How Encapsulation Works Within the TCP/IP Model 11:09 AM

In the previous section we reviewed the TCP/IP and OSI model. For the sake of clarity, we will be using the TCP/IP model to demonstrate encapsulation, as compared to the OSI model. If you are still struggling with grasping concepts of the TCP/IP model you may wish to view the previous section once more. Otherwise, onward to the encapsulation process!

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.

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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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OSI 7 Layers Reference Model For Network Communication 10:40 AM

Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984, as a conceptual framework of standards for communication in the network across different equipment and applications by different vendors. It is now considered the primary architectural model for inter-computing and internetworking communications. Most of the network communication protocols used today have a structure based on the OSI model. The OSI model defines the communications process into 7 layers, which divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.

The OSI 7 layers model has clear characteristics. Layers 7 through 4 deal with end to end communications between data source and destinations. Layers 3 to 1 deal with communications between network devices.

On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

Layer 7:Application Layer

• Defines interface to user processes for communication and data transfer in network
• Provides standardized services such as virtual terminal, file and job transfer and operations

Layer 6:Presentation Layer

• Masks the differences of data formats between dissimilar systems
• Specifies architecture-independent data transfer format
• Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data

Layer 5:Session Layer

• Manages user sessions and dialogues
• Controls establishment and termination of logic links between users
• Reports upper layer errors

Layer 4:Transport Layer

• Manages end-to-end message delivery in network
• Provides reliable and sequential packet delivery through error recovery and flow control mechanisms Provides connectionless oriented packet delivery

Layer 3:Network Layer

• Determines how data are transferred between network devices
• Routes packets according to unique network device addresses
• Provides flow and congestion control to prevent network resource depletion

Layer 2:Data Link Layer

• Defines procedures for operating the communication links
• Frames packets
• Detects and corrects packets transmit errors

Layer 1:Physical Layer

• Defines physical means of sending data over network devices
• Interfaces between network medium and devices
• Defines optical, electrical and mechanical characteristics

There are other network architecture models, such as IBM SNA (Systems Network Architecture) model . Those models will be discussed in separate documents.

The OSI 7 layer model is defined by ISO in document 7498 and ITU X.200, X.207, X.210, X.211, X.212, X.213, X.214, X.215, X.217 and X.800. The protocols defined by ISO based on the OSI 7 layer mode are as follows:

Application ACSE: Association Control Service Element

CMIP: Common Management Information Protocol

CMIS: Common Management Information Service

CMOT: CMIP over TCP/IP

FTAM: File Transfer Access and Management

ROSE: Remote Operation Service Element

RTSE: Reliable Transfer Service Element Protocol

VTP: ISO Virtual Terminal Protocol

X.400: Message Handling Service (ISO email transmission service) Protocols

X.500: Directory Access Service Protocol (DAP)

Presentation Layer ISO-PP: OSI Presentation Layer Protocol

ASN.1: Abstract Syntax Notation One

Session Layer ISO-SP: OSI Session Layer Protocol

Transport Layer ISO-TP: OSI Transport Protocols: TP0, TP1, TP2, TP3, TP4

Network Layer ISO-IP: CLNP: Connectionless Network Protocol

CONP: Connection-Oriented Network Protocol

ES-IS: End System to Intermediate System Routing Exchange protocol

IDRP: Inter-Domain Routing Protocol

IS-IS: Intermediate System to Intermediate System

Data Link HDLC: High Level Data Link Control protocol

LAPB: Link Access Procedure Balanced for X.25

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