCP/IP model, except that it has more layers. This is currently being pushed by Cisco since it aids in learning the TCP/IP stack in an easier manner. Likewise, you will see the OSI model on many Cisco exams.

Instead of four layers, the OSI model has seven. You can see a direct comparison of the two models below; notice that only the Application Layer and Network Layer are divided into smaller layers, and the Internet Layer is renamed to the “Network Layer.”

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• Physical Layer – They Physical Layer converts data into streams of electric or analog pulses- commonly referred to as “1’s and 0’s.” Data is broke down into simple electric pulses, and rebuilt at the receiving end.
• Data Link Layer – The Data Link layer provides an interface with the network adapter, and can also perform basic error checking. It also maintains logical links for subnets, so that subnets can communicate with other parts of the network without problem.
• Network Layer – Much like the Transport Layer of the TCP/IP model, the Network Layer simply supports logical addressing and routing. The IP protocol operates on the Network Layer.
• Transport Layer – Since we left out the error and flow control in the Network Layer, we introduce it into the Transport Layer. The Transport Layer is responsible for keeping a reliable end-to-end connection for the network.
• Session Layer – The Session Layer establishes sessions between applications on a network. This may be useful for network monitoring, using a login system, and reporting. The Session Layer is actually not used a great deal over networks, although it does still serve good use in streaming video and audio, or web conferencing.
• Presentation Layer – The Presentation Layer translates data into a standard format, while also being able to provide encryption and data compression. Encryption or data compression does not have to be done at the Presentation Layer, although it is commonly performed in this layer.
• Application Layer – The Application Layer provides a network interface for applications and supports network applications. This is where many protocols such as FTP, SMTP, POP3, and many others operate. Telnet can be used at this layer to send a ping request- if it is successful, it means that each layer of the OSI model should be functioning properly.

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Now, the Bad News

Now that we’ve reviewed each layer, you have to commit each layer and its function to memory. Most networking exams require that knowledge of each layer be present. We realize that remembering seven different layers is tough- so we use a mnemonic. A mnemonic is simply a tool we can use to remember all seven layers. Look at each beginning letter of each layer- it’s PDNTSPA, starting with the Physical Layer. You could come up with a phrase such as “Please Do Not Throw Sausage Pizza Away,” to help you remember each layer name.

It is important to remember that each layer is a standard- not an implementation. This means that not all network communication will necessarily use each layer. We partly covered this with the Session Layer, which isn’t always necessarily used. Some devices such as routers only operate at the third layer and below. Some devices are even more limited- repeaters only work at the physical layer of the OSI model.

Closing Comments

The OSI and TCP/IP model are fairly prevalent in networking- don’t be surprised if you see them more than you’d like. If you take anything from this section, remember to use a pneumonic to memorize each layer name in order. You can get as crazy as you’d like with the phrase you use, but “Please Do Not Throw Sausage Pizza Away” is generally the easiest to remember.

In the next article, we will be specifically looking at how data moves from one computer to another- and how it moves through the OSI model. Don’t worry if this seems new to you and you don’t quite take all of it in, simply review it some more and move on to the next section. And, as always, you can review the previous section if you didn’t quite grasp all the concepts in this one.